Mechanisms of Altitude-Related Cough

Transcription

Mechanisms of Altitude-Related Cough
Nicholas Paul Mason
Laboratoire de Physiologie et de Physiopathologie
Faculté de Médecine
Thèse présentée en vue de l’obtention du titre académique de
Docteur en Sciences Médicales
Année Académique 2011- 2012
Jury de Thèse
Président :
Prof. Pierre-Alain GEVENOIS
(Service de Radiologie, Hôpital Erasme, ULB, Bruxelles)
Membres Facultaires :
Dr. Guy DECAUX
(Service de Medecine Interne Génerale, Hôpital Erasme, ULB, Bruxelles)
Prof. André De TROYER
(Service de Pneumologie, Hôpital Erasme, ULB, Bruxelles)
Prof. Paul DE VUYST
(Service de Pneumologie, Hôpital Erasme, ULB, Bruxelles)
Prof. Alexandre LEGRAND
(Service de Pneumologue, Hôpital Erasme, ULB, Bruxelles.
Faculté de Médecine Université de Mons)
Experts Extérieurs :
Dr. James MILLEDGE
(Honorary Professor, Department of Physiology, University College London)
Prof. Jean-Paul RICHALET
(Laboratoire de Réponses Cellulaires et Fonctionnelles à l'Hypoxie,
Université Paris 13. Service de Physiologie, Hôpital Avicenne, Bobigny)
Secrétaire / Promoteur :
Prof. Robert NAEIJE
(Laboratoire de Physiologie et de Physiopathologie, ULB, Bruxelles)
MECHANISMS OF ALTITUDE-RELATED COUGH
Dr. Nicholas P. Mason
CONTENTS
Synopsis of Thesis
Chapter 1
Literature Review.
Chapter 2
Cough frequency and cough receptor sensitivity to citric acid challenge during a simulated
ascent to extreme altitude.
Chapter 3
Serial changes in spirometry during an ascent to 5300 m in the Nepalese Himalaya.
Chapter 4
Serial changes in nasal potential difference and lung electrical impedance tomography at high
altitude.
Chapter 5
Changes in plasma bradykinin concentration and citric acid cough threshold at altitude
Chapter 6
The citric acid cough threshold and the ventilatory response to carbon dioxide on ascent to
high altitude.
Acknowledgements
2
SYNOPSIS OF THESIS
The original work presented in this thesis investigates some of the mechanisms that may be
responsible for the aetiology of altitude-related cough. Particular attention is paid to its relationship
to the long recognised, but poorly understood, changes in lung volumes that occur on ascent to
altitude. The literature relevant to this thesis is reviewed in Chapter 1.
Widespread reports have long existed of a debilitating cough affecting visitors to high altitude that
can incapacitate the sufferer and, on occasions, be severe enough to cause rib fractures (22, 34, 35).
The prevalence of cough at altitude has been estimated to be between 22 and 42% at between 4200
and 4900 m in the Everest region of Nepal (10, 29). Traditionally the cough was attributed to the
inspiration of the cold, dry air characteristic of the high altitude environment (37) but no attempts
were made to confirm this aetiology. In the first formal study of cough at high altitude, nocturnal
cough frequency was found to increase with increasing altitude during a trek to Everest Base Camp
(5300 m) and massively so in 3 climbers on whom recordings were made up to 7000 m on Everest
(8). After 9 days at 5300 m the citric acid cough threshold, a measure of the sensitivity of the cough
reflex arc, was significantly reduced compared with both sea level and arrival at 5300 m.
During Operation Everest II, a simulated climb of Mount Everest in a hypobaric chamber, the
majority of the subjects were troubled above 7000 m by pain and dryness in the throat and an
irritating cough despite the chamber being maintained at a relative humidity of between 72 and 82%
and a temperature of 23ºC (18). This argued against the widely held view that altitude-related cough
was due to the inspiration of cold, dry air.
In the next major hypobaric chamber study, Operation Everest III, nocturnal cough frequency and
citric acid cough threshold were measured on the 8 subjects in the study. The chamber temperature
was maintained between 18 and 24ºC and relative humidity between 30 and 60% (24). This work is
presented in Chapter 2 and, demonstrated an increase in nocturnal cough frequency with increasing
altitude which immediately returned to control values on descent to sea level. Citric acid cough
threshold was reduced at 8000 m compared to both sea level and 5000 m values. Changes in citric
acid cough threshold at lower altitudes may not have been detected because of the constraints on
subject numbers in the chamber. The study still however demonstrated an increase in clinical cough
and a reduction in the citric acid cough threshold at extreme altitude, despite controlled
environmental conditions, and thus refuted the long held belief that altitude-related cough is solely
due to the inspiration of cold, dry air.
3
If altitude-related cough is not simply due to the inspiration of cold, dry air, other possible
aetiologies are:
• Acute mountain sickness (AMS).
• Sub-clinical high altitude pulmonary oedema.
• Changes in the central control of cough.
• Respiratory tract infections.
• Loss of water from the respiratory tract.
• Bronchoconstriction and asthma.
• Vasomotor-rhinitis and post-nasal drip.
• Gastro-oesophageal reflux.
It is unlikely that cough at altitude is due to AMS. Despite both AMS and cough occurring
commonly at high altitude no relationship has ever been demonstrated between them in any study of
altitude-related cough (8, 24, 27, 38) and cough has not been reported as a symptom in over 20
papers studying AMS (9).
There is considerable indirect evidence that the majority of subjects ascending to high altitude may
develop sub-clinical pulmonary oedema. This evidence includes changes in lung volumes and in
particular forced vital capacity (FVC) and changes in the nitrogen washout curve and closing
volume. The conflicting literature on this topic, and other possible mechanisms underlying the fall
in FVC on ascent to altitude, are reviewed fully in Chapter 1.
Chapter 3 presents a field study investigating the changes in FVC, forced expiratory volume in one
second (FEV1) and peak expiratory flow (PEF) in 55 subjects ascending to Everest Base Camp at
5300 m and addressing some of the methodological shortcomings of previous studies (25). Forced
vital capacity fell significantly on ascent to 5300 m. Peak expiratory flow increased, as predicted by
the fall in gas density with increasing altitude, while FEV1 was unchanged.
If sub-clinical pulmonary oedema is responsible for the fall in FVC on ascent to altitude then it
might also be an aetiological factor in altitude-related cough. Animal work has demonstrated that
even small changes in left atrial pressure can be sufficient to produce pulmonary venous congestion
sufficient to stimulate airway rapidly adapting receptors (RAR) that form part of the afferent limb
of the cough reflex arc (15-17). It is therefore possible that sub-clinical pulmonary oedema
occurring at altitude could stimulate airway RARs and provoke cough.
4
Two possible mechanisms could be responsible for sub-clinical pulmonary oedema at high altitude:
pulmonary hypertension secondary to hypoxic pulmonary vasoconstriction or a reduction in
respiratory epithelial fluid clearance. Chapter 4 presents a study investigating these two
mechanisms during a 14 day stay at 3800 m in 20 lowland volunteers (26). Forced vital capacity fell
on ascent to 3800m as did the normalized change in lung electrical impedance tomography
suggestive of an increase in extravascular lung water. There was a positive correlation between
FVC and the change in lung electrical impedance tomography. Respiratory epithelial ion transport
was studied using nasal potential difference measurements. Nasal potential difference
hyperpolarised at altitude which would be consistent with either increased transepithelial sodium
absorption or anion secretion, or a combination of both. If anion secretion predominated over
sodium reabsorption, it would be associated with the secretion of water into the respiratory lumen
as occurs in the fetal lung (4) and could cause sub-clinical pulmonary oedema. The increase in
pulmonary artery pressure estimated by echo-Doppler was insufficient to cause clinical pulmonary
oedema .
Cough is a recognised side effect of angiotensin converting enzyme (ACE) inhibitors and is thought
to be due to the sensitisation of airway RARs by increased levels of bradykinin and substance P
(21). Bradykinin is degraded by kininases of which the most important in human serum is ACE.
Little or nothing was known about the effects of hypoxia on ACE and bradykinin. Chapter 5
presents further work done on the 20 lowland subjects during their 2 week stay at 3800 m (27).
Citric acid cough threshold was reduced throughout the stay at altitude compared to low altitude
baseline measurements. Serum ACE activity was unchanged on ascent to 3800 m, although plasma
bradykinin fell significantly making it unlikely that bradykinin plays a role in the change in citric
acid cough threshold seen on ascent to altitude.
Respiratory control undergoes profound changes with acclimatisation (39) and the central control of
cough is complex and poorly understood (12, 13). A relationship has been demonstrated between
the hypercapnic ventilatory response (HCVR) and the cough threshold to hypotonic saline (1).
Those subjects who responded to the hypotonic saline challenge had a higher HCVR than the
subjects who did not respond. In addition post-hoc analysis of data from the 1994 British Mount
Everest Medical Expedition also demonstrated a relationship between the citric acid cough
threshold and the dynamic ventilatory response to CO2 (5).
Chapter 6 presents work which investigated the relationship between the citric acid cough
threshold and HCVR in 25 healthy subjects during a 9 day stay at 5200 m (38). Citric acid cough
threshold fell significantly on ascent to altitude and the HCVR increased significantly on ascent to
5
5200 m. There was, however, no demonstrable relationship between the citric acid cough threshold
and HCVR, or any change in these parameters on ascent to altitude. These findings argue against
altitude-related cough being mediated through changes in central control mechanisms.
Respiratory tract infections are the commonest cause of acute cough at sea level (20, 28, 32) and
occur commonly in visitors to altitude (11, 29). There is also evidence of impairment of mucociliary
transport, a crucial respiratory defence mechanism, at altitude (6). While there was no clinical
evidence of respiratory infection observed in any of the subjects during Operation Everest III (24),
cough associated with the production of purulent sputum is, anecdotally, a common finding at
altitude, particularly following prolonged vigorous exertion.
It is still possible, despite the controlled environmental conditions during Operation Everest III (24),
that water loss from the respiratory tract plays a role in the aetiology of altitude-related cough.
Hyperpnoea with cold air, in subjects susceptible to exercise-induced cough, and at respiratory rates
similar to those occurring with strenuous exercise, has been shown to be associated with an increase
in cough frequency (2). However increased coughing associated with hypernoea appears to depend
upon water loss rather than heat loss. Hypernoea with warm dry air produced more coughing than
hypernoea with cold air despite causing less heat loss (3). Hyperpnoea with ambient air also
produced an increase in cough frequency because it was associated with water loss. Increased
minute ventilation is a feature of the body’s response to hypobaric hypoxia and will increase further
with exercise (23). In addition there is evidence of subjective nasal blockage and an increase in
nasal resistance at altitude which may result in increased mouth breathing (6, 7) which will increase
water loss compared to nasal breathing (36).
Cough may be the only symptom of asthma (28) and bronchoconstriction can occur at altitude and
after hyperpnoea with cold air (14). However there was no demonstrable relationship between FEV1
or PEF and the change in the citric acid cough threshold at 5300 m altitude (8) or FEV1 and the
citric acid threshold during Operation Everest III (24). In addition no evidence of
bronchoconstriction could be found in healthy, non-asthmatic, subjects at Mount Everest Base
Camp (30).
Nasal blockage could also be a symptom of vasomotor rhinitis and post-nasal drip (recently
redesignated upper airway cough syndrome) and which is reported in some series to be one of the
most common causes of chronic cough at sea level (28, 31). Gastro-oesophageal reflux disease has
been reported in up to 40% of patients with chronic cough at sea level (19, 33). Nothing is known
about the relationship between post-nasal drip and cough, or the prevalence of gastro-oesophageal
reflux, at high altitude.
6
Conclusions and Future Perspectives
This thesis has investigated some of the potential aetiologies of altitude-related cough and
demonstrates that, contrary to the popularly held view, it may not solely be due to the inspiration of
the cold, dry air characteristic of the high-altitude environment. Data is presented that confirms the
fall in vital capacity on ascent to altitude and possible causes for this reduction are discussed. One
possible cause would be sub-clinical pulmonary oedema. This could also be a potential cause for
altitude-related cough and data is presented that suggests that this may be the result of changes in
respiratory epithelial ion and water transport. Evidence is presented that argues against altituderelated cough being due to changes in bradykinin or in the central control of cough.
While sub-clinical pulmonary oedema may be an aetiological factor in altitude-related cough, and
merits further study, it is likely that it is not the only cause and it is probable that cough at altitude is
a symptom of a number of unrelated conditions. Future work should focus on the role of water loss
from the respiratory tract at altitude, particularly during exercise as well as the association of upper
respiratory tract infection with cough and the place of vasomotor rhinitis and gastro-oesophageal
reflux in the aetiology of this fascinating condition.
REFERENCES
1.
Banner AS. Relationship between cough due to hypotonic aerosol and the ventilatory
response to CO2 in normal subjects. Am Rev Respir Dis 137: 647-650, 1988.
2.
Banner AS, Chausow A, and Green J. The tussive effect of hyperpnea with cold air. Am
Rev Respir Dis 131: 362-367, 1985.
3.
Banner AS, Green J, and O'Connor M. Relation of respiratory water loss to coughing
after exercise. N Engl J Med 311: 883-886, 1984.
4.
Barker PM and Olver RE. Invited Review: Clearance of lung liquid during the perinatal
period. J Appl Physiol 93: 1542-1548., 2002.
7
5.
Barry P, Mason N, Nickol A, Datta A, Milledge J, Wolffe C, and Collier D. Cough
receptor sensitivity and dynamic ventilatory response to carbon dioxide in man acclimatised
to high altitude (Abstract). J Physiol: 497P: 429-430, 1996.
6.
Barry PW, Mason NP, and O'Callaghan C. Nasal mucociliary transport is impaired at
altitude. Eur Respir J 10: 35-37, 1997.
7.
Barry PW, Mason NP, and Richalet JP. Nasal peak inspiratory flow at altitude. Eur
Respir J 19: 16-19., 2002.
8.
Barry PW, Mason NP, Riordan M, and O'Callaghan C. Cough frequency and coughreceptor sensitivity are increased in man at altitude. Clin Sci (Colch) 93: 181-186, 1997.
9.
Bartsch P and Roach R. Acute Mountain Sickness and High-Altitude Cerebral Edema. In:
High Altitude: an exploration of human adaptation, edited by Hornbein T and Schoene R.
New York: Marcel Dekker, 2001, p. 732-740.
10.
Basnyat B, Gertsch JH, Johnson EW, Castro-Marin F, Inoue Y, and Yeh C. Efficacy of
low-dose acetazolamide (125 mg BID) for the prophylaxis of acute mountain sickness: a
prospective, double-blind, randomized, placebo-controlled trial. High Alt Med Biol 4: 45-52,
2003.
11.
Basnyat B and Litch JA. Medical problems of porters and trekkers in the Nepal Himalaya.
Wilderness Environ Med 8: 78-81, 1997.
12.
Bolser DC and Davenport PW. Functional organization of the central cough generation
mechanism. Pulm Pharmacol Ther 15: 221-225, 2002.
13.
Bonham AC, Sekizawa SI, Chen CY, and Joad JP. Plasticity of brainstem mechanisms of
cough. Respir Physiol Neurobiol, 2006.
14.
Cogo A, Basnyat B, Legnani D, and Allegra L. Bronchial asthma and airway
hyperresponsiveness at high altitude. Respiration 64: 444-449, 1997.
15.
Gunawardena S, Bravo E, and Kappagoda CT. Effect of chronic mitral valve damage on
activity of pulmonary rapidly adapting receptors in the rabbit. J Physiol 511: 79-88., 1998.
16.
Gunawardena S, Bravo E, and Kappagoda CT. Rapidly adapting receptors in a rabbit
model of mitral regurgitation. J Physiol 521 Pt 3: 739-748., 1999.
17.
Hargreaves M, Ravi K, and Kappagoda CT. Responses of slowly and rapidly adapting
receptors in the airways of rabbits to changes in the Starling forces. J Physiol 432: 81-97.,
1991.
8
18.
Houston CS, Sutton JR, Cymerman A, and Reeves JT. Operation Everest II: man at
extreme altitude. J Appl Physiol 63: 877-882, 1987.
19.
Irwin RS. Chronic cough due to gastroesophageal reflux disease: ACCP evidence-based
clinical practice guidelines. Chest 129: 80S-94S, 2006.
20.
Irwin RS, Rosen MJ, and Braman SS. Cough. A comprehensive review. Arch Intern Med
137: 1186-1191, 1977.
21.
Israili ZH and Hall WD. Cough and angioneurotic edema associated with angiotensinconverting enzyme inhibitor therapy. A review of the literature and pathophysiology. Ann
Intern Med 117: 234-242, 1992.
22.
Litch JA and Tuggy M. Cough induced stress fracture and arthropathy of the ribs at
extreme altitude. Int J Sports Med 19: 220-222, 1998.
23.
Mason NP. The physiology of high altitude: an introduction to the cardiorespiratory
changes occurring on ascent to altitude. Current anaesthesia and critical care 11: 34-41,
2000.
24.
Mason NP, Barry PW, Despiau G, Gardette B, and Richalet JP. Cough frequency and
cough receptor sensitivity to citric acid challenge during a simulated ascent to extreme
25.
Mason NP, Barry PW, Pollard AJ, Collier DJ, Taub NA, Miller MR, and Milledge JS.
Serial changes in spirometry during an ascent to 5,300 m in the Nepalese Himalayas. High
Alt Med Biol 1: 185-195., 2000.
26.
Mason NP, Petersen M, Melot C, Imanow B, Matveykine O, Gautier MT, Sarybaev A,
Aldashev A, Mirrakhimov MM, Brown BH, Leathard AD, and Naeije R. Serial changes
in nasal potential difference and lung electrical impedance tomography at high altitude. J
Appl Physiol 94: 2043-2050, 2003.
27.
Mason NP, Petersen M, Melot C, Kim EV, Aldashev A, Sarybaev A, Mirrakhimov
MM, and Naeije R. Changes in plasma bradykinin concentration and citric acid cough
threshold at altitude. Wilderness and Environmental Medicine (in press) 20: 353-358, 2009.
28.
McGarvey LP and Morice AH. Clinical cough and its mechanisms. Respir Physiol
Neurobiol 152: 363-371, 2006.
29.
Murdoch DR. Symptoms of infection and altitude illness among hikers in the Mount
Everest region of Nepal. Aviat Space Environ Med 66: 148-151, 1995.
9
30.
Pollard AJ, Barry PW, Mason NP, Collier DJ, Pollard RC, Pollard PF, Martin I,
Fraser RS, Miller MR, and Milledge JS. Hypoxia, hypocapnia and spirometry at altitude
[published erratum appears in Clin Sci (Colch) 1997 Dec;93(6):611]. Clin Sci (Colch) 92:
593-598, 1997.
31.
Pratter MR. Chronic upper airway cough syndrome secondary to rhinosinus diseases
(previously referred to as postnasal drip syndrome): ACCP evidence-based clinical practice
guidelines. Chest 129: 63S-71S, 2006.
32.
Pratter MR. Cough and the common cold: ACCP evidence-based clinical practice
33.
Pratter MR. Overview of common causes of chronic cough: ACCP evidence-based clinical
practice guidelines. Chest 129: 59S-62S, 2006.
34.
Somervell T. After Everest. London: Hodder and Stoughton, 1936.
35.
Steele P. Medicine on Mount Everest 1971. Lancet 2: 32-39, 1971.
36.
Svensson S, Olin AC, and Hellgren J. Increased net water loss by oral compared to nasal
expiration in healthy subjects. Rhinology 44: 74-77, 2006.
37.
Tasker J. Everest the Cruel Way. London: Eyre Methuen Ltd., 1981.
38.
Thompson AA, Baillie JK, Bates MG, Schnopp MF, Simpson A, Partridge RW,
Drummond GB, and Mason NP. The citric acid cough threshold and the ventilatory
response to carbon dioxide on ascent to high altitude. Respir Med 103: 1182-1188, 2009.
39.
Ward M, Milledge JS, and West JB. Ventilatory response to hypoxia and carbon dioxide.
In: High altitude medicine and physiology (3rd ed.). London: Arnold, 2000, p. 50-64.
10
MECANISMES DE LA TOUX LIEE A L’ALTITUDE
Dr. Nicholas P. Mason
SOMMAIRE
Résumé de la thèse
Chapitre 1
Revue de la littérature
Chapitre 2
Fréquence de la toux et sensibilité du récepteur de la toux au test de provocation par l’acide
citrique au cours d’une ascension simulée vers une altitude extrême.
Chapitre 3
Modifications en série de la spirométrie durant une ascension jusqu’à 5300 m dans
l’Himalaya.
Chapitre 4
Modifications en série de la différence de potentiel nasale et de la tomographie électrique par
impédance du poumon à haute altitude.
Chapitre 5
Modifications de la concentration plasmatique de bradykinine et du seuil de toux à l’acide
citrique en altitude.
Chapitre 6
Seuil de la toux à l’acide citrique et réponse ventilatoire au gaz carbonique lors de l’ascension
jusqu’en haute altitude
Remerciements
11
RESUME DE LA THESE
L’étude d’une partie des mécanismes à l’origine de la toux liée à l’altitude, constitue un travail
original, présenté dans cette thèse. La relation entre cette toux et les modifications de volumes
pulmonaires survenant lors de la montée en altitude sont connues de longue date mais mal
élucidées ; elle fait l’objet d’une attention particulière. Une revue de la littérature pertinente est
exposée dans le chapitre 1.
Des rapports largement diffusés évoquent une toux fatigante, possiblement invalidante, qui touche
les personnes en haute altitude et qui, quand elle est suffisamment sévère, peut entraîner des
fractures de côtes (22, 34, 35). La prévalence de la toux en altitude a été estimée entre 22 et 42 % à
une altitude comprise entre 4 200 et 4 900 m dans la région de l’Everest népalais (10, 29).
Traditionnellement, la toux est attribuée à l’inspiration d’air froid et sec, caractéristique de
l’environnement à haute altitude (37), mais aucun essai n’a été réalisé pour confirmer cette
étiologie. Dans la première étude formelle sur la toux en haute altitude, il a été montré, au cours
d’un trek vers le camp de base de l’Everest (5 300 m), que la fréquence de la toux nocturne
augmentait avec l’altitude et, de façon massive, pour 3 alpinistes chez lesquels les enregistrements
ont été effectués jusqu’à 7 000 m sur l’Everest (8). Après 9 jours à 5 300 m, le seuil de toux à
l’acide citrique, mesurant la sensibilité de l’arc réflexe de la toux, était significativement diminué
comparé à celui, à la fois, du niveau de la mer et de l’arrivée à 5 300 m.
Au cours de l’Opération Everest II (ascension simulée du Mont Everest réalisée en chambre
hypobare), la majorité des sujets ont présenté des troubles à type de douleur et de sécheresse au
niveau de la gorge ainsi qu’une toux irritative, malgré une humidité relative comprise entre 72 % et
82 % et une température à 23° C (18). Ce résultat plaide contre l’idée largement répandue que l’air
froid et sec serait à l’origine de la toux liée à l’altitude.
Dans l’étude majeure suivante en chambre hypobare, Opération Everest III, la fréquence de la toux
nocturne et le seuil de toux à l’acide citrique ont été mesurés chez 8 sujets. La température de la
chambre et l’humidité relative ont été maintenues respectivement entre 18 et 24°C et entre 30 et
60% (24). Dans ce travail, présenté au chapitre 2, la fréquence de la toux nocturne s’élève avec
l’altitude revenant immédiatement aux valeurs de base en descendant vers le niveau de la mer. Le
seuil de toux à l’acide citrique est abaissé à 8 000 m par rapport aux valeurs à la fois au niveau de la
mer et à 5 000 m. Les modifications du seuil de toux à l’acide citrique à de plus basses altitudes
peuvent ne pas avoir été détectées du fait de la limitation du nombre de sujets dans la chambre.
Néanmoins, l’étude montre un accroissement de la toux clinique et une réduction du seuil de la toux
12
à l’acide citrique à une altitude extrême, malgré des conditions environnementales contrôlées, et
réfute, par conséquence, la vieille croyance selon laquelle la toux liée à l’altitude est uniquement
due à l’inspiration d’air froid et sec.
Si la toux liée à l’altitude n’est pas simplement la conséquence de l’inspiration d’air froid et sec,
quelles sont les autres étiologies possibles ? Il existe plusieurs mécanismes potentiels comprenant :
•
mal aigu des montagnes (MAM)
•
sub-œdème pulmonaire de haute altitude
•
modifications du contrôle central de la toux
•
infections des voies aériennes
•
perte d’eau des voies aériennes
•
bronchoconstriction et asthme
•
rhinite vasomotrice et secrétions post-nasales
•
reflux gastro-œsophagien.
Il est peu probable que ce type de toux soit dû au MAM. Bien que toux et MAM surviennent tous
deux, à haute altitude, aucune relation n’a jamais été démontrée entre eux, dans quelque étude que
soit sur la toux liée à l’altitude (8, 24, 27, 38). Dans plus d’une vingtaine d’articles sur le MAM, la
toux n’a jamais été signalée comme un symptôme (9).
Il a largement été prouvé, de façon indirecte, que la majorité des personnes montant en haute
altitude peuvent développer un sub-œdème pulmonaire. Cette preuve comporte des modifications
des volumes pulmonaires, en particulier de la capacité vitale forcée, de la courbe de rinçage de
l’azote et du volume de fermeture. La littérature contradictoire sur le sujet, et d’autres mécanismes
possibles à l’origine de la baisse de la capacité vitale forcée lors de la montée en altitude, sont
largement examinés au chapitre 1.
Le chapitre 3 présente un travail de terrain étudiant les modifications de la capacité vitale forcée,
du volume expiratoire forcé sur une seconde (VEF1) et du débit expiratoire de pointe (DEP) de 55
sujets en ascension vers le Camp de Base de l’Everest à 5 300 m. Ce travail répond à certaines des
insuffisances méthodologiques des études précédentes (25). La capacité vitale forcée a chuté, de
façon significative, en montant à 5 300 m. Le débit expiratoire de pointe a augmenté, comme
attendu puisque la densité d’un gaz baisse quand l’altitude s’élève, tandis que VEF1 restait
inchangé.
13
Si le sub-œdème pulmonaire est responsable de la baisse de capacité vitale forcée en montant en
altitude, il pourrait être aussi un facteur étiologique de la toux liée à l’altitude. Des travaux chez
l’animal, ont montré que même de faibles modifications de la pression de l’oreillette gauche
suffisaient à produire une congestion veineuse pulmonaire entraînant une stimulation des récepteurs
ventilatoires à adaptation rapide qui constituent une partie de la branche afférente de l’arc réflexe de
la toux (15-17). C’est pourquoi, il est possible que ce sub-œdème pulmonaire survenant en altitude
stimule les récepteurs ventilatoires à adaptation rapide et provoque la toux.
Deux mécanismes pourraient être responsables du sub-oedème pulmonaire en haute altitude :
l’HTAP secondaire à la vasoconstriction pulmonaire hypoxique ou la diminution de la clairance du
liquide alvéolaire. Le travail présenté dans le chapitre 4 analyse ces deux mécanismes pendant un
séjour de 14 jours à 3 800 m, chez 20 volontaires venus des plaines (26). En montant à 3 800 m, la
capacité vitale forcée a diminué tout comme la modification normalisée de la tomographie
pulmonaire d’impédance électrique, évocatrice d’une élévation de liquide pulmonaire
extravasculaire. Il existe une corrélation positive entre la capacité vitale forcée et la modification de
tomographie d’impédance électrique du poumon. Le transport ionique dans l’épithélium respiratoire
a été étudié en utilisant des mesures de différence de potentiel nasale. Cette dernière s’hyperpolarise
en altitude, ce qui pourrait être compatible avec soit une absorption sodique transépithéliale
augmentée, soit une sécrétion d’anions soit une combinaison des deux. Si la sécrétion d’anions
prédominait sur la réabsorption sodique, elle aurait été associée à la sécrétion d’eau dans la lumière
respiratoire comme c’est le cas dans le poumon fœtal (4) et pourrait causer un sub-œdème
pulmonaire. L’augmentation de pression artérielle pulmonaire estimée par échodoppler, n’a pas été
suffisante pour déclencher un sub-œdème pulmonaire.
La toux est un effet secondaire connu des inhibiteurs de l’enzyme de conversion de l’angiotensine
qui serait due à la sensibilisation des récepteurs ventilatoires à adaptation rapide par des taux élevés
de bradykinine et de substance P (21). La bradykinine est dégradée par des kinases dont la plus
importante est l’enzyme de conversion de l’angiotensine dans le sérum humain. Très peu de
données sont disponibles concernant les effets de l’hypoxie sur l’enzyme de conversion de
l’angiotensine et la bradykinine. Dans le chapitre 5, est présenté un autre travail, réalisé sur les 20
sujets venus des plaines, pendant leur séjour de 2 semaines à 3 800 m (27). Le seuil de toux à
l’acide citrique était réduit tout au long du séjour en altitude comparé aux mesures de référence en
basse altitude. L’activité sérique de l’enzyme de conversion de l’angiotensine n’était pas modifiée
en montant à 3 800 m, alors que la bradykinine plasmatique a chuté de façon significative. Il est
14
donc improbable que la bradykinine joue un rôle dans la modification du seuil de la toux à l’acide
citrique relevée lors de la montée en altitude.
Le contrôle respiratoire subit de profonds changements en s’adaptant au climat (39) et le contrôle
central de la toux est d’une part complexe, d’autre part mal élucidé (12, 13). On note une relation
entre la réponse ventilatoire à l’hypercapnie et le seuil de la toux au sérum hypotonique (1). Ces
sujets qui répondent au test de provocation au sérum hypotonique ont une réponse ventilatoire à
l’hypercapnie supérieure à ceux qui ne répondent pas. De plus, l’analyse post-hoc des données de
l’expédition britannique médicale de 1994 sur le Mont Everest a aussi montré une relation entre le
seuil de toux à l’acide citrique et la réponse ventilatoire dynamique au CO2 (5).
Le travail présenté dans le chapitre 6 analyse la relation entre le seuil de la toux à l’acide citrique et
la réponse ventilatoire à l’hypercapnie chez 25 sujets en bonne santé, au cours d’un séjour de 9
jours à 5 200 m (38). Le seuil de toux à l’acide citrique a chuté, de façon significative, lors de la
montée en altitude et la réponse ventilatoire à l’hypercapnie a augmenté, de façon significative, lors
de l’ascension à 5 200 m. Pourtant, il n’existait aucune relation évidente entre le seuil de la toux à
l’acide citrique et la relation ventilatoire hypercapnique ou quelque modification de ces paramètres
lors de la montée en altitude. Ces résultats plaident contre la toux liée à l’altitude impliquant des
modifications des mécanismes centraux de contrôle.
Les infections des voies aériennes sont la cause la plus fréquente de toux aiguë, au niveau de la mer
(20, 28, 32) et survient généralement en altitude chez les touristes (11, 29). Un dysfonctionnement
du transport mucociliaire, mécanisme de défense respiratoire crucial, en altitude est prouvé
également (6). Bien qu’il n’ait pas été relevé de preuve clinique d’infection respiratoire chez les
sujets pendant l’Opération Everest III (24), la toux associée à la production d’expectoration
purulente a été fréquemment constatée en altitude, en particulier à la suite d’un effort vigoureux
prolongé.
Il est toujours possible, malgré des conditions environnementales contrôlées au cours de l’opération
Everest III (9), que la perte d’eau des voies aériennes joue un rôle dans l’étiologie de la toux liée à
l’altitude. Il a été montré que l’hyperpnée due à l’air froid, à une fréquence ventilatoire similaire de
celle survenant lors d’un effort énergique, était associée à une élévation de la fréquence de la toux
(2). Néanmoins, l’élévation de la toux associée à l’hyperpnée semble plus dépendre de la perte
d’eau que de la perte de chaleur. L’hyperpnée liée à l’air chaud et sec a entraîné plus de toux que
l’hyperpnée liée à l’air froid bien qu’entraînant moins de perte de chaleur (3). L’hyperpnée en air
ambiant a également eu pour conséquence une augmentation de la fréquence de la toux car associée
15
à une perte d’eau. L’augmentation de la ventilation-minute est une réponse à l’hypoxie hypobare
qui augmente un peu plus avec l’exercice (23). De plus, il est démontré un blocage nasal subjectif et
une augmentation de la résistance nasale en altitude qui peuvent entraîner une respiration buccale
(6, 7) et donc une élévation de la perte d’eau comparé à la respiration nasale (36).
La toux peut être le seul symptôme de l’asthme (28) et la bronchoconstriction peut survenir en
altitude et après hyperpnée liée à l’air froid (14). Pourtant, il n’existait pas de relation évidente
entre le volume expiratoire forcé en une seconde ou le débit expiratoire de pointe et la modification
du seuil de toux à l’acide citrique, à 5 300 m d’altitude (8), ni entre le volume expiratoire forcé en
une seconde et le seuil de toux à l’acide citrique au cours de l’Opération Everest III (24). De plus,
aucune preuve de bronchoconstriction n’a été retrouvée chez les sujets en bonne santé, non
asthmatiques, au camp de base du Mont Everest (30).
Le blocage nasal pourrait être aussi un symptôme de rhinite vasomotrice et de, considéré, dans
quelques séries, comme l’une des plus fréquentes causes de toux chronique au niveau de la mer (28,
31). Le reflux gastro-œsophagien est retrouvé chez presque 40 % des patients présentant une toux
chronique, au niveau de la mer (19, 33). On ne connait ni la relation entre les secrétions postnasales et la toux, ni la prévalence du reflux gastro-œsophagien à haute altitude.
Conclusions et perspectives futures
Cette thèse a passé en revue quelques causes potentielles de toux liée à l’altitude et a montré que,
contrairement à une idée populaire répandue, elle n’est pas seulement due à l’inspiration d’air froid
et sec, caractéristique de l’environnement en haute altitude. Les données présentées confirment la
chute de la capacité vitale lors de la montée en altitude et les causes possibles de cette chute sont
discutées. Une cause possible pourrait être le sub-œdème pulmonaire. Cela pourrait être aussi une
cause potentielle à la toux liée à l’altitude et les données présentées suggèrent qu’il résulte de
modifications du transfert ionique et d’eau à travers l’épithélium respiratoire. Il est également
prouvé que la toux liée à l’altitude n’est la conséquence des modifications ni des taux de
bradykinine ou ni du centre de contrôle de la toux.
Alors que le sub-œdème pulmonaire peut être un facteur étiologique de la toux liée à l’altitude, et
mérite des études complémentaires, il est vraisemblable qu’il ne s’agit pas de la seule cause, la toux
étant un symptôme retrouvé dans bon nombre de situations sans rapport avec l’altitude. Un travail
futur devrait se concentrer sur le rôle de la perte d’eau des voies aériennes en altitude,
particulièrement au cours de l’exercice, mais aussi sur l’association infections des voies aériennes
16
supérieures et toux, et envisager la place de la rhinite vasomotrice et du reflux gastro-œsophagien
dans l’étiologie de cette passionnante situation.
REFERENCES
1.
response to CO2 in normal subjects. Am Rev Respir Dis 137: 647-650, 1988.
2.
Banner AS, Chausow A, and Green J. The tussive effect of hyperpnea with cold air. Am
Rev Respir Dis 131: 362-367, 1985.
3.
Banner AS, Green J, and O'Connor M. Relation of respiratory water loss to coughing
after exercise. N Engl J Med 311: 883-886, 1984.
4.
Barker PM and Olver RE. Invited Review: Clearance of lung liquid during the perinatal
period. J Appl Physiol 93: 1542-1548., 2002.
5.
receptor sensitivity and dynamic ventilatory response to carbon dioxide in man acclimatised
to high altitude (Abstract). J Physiol: 497P: 429-430, 1996.
6.
Barry PW, Mason NP, and O'Callaghan C. Nasal mucociliary transport is impaired at
7.
Respir J 19: 16-19., 2002.
8.
Barry PW, Mason NP, Riordan M, and O'Callaghan C. Cough frequency and coughreceptor sensitivity are increased in man at altitude. Clin Sci (Colch) 93: 181-186, 1997.
9.
Bartsch P and Roach R. Acute Mountain Sickness and High-Altitude Cerebral Edema. In:
High Altitude: an exploration of human adaptation, edited by Hornbein T and Schoene R.
New York: Marcel Dekker, 2001, p. 732-740.
10.
Basnyat B, Gertsch JH, Johnson EW, Castro-Marin F, Inoue Y, and Yeh C. Efficacy of
low-dose acetazolamide (125 mg BID) for the prophylaxis of acute mountain sickness: a
prospective, double-blind, randomized, placebo-controlled trial. High Alt Med Biol 4: 45-52,
2003.
17
11.
Basnyat B and Litch JA. Medical problems of porters and trekkers in the Nepal Himalaya.
12.
Bolser DC and Davenport PW. Functional organization of the central cough generation
mechanism. Pulm Pharmacol Ther 15: 221-225, 2002.
13.
Bonham AC, Sekizawa SI, Chen CY, and Joad JP. Plasticity of brainstem mechanisms of
cough. Respir Physiol Neurobiol, 2006.
14.
Cogo A, Basnyat B, Legnani D, and Allegra L. Bronchial asthma and airway
hyperresponsiveness at high altitude. Respiration 64: 444-449, 1997.
15.
Gunawardena S, Bravo E, and Kappagoda CT. Effect of chronic mitral valve damage on
activity of pulmonary rapidly adapting receptors in the rabbit. J Physiol 511: 79-88., 1998.
16.
Gunawardena S, Bravo E, and Kappagoda CT. Rapidly adapting receptors in a rabbit
model of mitral regurgitation. J Physiol 521 Pt 3: 739-748., 1999.
17.
Hargreaves M, Ravi K, and Kappagoda CT. Responses of slowly and rapidly adapting
receptors in the airways of rabbits to changes in the Starling forces. J Physiol 432: 81-97.,
1991.
18.
Houston CS, Sutton JR, Cymerman A, and Reeves JT. Operation Everest II: man at
extreme altitude. J Appl Physiol 63: 877-882, 1987.
19.
Irwin RS. Chronic cough due to gastroesophageal reflux disease: ACCP evidence-based
clinical practice guidelines. Chest 129: 80S-94S, 2006.
20.
Irwin RS, Rosen MJ, and Braman SS. Cough. A comprehensive review. Arch Intern Med
137: 1186-1191, 1977.
21.
Israili ZH and Hall WD. Cough and angioneurotic edema associated with angiotensinconverting enzyme inhibitor therapy. A review of the literature and pathophysiology. Ann
Intern Med 117: 234-242, 1992.
22.
Litch JA and Tuggy M. Cough induced stress fracture and arthropathy of the ribs at
extreme altitude. Int J Sports Med 19: 220-222, 1998.
23.
changes occurring on ascent to altitude. Current anaesthesia and critical care 11: 34-41,
2000.
18
24.
Mason NP, Barry PW, Despiau G, Gardette B, and Richalet JP. Cough frequency and
cough receptor sensitivity to citric acid challenge during a simulated ascent to extreme
25.
Mason NP, Barry PW, Pollard AJ, Collier DJ, Taub NA, Miller MR, and Milledge JS.
Serial changes in spirometry during an ascent to 5,300 m in the Nepalese Himalayas. High
Alt Med Biol 1: 185-195., 2000.
26.
Mason NP, Petersen M, Melot C, Imanow B, Matveykine O, Gautier MT, Sarybaev A,
Aldashev A, Mirrakhimov MM, Brown BH, Leathard AD, and Naeije R. Serial changes
in nasal potential difference and lung electrical impedance tomography at high altitude. J
Appl Physiol 94: 2043-2050, 2003.
27.
Mason NP, Petersen M, Melot C, Kim EV, Aldashev A, Sarybaev A, Mirrakhimov
MM, and Naeije R. Changes in plasma bradykinin concentration and citric acid cough
threshold at altitude. Wilderness and Environmental Medicine (in press) 20: 353-358, 2009.
28.
Neurobiol 152: 363-371, 2006.
29.
Everest region of Nepal. Aviat Space Environ Med 66: 148-151, 1995.
30.
Fraser RS, Miller MR, and Milledge JS. Hypoxia, hypocapnia and spirometry at altitude
[published erratum appears in Clin Sci (Colch) 1997 Dec;93(6):611]. Clin Sci (Colch) 92:
593-598, 1997.
31.
(previously referred to as postnasal drip syndrome): ACCP evidence-based clinical practice
32.
33.
Pratter MR. Overview of common causes of chronic cough: ACCP evidence-based clinical
practice guidelines. Chest 129: 59S-62S, 2006.
34.
35.
Steele P. Medicine on Mount Everest 1971. Lancet 2: 32-39, 1971.
19
36.
Svensson S, Olin AC, and Hellgren J. Increased net water loss by oral compared to nasal
expiration in healthy subjects. Rhinology 44: 74-77, 2006.
37.
38.
response to carbon dioxide on ascent to high altitude. Respir Med 103: 1182-1188, 2009.
39.
Ward M, Milledge JS, and West JB. Ventilatory response to hypoxia and carbon dioxide.
In: High altitude medicine and physiology (3rd ed.). London: Arnold, 2000, p. 50-64.
CHAPTER 1: LITERATURE REVIEW
CHANGES IN LUNG VOLUMES AT ALTITUDE
The first report of a change in lung volume on exposure to hypobaric hypoxia is credited to
von Vivenot in 1868 when 2 subjects who were acutely decompressed in a hypobaric chamber
to an equivalent altitude of 4650 m had a reduction in vital capacity (VC) of 9 and 13%
(quoted by Bert (51) who demonstrated a reduction of 32% in his own VC on exposure to a
barometric pressure of 430 mm Hg, equivalent to an altitude of around 4500m). A number of
studies during the first half of the 20th Century seemed to confirm the reduction in VC on
exposure to hypobaric hypoxia although the change in volume was extremely variable ranging
from 0 to 14% with acute exposure and up to 50% with longer exposure (422). However these
studies frequently involved small numbers of subjects and invariably volumes were not
corrected to BTPS. As a result the reported changes in VC may simply represent the fall in
temperature between the subjects and the spirometer (350). Early studies are here arbitrarily
defined as studies performed before the end of the Second World War and are summarised in
Table 1.
Author
Barometric
pressure
(mm hg)
Reduction in
VC (%)
Altitude (m)
Comments
1) Acute exposure in hypobaric chamber or breathing hypoxic gas mixture
V. Vivenot (1868)
424
4572
10 & 14
2 subjects.
Bert ( 1878)
430
~ 4500
Bohr (1907)
Sea level
--
32
1 subject.
Insignificant
Hypoxic gas mixtures of FiO2 0.8 to 0.12.
Schneider (1932)
350
Hurtado et al (1934)
419
6096
8
24 subjects.
4877
4, 8 & 14
3 subjects.
2) Longer exposure to high altitude
2 subjects. Returned to baseline values after 3
days.
1 subject. Monte Rosa, Switzerland.
Hewett (1875)
682
914
11 & 12
Bert (1878)
440
4389
Mosso (1889)
440
4389
Schumburg & Zuntz
(1896)
Durig (1911)
626
480
440
1524
3658
4389
50
3, 6, 9, 10, 13 &
20
11 & 8
7&3
14 to 16
Barcroft et al (1923)
460
4023
2, 3, 4 & 13
5 subjects. 1 week Cerro de Pasco, Peru.
Grollman (1930)
--
4298
11.2
Schneider (1932)
--
4298
7 to 15
Verzar (1933)
492
3475
2 to 24
Verzar (1945)
492
3475
9 to 12
1 subject. Pike’s Peak, Colorado.
9 subjects. Maximum decrease on first day.
Pike’s Peak, Colorado.
5 subjects. Return towards normal after 5 days.
Jungfaujoch, Switzerland.
9 subjects. Jungfraujoch, Switzerland.
6 subjects. Monte Rosa, Switzerland.
2 subjects.
4 subjects. No change over 3 weeks.
Table 1: Early chamber and field studies of change in vital capacity (VC) on exposure to hypoxia or high altitude.
Adapted from Rahn & Hammond (350).
Only Durig (1911) and Barcroft (1923) mention that corrections were applied to spirometric measurements to account for the temperature
change between the subjects and the spirometer, but the factors are not given.
2
Subsequently there have been many reports of changes in lung volumes on exposure to
hypoxia and hypobaria but the interpretation of this body of literature is again difficult
because of the widely varying conditions under which the measurements have been made:
•
Hypobaric hypoxia in hypobaric chambers
•
Normoxic hypobaria in hypobaric chambers
•
Normobaric hypoxia using hypoxic gas mixtures
•
Hypobaric hypoxia in field studies at high altitude
In addition interpretation is further complicated by the durations of exposure which range
from a few hours to several weeks and even on occasions months to years. These later studies
are summarised in Tables 2a and 2b and discussed below.
1) Hypobaric chamber studies
The first study of what may considered the modern era addressing the fall in VC on exposure
to hypobaric hypoxia is that of Rahn and Hammond (350). In this complex protocol
between 4 and 18 subjects were exposed to a variety of environments for varying durations. In
summary the authors showed that VC fell on acute exposure to hypobaric hypoxia in 18
subjects1. Although they claim that there was less of a fall with normobaric hypoxia sufficient
to produce an equivalent partial pressure of oxygen (PO2) in 9 subjects, suggesting that
hypobaria per se played a role, re-analysis of their data shows it to be underpowered
invalidating this claim2. Approaching the data from this often cited paper critically, the only
conclusion that can be drawn is that VC fell on exposure to an altitude equivalent of 4267 m
and 5468 m by 2.4 and 3.8% respectively, and at 5468 m this fall in VC was reversed by
breathing supplementary oxygen.
In a further experiment on, “an average of 4 subjects,” to investigate the effects of acute
extreme hypobaria, with oxygen supplementation to normalise or limit the effects of hypoxia,
VC fell only at 9144 and 12192 m. No comment is made as to whether these reductions were
statistically significant and at 12192 m, even with supplementary oxygen, the PiO2 was
1 : Rahn and Hammond (350). Effect of acute hypobaric hypoxia in 18 subjects: 5 min ascents to 3048, 4267, 5486 m (10,
14 and 18 000 ft); 5 mins adjustment at new altitude; 5 mins for measurements before ascent to new altitude. Supplementary
O2 given at the end of measurement at 5486m. No information is given on statistical methods. However ANOVA with
Bonferroni t-test for multiple comparisons on data in the paper gives a significant difference between 4267 m and SL and
2 : Rahn and Hammond (350). Effect of acute normobaric hypoxia using O2:N2 mixtures to give the equivalent PO2 as
acute hypobaric hypoxia for the same duration in 9 subjects. ANOVA shows no significant difference, however the power of
the test with α at 0.05 is 0.408 (i.e. < 0.8).
3
Authors (ref)
Rahn &
Hammond,
1952 (350)
Tenney et al,
1953 (422)
Shields et al,
1968 (397)
Cruz, 1973
(105)
Gray et al,
1973 (167)
Conditions
Chamber
446 & 379 mm Hg
= 4267 & 5486m
4300 m
(Mount Evans,
Colorado)
4298m (Pike's Peak)
4350m
(Cerro de Pasco,
Peru)
Chamber
= 4900m
Saunders et al, Normobaric hypoxia
1977 (379)
= 3700 - 4300m
Coates et al,
1979 (92)
Chamber
446 mm Hg = 4268m
Length of stay
Subjects
Relevant
Methodology
VC or FVC
TLC
RV
FRC
5 mins
18
Bell spirometer
↓ 2.4 & 3.8%
N/A
N/A
N/A
8 days
4
Spirometer
↔ underpowered
65 days
8
Stead-Wells
Spirometer
↓ 3.7% at 7 days.
N/A
N/A
N/A
↓ 3.7% (pooled
data)
N/A
N/A
↑ 5.2%
(pooled data)
↓ 7%
N/A
N/A
N/A
↑5%
↑31%
↑9%
↑ at 5h
↑ 18% at 5h
but NS
72hrs
5
20 mins
7
Plethysmography
↔ underpowered
Pneumotach
24hrs
4
Wedge spirometer ↓ 10% at 20 hrs cf ↑ 21% at 5h
He dilution
5hrs
NS at 20 hrs
6
Wedge spirometer
He & Ar dilution
↔
↔
N/A
N/A
up to 25
Dry rolling seal
spirometer
↓ ~1% cf SL
↔
↑ 11.2% at
72hrs cf
SL72hrs
N/A
↑ 18%
↑ 76%
↑ 40%
↔
↔
↓ 280ml on D1
Pollard et al,
1996 (344)
Cogo et al,
1997 (93)
Chamber
maximum = 8844m
N/A
N/A
N/A
40 days
7
9
6
Spirometer
N/A
4 hrs
Goldstein et al, Normobaric hypoxia
20 mins
1979 (160)
= 3700 - 4300m
3000 - 4300m
Jaeger et al,
(Pike's Peak,
72hrs
1979 (218)
Colorado)
Mansell et al,
5366m (Mt Logan) up to 6 weeks
1980 (274)
Gautier et al,
3457m
6 days
1982 (156)
(Jungfraujoch)
Welsh et al,
1993 (452)
8 alt residents
Cournand's method
6 SL residents
↔
↑ 22% day 1
underpowered
Wedge spirometer
↔ underpowered
He dilution
Pneumotach
↓ 250ml on D2 &
Plethysmography
D3
↓ ~7.5% at 6096m
Water filled
↓ ~12% at 7620m
spirometer
↓13.6% at 8844m
12 day trek to
EBC.
5300m
Measurements
(Everest Base Camp)
up to 5 days
after arrival.
51
Micromedical
Turbine
↓ 5.2%
N/A
N/A
N/A
5 days to 4559
4559 m (Margherita
m
and
Hut)
then 4 day stay
5
Micromedical
Turbine
↓ 3.6% D2 3500 m
↓ 4.3% D1 & D2
4559 m
N/A
N/A
N/A
9 days to 5050
5050 m
m
and
(Pyramid Lab, Nepal) then 10 day
stay
12
Micromedical
Turbine
↓ 8.6% at 3500m
↓ 11.5 % at 5050m
N/A
N/A
N/A
Table 2a: Post-War chamber and field studies on lung volumes.
↓: decrease; ↔: unchanged. Results are shown without comment if statistically significant.
VC: vital capacity; FVC: forced vital capacity; TLC: total lung capacity; RV: residual volume; FRC: functional residual capacity; D: day;
4
Chamber
460 mm Hg = 4300 Maximum of 5 hrs
m
4300 m
Weekly
(Pikes Peak,
measurements
Colorado)
over 3 weeks
↓ 3%
NB: Pooled group mean
data
↓ 2.8%
NB: Pooled group mean
data
12
Collins water
seal spirometer
9
Collins water
seal spirometer
Mason et al,
Serial to 5300m
12 days to EBC
2000 (281)
(Everest Base Camp)
46
Micromedical
Turbine
↓ 4% at 2800m
↓ 8.6% at 5300m
Cremona et 4559 m (Margherita
al, 2002 (104)
Hut)
Within 1 hr of
arriving
262
Cosmed
Spirometer
Turbine or
Pneumotach
↔ in healthy subjects
↓ ~2% in 39 with HAPE
3800m
(Tien Shan,
Kyrgyzstan)
Over 15 days
20
DeBoeck,
2005 (118)
Chamber
451 mm Hg = 4267m
Up to 12hrs
Senn et al,
2006 (393)
4559 m (Margherita
Hut)
Fasano et al, 4559 m (Margherita
2007 (145)
Hut)
Forte et al,
1997 (149)
Mason et al,
2003 (282)
Dehnert et al, 4559 m (Margherita
2010 (120)
Hut)
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
↓ 4.3% D1; ↓ 5.7% D2;
Micromedical
↓ 5.9% D5; ↓ 6.5% D10;
Turbine
↓ 4.8% D15
N/A
N/A
N/A
15
Micromedical
Turbine
↓ 3%(300ml) at 6hrs
↓ 4%(350ml) at 12hrs
N/A
N/A
N/A
24 hrs
26
SensorMedics
Vmax 2900
↓ 6.4% D1
↓ 7% D2
N/A
N/A
N/A
7 days
8
Micromedical
Spirometer
↓ ~5% (estimated)
N/A
N/A
N/A
34
Pneumotach ↔ in non-HAPE subjects
↔?
6 with previous
N/A
Plethysmography
underpowered
underpowered
HAPE
N/A
48 hrs
Table 2b: Post-War chamber and field studies on lung volumes.
↓: decrease; ↔: unchanged. Results are shown without comment if statistically significant
VC: vital capacity; FVC: forced vital capacity; TLC: total lung capacity; RV: residual volume; FRC: functional residual capacity; D: day;
5
equivalent to an altitude of 3353 m. In a further experiment reported in their paper the effect
on VC of an 8 day stay at 14200 ft (4328 m) on the summit of Mount Evans in Colorado was
studied in 4 subjects. The data is again hevily underpowered and it is not possible to draw any
meaningful conclusions from it.
Gray et al (168) studied 5 subjects exposed to a barometric pressure equivalent to 4900 m for
4 hours. Measurements were made 4 times at roughly 1 hour intervals throughout the study.
Vital capacity had fallen by a mean of 11% from control after 1 hour in the chamber and by a
mean of 7% after 4 hours.
Coates et al (92) despite studying only 4 subjects decompressed to 446 mm Hg (equivalent to
an altitude of 4268 m) found a significant 10% fall in VC at 20 hours compared to the value at
5 hours (p <0.05) although there was no difference between control values and 5 hours. The
data at 20 hours is not given but re-analysis of the control and 5 hour data shows them to be
underpowered to detect a significant change3. Total lung capacity (TLC) measured by helium
dilution increased by a mean of 21% at 5 hours compared to control (p<0.02) but had fallen
again so that it was not significantly different from control values at 20 hours. Residual
volume (RV) and closing capacity (CC) both showed large increases between control and 5
hours (p < 0.02 and 0.05 respectively. Absolute values are not given but are of the order of
30-40%) and then did not change between 5 and 20 hours. As both RV and CC increased by
similar amounts closing volume did not change4. Functional residual capacity (FRC)
increased by 19% between control and 5 hours but was not statistically significant because of
the underpowered nature of the data. In summary, despite being considerably underpowered,
this study demonstrated a 21% increase in TLC and significant increases in RV and CC
between control and 5 hours exposure to a barometric pressure of 446 mm Hg. Vital capacity
fell by 10% between 5 and 20 hours.
Welsh et al (451) during the hypobaric chamber experiment Operation Everest II, in which 8
subjects underwent a simulated ascent of Mount Everest, demonstrated significant decreases
(p<0.05) in forced vital capacity (FVC) at barometric pressures of 347, 282 and 240 mm Hg
(equivalent to 6096, 7620 and 8844 m respectively, the last value approximating the summit
of Mount Everest). The mean value of FVC on the “summit” was 13.6% less than the control
values. Measurements made within 30 minutes of a return to sea level pressures showed that
FVC had normalised by approximately 50% and had normalised completely after 19 hours.
3 : Coates et al (92). Comparing control with 5 hours at 4268m using paired t-test, p = 0.37. With α of 0.05 the power of the
performed test is 0.059 (i.e. significantly < 0.8).
4 : closing volume (CV) = closing capacity (CC) – residual volume (RV)
6
Forte et al (149) studied 12 subjects, 3 of whom also participated in the field component of
the study at Pikes Peak, Colorado (4300 m), discussed below, who were decompressed to a
barometric pressure of 460 mm Hg (equivalent to an altitude of 4300 m). The subjects were
studied on 5 occasions at sea level and on 5 occasions at 460 mm Hg. The FVC fell by 2.8%
at 460 mm Hg compared with sea level (p<0.05) which is very similar to the magnitude of the
fall observed at Pikes Peak. It is unclear from the paper if the values reported represent the
average of the 5 decompressions.
Dillard et al (123), in a study designed to investigate the changes in lung function which
occur during commercial airline flights, looked at 18 patients with severe chronic obstructive
pulmonary disease (COPD - defined as a forced expiratory volume in 1 second (FEV1) ≤ 50%
predicted; > 15% increase in FEV1 after bronchodilator therapy and normal TLC) and 9
healthy controls, during 2 hours exposure to a barometric pressure of 565 mm Hg, equivalent
to an altitude of 2438 m. In the healthy controls FVC fell by 3% (p<0.05) and by 4.3% in the
COPD group but did not reach statistical significance.
During Operation Everest III (COMEX ‘97) which followed a similar design to Operation
Everest II, Mason et al (280) demonstrated a 6.8% fall in FVC at a simulated altitude of 8000
m compared with sea level in 8 subjects. There was no change between sea level and 5000 m
but because of the limitations of the study (the restriction of subject numbers by chamber
size) the data was underpowered.
Deboeck et al (118) reported a significant fall in FVC of 3% after 6 hours and 4% after 12
hours of exposure to a barometric pressure of 451 mm Hg equivalent to an altitude of 4267 m
in the Belgian Air Force hypobaric chamber at Evere, Brussels.
2) Normobaric hypoxia
Saunders et al (379) in an attempt to isolate the effects of hypoxia from the concomitant
hypocapnia that accompanies it at altitude, studied 7 subjects using a re-breathing method to
give an alveolar partial pressure of oxygen (PAO2) of 40-50 mm Hg (5.3-6.4 kPa, equivalent
to an altitude of between 3700 and 4300 m) for 20 minutes while maintaining alveolar
normocapnia (alveolar partial pressure of CO2 (PACO2): 38 to 42 mm Hg (5.1 to 5.6 kPa)).
Measurements using body plethysmography were made before and after hypoxia, and every 5
minutes during hypoxia. There was no change in VC, although analysis of the raw data in the
7
paper shows it to be underpowered5, whereas RV, TLC and FRC all increased significantly by
31% (p<0.02), 5% (p<0.005) and 9% (P<0.01) respectively. All volumes had returned to their
equivalent to an altitude of 3353 m. In a further experiment reported in their paper the effect
on VC of an 8 day stay at 14200 ft (4328 m) on the summit of Mount Evans in Colorado was
studied in 4 subjects. The data is again heavily underpowered and it is not possible to draw
any meaningful conclusions from it. control values within 3 minutes of re-oxygenation by the
addition of 100% O2 to the breathing circuit.
Goldstein et al (160), using the same re-breathing technique as Saunders et al (379) and
identical normobaric isocapnic hypoxic conditions, found no changes in TLC or VC using
helium and argon dilution. Unfortunately data for individual subjects are not given and it is
not possible to see if the study was adequately powered to demonstrate a significant
difference although this is unlikely in view of the small numbers of subjects involved (9 for
VC measurements). In their discussion the authors suggest that the discrepancy between their
results and those of Saunders are due to artefacts from body plethysmography, although
Coates et al (92) in hypobaric hypoxia at a roughly equivalent PAO2 demonstrated a 21%
increase in TLC using the same technique of helium dilution.
3) Hypobaric normoxia
In an attempt to simulate the conditions of space flight, Ulvedal et al (431) studied the effects
of prolonged hypobaria without hypoxia for between 14 and 17 days at simulated altitudes of
5486, 8230 and 10211 m on 2, 4 and 8 subjects respectively. Unfortunately only average
summary changes for these simulated altitudes are given and so it is not possible to make any
informed comments, The authors report a fall in FVC of 3.1%, 2.9% and 7.6% at 5486, 8230
and 10211 m respectively despite the absence of hypoxia but it is not known if these results
were statistically significant.
4) Field studies
Tenney et al (422) studied 4 subjects during a 7 day stay at 4300 m on Mount Evans in
Colorado. The authors claim that VC fell over the first 3 days at altitude with the maximum
fall on day 3, before increasing back towards sea level values during the rest of the stay at
altitude. Reanalysis of the data for VC shows that this change was not statistically significant
5: Saunders et al (379). Using ANOVA on VC data in the paper, p = 0.25. Power of the performed test, with α of 0.05 =
0.121
8
and, unsurprisingly in view of the small sample size, that the study was markedly
underpowered6. The increase in RV that occurred from day 1 at altitude was claimed by the
authors to show a definite trend but no statistical analysis was performed. Reanalysis of the
data in the paper shows that the 22% increase in RV was only statistically significant on day 1
at altitude compared with the sea level control7 and not significant on the other days at
altitude. Data is not given for TLC, however the authors state that in 3 out of 4 subjects TLC
had increased on day 1 at 4300 m but had fallen to well below control values by day 3.
Calculation of TLC from the data for RV and VC in the paper allows statistical analysis of
these claimed changes and shows them not to be statistically significant, and also to be
underpowered8. Functional residual capacity was estimated from measures of RV and
expiratory reserve volume (ERV). As RV was measured in the sitting position and ERV
supine, and as the data is underpowered, it is not possible to draw any conclusions on changes
in FRC from this study.
Shields et al (397) studied 8 female subjects during a 65 day stay at Pike’s Peak, Colorado
(4298 m). Measurements were made on days 1, 7, 30 and 65 on Pike’s Peak. FVC was
significantly reduced compared to sea level controls by 3.7% on day 7. By day 30 FVC
although still reduced compared to baseline was no longer statistically different. This study is
important because it is one of the longest field studies and shows a return to baseline values of
FVC with time at altitude.
In a complicated series of experiments Cruz et al (105) studied 8 Peruvian high altitude
natives at Cerro de Pasco (4350 m). Four of these high altitude natives were then studied
again after descent to Lima (150 m). Six sea level subjects were also studied first at Lima and
then at 4350 m. Vital capacity decreased and FRC increased in the lowlanders on ascent to
4350 m but because of the small group size these changes were not statistically significant9.
In the high altitude natives, descent to Lima produced an increase in VC while FRC
decreased. Despite being only 4 subjects the 4.6% increase in VC with descent to Lima was
6: Tenney et al (422). Using one way repeat measure ANOVA on the raw data in the paper the change in VC is not
statistically significant (p = 0.582) but is underpowered with a power for α = 0.05 of 0.05 (i.e. well below 0.8). Even using a
paired t-test comparing VC for the sea level control with high altitude day 3 when the maximum fall occurred, the data is still
not statistically significant (p = 0.343).
7: Tenney et al (422). Using one way repeat measure ANOVA with Bonferroni t-test for multiple comparisons vs sea level
control on RV data, the difference between sea level and day 1 at altitude was statistically significant (p = 0.004).
8: Tenney et al (422). Using one way repeat measure ANOVA the power for α = 0.05 is 0.05.
9 : Cruz et al (105). Using a paired t-test, FVC in 6 lowlanders fell by 3.1% on ascent to 4350 m, but p = 0.07 and power for
α = 0.05 is 0.38. FRC increased by 5.3% but p = 0.08 and power for α = 0.05 is 0.349.
9
statistically significant. The fall in FRC on descent however was again underpowered10.
Pooling the data from the 6 sea level subjects who ascended and 4 altitude natives’ who
descended gives a significant decrease of 3.7% in VC and a significant increase of 5.2% in
FRC at 4350 m compared with 150 m.
Jaeger et al (218) studied up to 25 soldiers on manoeuvres for 72 hours, first at a control
altitude of between 200 and 875 m and then one week later during two 15 km hikes from
3000 to 4300m on Pike’s Peak, Colorado. The routes taken at the control altitude and on
Pike’s Peak were chosen to be as similar as possible and measurements were made in both
arms of the study at 0, 36 and 72 hours. Forced vital capacity was reduced at altitude by
around 1% compared with control at all times (p<0.05). Total lung capacity was unchanged at
altitude compared with control but it increased significantly with time at both altitude and
under control conditions. Residual volume increased significantly with time both at sea level
and at altitude (11.2% increases at altitude c.f. sea level at 72 hours). Closing capacity
increased with time both under control conditions and at altitude. Interpretation of these
results is made difficult because of the confounding effect of exercise which can precipitate
pulmonary oedema both at sea level (296, 297, 469) and at altitude (9) with the resultant
changes in respiratory mechanics discussed below.
Mansell et al (274) studied 7 subjects after either 9 or 30 days on the summit of Mount Logan
(5366 m). In addition the subjects had been allowed to acclimatise at an intermediate camp of
unspecified height, “for at least 4 days,” during their ascent to the summit. Although VC fell
5% at 5300 m compared with sea level this change did not reach statistical significance but
again was underpowered to demonstrate a difference11. Using helium dilution, TLC, RV and
FRC were all significantly increased at 5300 m by 18%, 76 and 40% respectively.
Gautier et al (156) studied 9 subjects each day during a 6 day stay at 3457 m using
plethysmography. Vital capacity was significantly reduced by around 250 ml on days 2 and 3
of the stay at altitude compared with control values. Altitude data for individual subjects are
not given in the paper and initial control values are represented only as percentages of
predicted values. Functional residual capacity was significantly reduced by 280 ml on day 1 at
altitude. The fall in VC on day 3 and in FRC on day 1 were both reversed by supplementary
O2. This was given for at least 5 minutes to a PiO2 of 150 to 160 mm Hg (i.e. sea level PiO2)
daily and measurements repeated while the subjects continued to breathe supplementary O2.
10 : Cruz et al (105). On descent to Lima from 4350 m FRC fell by 5.1%, p = 0.341 using a paired t-test and power for α =
0.05 is 0.068.
11 : Mansel et al (274) Using a paired t-test to compare VC at SL and 5366 m, p = 0.145 and power for α = 0.05 is 0.198.
10
There was a non-significant fall in TLC and a non-significant increase in RV during the stay
at altitude. It is not possible to assess the power of the study because of the lack of individual
data but it is very likely in view of the small number of subjects and the relatively low altitude
that the study was underpowered.
Forte et al (149) in the field component of their chamber study discussed above measured
FVC weekly in 9 subjects during a 3 week stay at Pikes Peak (4300 m).
Forced vital
capacity, which fell by 3% at 4300 m compared with sea level (p<0.05), is reported as the
mean of the 3 measurements made at weekly intervals. However the authors state that the
results did not vary significantly during the stay.
Cogo et al (93) in what were effectively 2 studies reported the effect of altitude on FVC, peak
expiratory flow (PEF) and mid-expiratory flow at 25% of FVC (MEF25) measured with a
Micro Medical turbine spirometer in 5 subjects during a 5 day ascent in the European Alps to
the Cappana Regina Margherita hut on Mount Rosa at 4559 m where the subjects then stayed
for 4 days before descent, and in 12 subjects during a 9 day ascent, and subsequent 10 day
stay, at the Pyramid Laboratory at 5050 m in the Nepalese Himalaya.
During the ascent to the Regina Margherita Hut FVC had fallen significantly by 3.6%
compared with sea level values on the second day at 3500 m (subjects spent 2 nights at 1800
m and 3 nights at 3500 m during the ascent) although was initially unchanged on arrival at
this altitude. On the first and second days at the Regina Margherita Hut at 4559 m, FVC was
significantly reduced by 4.3% but had increased and was unchanged from sea level values by
the fourth day. Peak expiratory flow was significantly increased by 15% at 3600 m and 26%
at 4559 m while MEF25 was reduced at 3600 m but did not reach statistical significance (and
may well have been underpowered in view of the small number of subjects), but was
significantly reduced by 5% on day 2 and 7.6% on day 3 at 4559 m. The reductions in FVC
and MEF25 were both greater in the mornings compared to the afternoons at the Margherita
hut although this was only statistically significant on the second day.
In the Nepalese part of the study, FVC had fallen significantly by 8.6% on the second day and
7% on the third day at 3500 m (it was unchanged from sea level values on day 1 at 3500 m);
by 5% on day 1, 5.5% on day 2 and 8.7% on day 3 at 4550 m and by 11.5% on arrival at
5050m. The fall in FVC gradually returned toward sea level values with time at 5050 m so
that by day 7 it was unchanged from sea level values. Peak expiratory flow increased with
ascent, being 121.4% of sea level values on day 2 at 3500 m; 129.4% at 4240 m and 135.2%
at 5050 m. MEF25 was reduced on the second day at 3500 m by 12%; by between 12 and
11
16.5% at 4240 m and by 15.5% on arrival at 5050 m returning towards sea level values with
time at 5050 m. There was no diurnal variation in either FVC or MEF25 but the fall in both
parameters was greater in those subjects with more severe acute mountain sickness (AMS).
While this study is a more realistic reflection of an ascent to altitude it illustrates how the
effect of exercise; the different altitudes at which measurements were made and time spent
acclimatising at a given altitude before continuing to ascend to a new altitude all confound the
interpretation of the results.
During the 1994 British Mount Everest Medical Expedition Pollard et al (344) compared
measurements of PEF made with a Mini-Wright peak flow meter with those made using a
Micro Medical turbine spirometer at sea level and within 5 days of arrival at Mount Everest
Base Camp (5300 m). A number of previous studies had reported the effects of altitude on
PEF but measurements had often been made using variable orifice meters such as the MiniWright which in addition to having a non-linear response to flow (298) are affected by the fall
in gas density which occurs with increasing altitude (335). The fixed orifice Micro Medical
turbine device is unaffected by changes in gas density (335). In addition to PEF the Micro
Medical device also records FVC and FEV1. Peak expiratory flow increased by 25.5% with
the Micro Medical device compared to sea level while when measured with the Mini Wright it
fell by 6.6%, thus under reading true PEF by 32%. Forced expiratory volume in 1 second was
unchanged compared with sea level while FVC fell by 5.2%.
Lower PEF readings were associated with higher Lake Louise acute mountain sickness
(AMS) scores (360) (p<0.05) and morning readings (3.3% lower than afternoon readings
p<0.01). Forced vital capacity was also significantly lower in the mornings compared to the
afternoons (p<0.01) but was not related to AMS score. Data was recorded in 14 subjects who
remained at Everest Base Camp for between 1 and 45 days. In these subjects there was a trend
for FVC to return towards baseline values with time at altitude but it did not reach statistical
significance.
As part of the same expedition Pollard et al (343) tested the hypothesis that the hypobaric
hypoxia at altitude and the hypocapnia resulting from hyperventilation might produce
bronchoconstriction in healthy human subjects as occurs in animals exposed to acute hypoxia;
humans with chronic lung disease; asthmatics, or patients receiving mechanical ventilation for
neurological injury. Using supplementary O2 and inhaled β2 agonists the authors were unable
to demonstrate the presence of bronchoconstriction, and PEF actually fell with supplementary
O2. However a very low flow rate of O2 was given (1 l / min) for only 5 minutes using an
12
open circuit mask and O2 saturations were only increased to a mean of 94%. Lower O2
saturation was significantly associated with lower FVC and FEV1; higher AMS scores were
again associated with lower PEF readings, and PEF and FVC were again lower in the morning
compared to readings made in the afternoon.
Mason et al (281) studied the same subjects during their walk in to Everest Base Camp at
5300 m over a period of between 9 and 14 days. Fifty-five subjects were studied. Of these, 46
had complete data sets and were analysed. FVC was shown to fall by a mean of 4% at 2800 m
and 8.6% at 5300 m. Forced vital capacity tended to increase by 0.5% of the sea level value
during each stay at a given altitude (never more than 2 nights) but did not reach statistical
significance. This tendency for FVC to increase at a given altitude could explain the
difference between the 8.6% reduction in FVC at 5300m in this study with Pollard et als’
5.2% fall (344) in the same subjects at the same altitude. The recordings in the present study
were taken on arrival at Everest Base Camp, whereas Pollard et als’ were taken on average 2
to 3 days after arrival. No significant relationship was demonstrable between the fall in FVC
and either O2 saturation or AMS score unlike in Pollard et als’ study.
Cremona et al (104) studied 262 climbers ascending the Monte Rosa (4559 m) making
control measurements at 1200 m and again at 4559 m, 24 hours later. The FVC was
unchanged at 4559 m compared with control. However 39 of the subjects developed either
clinical or radiological signs of mild pulmonary oedema or both on ascent to 4559 m but did
not have symptoms of HAPE. When a subgroup analysis is made of the change in FVC in
these 39 subjects with signs but no symptoms of pulmonary oedema, FVC fell significantly
by around 2% (p=0.03) although absolute values are not given. This study is discussed further
in the section on changes in the nitrogen washout curve, below.
Mason et al (282) studied 20 subjects rapidly transported by road in 7 hours to 3800m where
they remained for 2 weeks. Compared with sea level values, FVC was significantly reduced
by a mean of 4.3% within 24 hours of arrival at 3800m and remained reduced throughout the
2 weeks at altitude with a maximum reduction of 6.5% on day 10. Measurements were made
at baseline within 24 hours of the subjects’ departure from altitude when FVC was still
significantly reduced by 4%.
Senn et al (393) studied 26 subjects who ascended to 3200 m by cable car; walked to the
Gnifetti Hut at 3611 m where they stayed the night before walking the next day to the Regina
Margherita Hut at 4559 m. Measurements were made within 3 hours of arrival at 4559 m and
on the following morning. Forced vital capacity fell by 6.4% (300ml), compared with low
13
altitude, on arrival at 4559 m and had fallen by 7% (330 ml) the following day. Significant
changes in the nitrogen washout curve on ascent to 4559 m are discussed below.
Fasano et al (145) studied 8 subjects who also ascended to the Regina Margherita Hut at
4559 m although in this study over 4 days, spending 3 nights at the Gnifetti Hut before
ascending to 4559 m. A number of variables were measured including FVC; peak inspiratory
and expiratory flows (PIF and PEF) and the maximal inspiratory pressure generated by the
respiratory muscles (MIP) which is discussed further below (see: Respiratory Muscle
Function). The FVC is reported to have fallen significantly (p<0.05) at 3600 and 4559 m
compared to sea level but no values are given. Estimating the fall from a graph in the paper
gives a value of ~5% at 4559 m.
Sharma and Brown (395) studied 7 “accomplished mountaineers,” climbing Mount Everest
in an unusual protocol in which spirometry, maximal voluntary ventilation and inspiratory
and expiratory muscle pressures were performed. Because of what was described as
expedition logistics, no sea level control measurements were made and the first measurements
of the study took place at 3450 m, above the altitude at which falls in FVC have been
reported, and after the subjects had flown into this altitude by helicopter. Measurements were
made at 3450 m and at Everest Base Camp (5350 m) after a 9 day walk in; after 8 days on
Everest during which time the subjects had ascended to 6300 m and returned to Base Camp,
and after 27 days on the mountain during which time the subjects had ascended to 7200 m.
Although the paper states that subjects were evaluated for AMS and high altitude pulmonary
oedema (HAPE), no formal AMS scoring was performed.
Compared to control measurements made on arrival at 3450 m, FVC fell by 14% on arrival at
5350 m and was reduced by 12% and 17.9% after visits to 6300 and 7200 m respectively.
These results are difficult to interpret because the FVC used for the control had already fallen
to 96.2% of the predicted sea level value and there is a large variation in the measurements
made on subsequent days: on the second and fourth days at 3450 m FVC was found to be
respectively 105.1% and 79.8% of the predicted sea level values. There is a similar
discrepancy between the values of FVC found on arrival at and after 2 days at 5350 m
(110.4% and 84.6% of predicted sea level values respectively). This is in marked contrast to
the consistency of measurements in other papers such as those of Pollard et al (344) and
Mason et al (281). Because of this; the small number of subjects involved; lack of a sea level
control and the lack of formal AMS scoring, it is impossible to draw any meaningful
conclusions from this paper.
14
Dehnert et al (120) carried out extensive pulmonary function tests on 34 subjects during a 2
day stay at the Regina Margherita Hut at 4559 m. The 34 subjects included 8 who regularly
suffered AMS and 6 with a previous history of HAPE. Measurements were made at sea level
and after 4, 20 and 44 hours at 4559 m. In the 30 subjects who did not develop HAPE there
was no change in FVC or TLC at altitude compared with sea level. The authors state that a
power calculation based on the closing volume (CV) changes reported by Cremona et al (104)
indicated that a group size of 30 would give a power of 0.8 for an α value of 0.05 to detect a
difference of 25% between groups. While this may have been an adequate power for changes
in CV, the maximum documented fall in FVC at this altitude is of the order of 6% (393)
rendering the study underpowered to detect any change in FVC. There is not enough available
data on which to base power calculations for TLC. Changes in closing volume (CV);
compliance; respiratory muscle strength; The diffusing capacity for CO (DLCO); the alveolararterial oxygen difference (
sections below.
!
A ! aDO
2
) and airways conductance are discussed in the relevant
15
SUMMARY OF LUNG VOLUME CHANGES
The following summary includes only those studies which took place under conditions of
hypobaric hypoxia either in a hypobaric chamber or in the field. The normobaric studies of
Saunders et al (379) and Goldstein et al (160) are not discussed further. Interpretation of the
remaining hypobaric studies is difficult because of the markedly different circumstances
under which they were performed. Chamber studies should provide the most controlled
environment and be most free from confounding factors such as differing rates of ascent;
exercise; cold and humidity.
Vital capacity: hypobaric chamber studies
Figure 1 shows the pooled data for the 7 chamber experiments in the literature with
significant changes in VC compared with normobaric controls (118, 123, 149, 168, 280, 350,
451). For this reason the study of Coates et al (92) has not been included as while it shows a
fall in VC at 20 hours compared with 5 hours, there is no difference between 5 hours and
normobaria.
Figure 1: Change in vital capacity (VC) with simulated altitude.
Combined data from 7 published hypobaric chamber studies with statistically significant changes in vital capacity
16
There is good agreement between the 7 studies and VC appears to fall by 1.7% for every
1000m of ascent. The regression equation for this data is:
% Fall in VC = -3.382 + (0.0017 x altitude (m))
r2 = 0.724
p < 0.001
Vital capacity: field studies
Interpretation of the field studies is more difficult. Pooling studies and performing linear
regression analysis is complicated by the repeated measures which occur during prolonged
stays at altitude. To ensure consistency in this analysis only significant changes within the
first 24 hours of arrival at a new altitude have been included in the pooled data.
Figure 2: Change in vital capacity (VC) with altitude.
Combined data from 6 published field studies with statistically significant changes in VC with 24 hours of arriving at a new altitude.
The study of Cogo et al (93) has been excluded because it has such wide variation in the
results at the same altitude while the study of Cremona et al (104) has not been included
because a fall in FVC was only present in a subgroup analysis and actual values for the
change are not given. Two other studies have been excluded from the regression analysis
although their data are shown on the graph: the study of Forte et al (149) because the result is
17
the mean of weekly measurements during a 3 week stay at 4300 m and that of Fasano et al
(145) because no actual results are given in the paper and the result has been estimated from a
plot. Figure 2 shows the regression line for the 6 pooled field studies with statistically
significant results. There is much poorer agreement compared with the chamber studies and
the result is not statistically significant. This reflects the smaller number of studies (power for
α = 0.05 is 0.31) and the difference in conditions under which measurements were made.
Vital capacity appears to fall by 1.3% for every 1000m of ascent. The regression equation for
this data is:
r2 = 0.39
p = 0.134
Vital capacity: combined chamber and field studies
Figure 3 shows the results of all statistically significant chamber and field studies combined.
From this pooled data VC appears to fall by 1.7% for every 1000m of ascent. The regression
equation for this data is:
r2 = 0.67
p < 0.001
The regression equations for the chamber and combined chamber and field studies are
remarkably similar. The equation for the field studies is underpowered and does not reach
statistical significance. Despite this it is of a similar order of magnitude to the chamber and
combined data. A fall in FVC is apparent at altitudes as low as 2800 m in both a chamber
study (123) and in the field (281). With time at altitude, VC appears to return to (93, 397) or
towards control values (281, 344).
18
Figure 3: Change in vital capacity (VC) with altitude and simulated altitude
Combined data from 7 hypobaric chamber and 6 field studies with statistically significant changes in VC within 24 hours of arrival at a new
altitude.
Welsh et al (451) demonstrated that within 30 minutes of return to sea level from extreme
altitude, FVC had only normalised by around 50% and had only returned to control levels
after 19 hours. At the relatively low altitude of 3457 m, Gautier et al (156) demonstrated that
VC could be normalised with supplementary oxygen although this was demonstrable on only
the third day of a 6 day sojourn. These findings could not be repeated by Pollard et al (343) at
the higher altitude of 5300m although in this study lower VC was associated with lower O2
saturation. The fall in VC compared to control would appear to be greater in the mornings
than in the afternoons (93, 343, 344).
19
Residual volume
Data on changes in RV are available from only 5 studies in hypobaric hypoxia (1 chamber
and 4 field studies). Significant changes are found in 4 studies (92, 218, 274, 422). The study
of Jaeger et al (218) was complicated by the effects of exercise. The study of Tenney et al
(422) only shows a significant increase in RV on day 1, but this may reflect the small sample
size and resultant lack of power. Gautier et al (156) did not demonstrate a change on exposure
to 3457 m. It is not known if this was because the study was underpowered. Residual volume
may increase on exposure to altitude but there is such wide variation between the results of
Tenney et al (422), Coates et al (92) and Jaeger et al (218) made at 4300 m, or its barometric
equivalent, that it is not possible to draw a meaningful conclusion.
Total lung capacity
Data on changes in TLC are available from only 3 studies in hypobaric hypoxia (92, 156, 274)
and one field study (120) although they have also been calculated for the field study of
Tenney (422). Three studies (120, 156, 422) did not show significant change although it is
likely that all 3 were underpowered. Coates et al (92) found an increase of 21% from control
in 4 subjects at an altitude equivalent of 4268 m at 5 hours but by 20 hours this difference
from control had disappeared. Mansell (274) demonstrated an 18% increase in 7 subjects at
5366 m. It is impossible to draw any physiologically meaningful conclusions from this limited
data.
Functional residual capacity
Functional residual capacity was measured in only 5 studies (92, 105, 156, 274, 422). Tenney
et al
(422) estimated FRC from RV and ERV but as discussed above this was
methodologically unsound and will not be discussed further. Three studies show an increase
in FRC (92, 105, 274) while the study of Gautier et al (156) shows a decrease of 4% at 3457
m. It is impossible to draw any physiologically meaningful conclusions from this data or
indeed to imagine any mechanism that would cause a fall in FRC at 3457 m but an increase in
FRC above 4000 m.
20
CONCLUSIONS ON CHANGES IN LUNG VOLUMES
Vital capacity
Vital capacity decreases by between 1.6 and 1.7% of sea level values for every 1000m of
ascent and returns to, or towards, control values with time at altitude.
Residual volume
Residual volume may increase on exposure to altitude but there is such wide variation
between the results obtained at the same altitude that further interpretation is not possible.
Total lung capacity
It is impossible to draw any meaningful conclusion from the available data.
Functional residual capacity
It is impossible to draw any meaningful conclusion from the available data.
21
5) Mechanisms to explain the changes in lung volumes
Vital capacity is defined as the difference between TLC and RV (i.e. VC = TLC – RV).
Residual volume is the minimum lung volume that can be achieved by voluntary expiration. It
is determined by the balance between the expiratory muscles and the elastic recoil of the
respiratory system, predominantly of the chest wall. In addition RV can be influenced by
expiratory flow limitation when expiration is interrupted before lung volume reaches the point
at which equilibrium exists between the expiratory muscles and elastic recoil of the
respiratory system. This is the predominant mechanism controlling RV in adults older than 35
years and in subjects with obstructive lung disease.
Total lung capacity is the maximum lung volume that can be achieved in conscious subjects
by voluntary inspiration. It is determined by the balance between inspiratory muscle strength
and the elastic recoil of the respiratory system, predominantly of the lung. Because RV and
TLC are controlled by different mechanisms they can vary independently from one another
(100, 261). If changes in either RV or TLC occur independently from the other then VC will
change.
Unfortunately there are insufficient data to see if changes in VC correlate with changes in RV
or TLC. There are only 4 studies in which both VC and RV are measured (92, 218, 274, 422).
In 2 studies while RV increased there was no change in VC (274, 422). However the studies
were underpowered. In the study of Coates et al (92) while there was an increase in RV
between control and 5 hours exposure to an equivalent altitude of 4268 m, the fall in VC only
occurred between 5 and 20 hours. This only leaves the study of Jaeger et al (218) which is
complicated by exercise.
There are only 2 studies in which both VC and TLC were measured (92, 274). The study of
Mansell et al (274) was underpowered to demonstrate a change in VC. In the study of Coates
et al (92) the changes in TLC and VC again occurred at different times.
A number of mechanisms have been suggested to explain the change in VC which occurs at
altitude. These are:
•
Changes in the mechanics of the respiratory system
•
Increase in pulmonary blood volume
•
Alterations in respiratory muscle function
•
Abdominal distension
22
•
Oxygen absorption
•
Early airways closure secondary to sub-clinical pulmonary oedema
a) Respiratory mechanics
As discussed above, changes in the balance between the elastic recoil forces of the respiratory
system and the inspiratory or expiratory muscles will affect TLC or RV respectively.
Independent changes in either TLC or RV will influence VC. The elastic recoil of the
respiratory system is represented by static compliance (Cst) which expresses the relationship
between the applied pressure and the resultant lung volume under equilibrium conditions –
i.e. zero or effectively zero flow. The static compliance of the lung (Cst(L)) is thus the change
in lung volume per unit change in transpulmonary pressure. Compliance may also be
estimated during tidal breathing from pressure and volume measurements made at the end of
inspiration and expiration when the lung is apparently static. In normal lungs this dynamic
compliance (Cdyn) and Cst are usually similar but Cdyn may be reduced due to small airway
obstruction as can occur in obstructive airway disease or pulmonary oedema (100). It should
also be noted that because dynamic lung compliance (Cdyn(L)) demonstrates frequency
dependence (97), interpretation is complicated by the marked increase in minute ventilation
which occurs at altitude, and is further complicated by the fall in air density (88, 433) and
thus in airway resistance (156, 274) which occur with increasing altitude. For these reasons,
although reported changes in Cdyn(L) will be included in the following discussion for
completeness, conclusions will only be drawn from changes in Cst(L).
Lefrancois et al (260) could not demonstrate a change in quasi-static compliance (Cst(L)) in
10 subjects after 1 month at 3660 m although the high altitude pressure-volume (P-V) curves
were shifted to the left compared with control, consistent with a loss of lung elastic recoil.
Kronenberg et al (243) measured Cst(L) in 4 subjects at 3800 m at the Barcroft Laboratory on
White Mountain. The airway was occluded for 3 seconds at incremental inspired steps of
500ml from RV to 70-80% of TLC. Between 4 and 6 pressure volume curves were obtained
in each subject at control and after 24 and 72 hours at Barcroft. Control measurements were
also made at Barcroft, on arrival, while breathing supplementary oxygen sufficient to
maintain the arterial oxygen tension (PaO2) above 150 mm Hg (20 kPa). The mean (± S.E.M)
quasi-static inspiratory lung compliance at control and after 24 and 72 hours at 3800 m were
176 ± 7.9; 169 ± 15.7 and 141 ± 8.2 ml cm H20-1 respectively. The 17% decrease between 24
23
and 72 hours was significant (p < 0.025). No comment was made as to whether the P-V
curves were displaced or not.
Raymond and Severinghaus (352) using the same technique as Kronenberg et al (243) (i.e.
an inspiratory volume history) studied 4 subjects during a 10 day stay on the summit of White
Mountain at an altitude of 4342 m and found no change in Cst(L). Multiple P-V loops were
recorded at each altitude and only the mean values for each subject’s P-V loops are given in
the paper. However reanalysis of the means reveals the data to be underpowered to detect a
change over repeated measurements in such a small sample size12.
Cruz (105) measured dynamic lung compliance (Cdyn(L)) in 4 high altitude natives at 4350 m
before repeating the measurements at 150m and in 6 sea level subjects at 150 m before
transporting them to 4350 m. There was no change in Cdyn(L) in either group. The changes in
VC and FRC in the lowlander group and FRC in the high altitude natives were underpowered
(see above). Individual data are not available but this is very likely to be the case for Cdyn(L)
too.
Jaeger et al (218) in their study described above, measured Cst(L) and obtained acceptable
expiratory P-V loops during all phases of their study in 11 subjects. There was a clockwise
rotation of the P-V curves around FRC (i.e. a rightward shift above FRC and a leftward shift
below FRC) at high altitude compared to low altitude suggestive of a reduction in pulmonary
compliance. This change was interpreted as being due to interstitial oedema. It should be
noted that unlike the other studies investigating Cst(L) discussed here considerable physical
exercise formed an integral part of this study’s protocol.
Mansell et al (274) measured both Cst(L) and Cdyn(L) in 7 subjects after either 9 or 30 days at
5366 m. Expiratory Cst(L) did not change at altitude although the P-V curves were shifted up
and to the left compared with the sea level curves. When the mean data for Cst(L) was
standardised for lung volume, by plotting volume as a percentage of TLC, the static elastic
recoil pressure13 at TLC did not change at altitude, while at 60% of TLC it was more than 2
standard deviations to the left of the range of normal values established by Turner et al (430).
This suggests a loss of elastic recoil at 60% of TLC. There was a 26% fall in Cdyn(L) at 5366
m but this did not reach statistical significance. Reanalysis of the data however reveals it to be
underpowered14. Interpretation of these results is further complicated by the difference in time
12: Using one way repeat measure ANOVA, p = 0.078 and power for α = 0.05 is 0.38.
13: elastic recoil pressure ≡ lung recoil pressure ≡ transmural pressure
14: P = 0.055 and power for α = 0.05 is 0.438
24
that the subjects spent at altitude. Three subjects were measured after 9 days and 4 subjects
after 30 days at 5366m which represent very different time periods for acclimatisation.
Gautier et al (156) in their study of 9 subjects at 3457 m measured expiratory P-V curves at
sea level and on days 1, 3 and 5 at altitude. There was no change in Cst(L) but the P-V curve
was shifted progressively to the left during the stay at altitude. This shift of the curve to the
left, consistent with a loss of lung elastic recoil, was statistically significant on days 4-6 at
altitude (p ranging from 0.05 to 0.01). There was no change in Cdyn(L). In view of the small
number of subjects at the relatively low altitude of 3457 m it is likely that the study was
underpowered to detect changes in either Cst(L) or Cdyn(L).
Saunders et al (379) in the study described above (in Section 2, Normobaric Hypoxia)
measured both Cst(L)
and
Cdyn(L) in acute isocapnic, normobaric hypoxia. Quasi-static lung
compliance increased by 26% during the 20 minutes of isocapnic hypoxia (p<0.005) and the
P-V curves were shifted upwards. These changes reversed within 3 minutes of reoxygenation. Interestingly if the published data is corrected for the increase in lung volume
that occurred under hypoxic conditions (see above) and specific compliance is calculated (i.e.
Cst(L) / FRC) there is still a statistically significant increase in specific quasi-static compliance
between control and hypoxia of around 14% (p=0.008). There was no change in Cdyn(L)
between control and normobaric hypoxia but the study was underpowered to detect a
difference15.
Dehnert et al (120) measured Cst(L) in 30 subjects ascending to the Regina Margherita Hut at
4559 m. Analysis was made of all subjects and also those subjects who developed AMS
(14/30) and HAPE (4/30). There was no statistically significant change in Cst(L) in any group.
15: P = 0.177 and power for α = 0.05 is 0.159
25
SUMMARY OF CHANGES IN STATIC LUNG COMPLIANCE
In the 8 studies which investigated Cst(L) under conditions of hypobaric hypoxia (120, 156,
218, 243, 260, 274, 352) only the study of Kronenberg et al (243) demonstrated a statistically
significant change in Cst(L): a fall of 17% after 72 hours at 3800 m compared with 24 hours at
3800 m, but not when compared with the control measurements. In all of the remaining
studies, apart from that of Raymond and Severinghaus (352) which was definitely
underpowered, it is not possible to exclude type 2 error as the cause of the lack of a
demonstrable change in Cst(L).
In the study of Jaeger et al (218), although Cst(L) did not change, there was a clockwise
rotation of the P-V curve about FRC with the portion of the curve above FRC being shifted to
the right. In 3 studies (156, 260, 274) the P-V curve was shifted to the left consistent with a
loss of elastic recoil. In the 2 remaining studies (243, 352) it is not known if the position of
the P-V curve changed. It should also be noted that in these 2 studies the P-V curves were
recorded during an inspiratory manoeuvre as opposed to the more normal expiratory
manoeuvre from TLC. The discrepancy between the study of Jaeger et al (218) where the P-V
curve shifted to the right above FRC and the 3 studies in which the P-V curves shifted left
(156, 260, 274) may be explained by the exercise, which may precipitate pulmonary oedema
and which formed part of the Jaeger et al protocol. A difference in volume history could also
account for differences between studies.
CONCLUSIONS ON CHANGES IN STATIC LUNG COMPLIANCE
There is no evidence of a change in Cst(L) on ascent to altitude although a change cannot be
excluded because it is not possible to exclude type 2 error in the published studies. The P-V
curve appears to shift to the left so that Pst(L) is reduced for a given lung volume, consistent
with a loss of lung elastic recoil. This would result in an increase in TLC, RV and FRC as
occurs in emphysema (454). Vital capacity would be reduced if RV increased more than TLC.
There is however insufficient data on the change in TLC, RV and FRC on ascent to altitude to
permit interpretation. The reason for a loss of elastic recoil at altitude is unclear. The β2
agonist fenoterol has been shown by De Troyer et al to shift the P-V curve to the left (116)
who speculated that in addition to its bronchodilator effect fenoterol may relax some
contractile elements in the lung parenchyma and reduce Pst(L). Increased sympathetic activity
is known to occur at altitude (392) which may explain the observed change in Pst(L).
26
b) Pulmonary blood volume changes
The effects of changes in pulmonary blood volumes on respiratory mechanics are complex but
potentially could produce changes in respiratory mechanics at altitude and thus in lung
volumes. The majority of experiments are descriptive with little or no statistical analysis and
the findings have not been subjected to modern investigational techniques or analysis. Von
Basch as early as 1887 had reported that pulmonary vascular congestion in alive dogs altered
the mechanical properties of their lungs (quoted in (66, 189)). Borst et al (66) in 1957
reported that in anaesthetised dogs increasing pulmonary blood flow from 0 to 450 ml / kg
body weight /min, while left atrial pressure (LAP) was maintained constant, had no effect on
Cst(L). Increasing LAP produced a small, reversible, fall in Cst(L) prior to the development of
pulmonary oedema. The study of Giannelli et al (157) in 1967, using an isolated perfused
and ventilated dog heart-lung preparation, suggests that a perfused preparation, when
compared with a non-perfused preparation, had both lower peak and equilibrium inflation
pressures (a lower equilibrium pressure equating to an increased Cst(L)). The effect was not
dependent upon venous distenstion (i.e. increased LAP) or variations in pulmonary blood
flow between 100 and 220 ml / kg. Hauge et al (189) in 1975 studied the effects of
pulmonary blood volume on Cdyn(L) in an isolated perfused and ventilated rabbit heart-lung
preparation. Pulmonary blood volume, which could be increased by either an increase in LAP
or in pulmonary blood flow, was assessed by means of weighing the heart-lung preparation. It
was assumed that the rapid change in weight occurring within the first 30 seconds of any
increase in LAP was due to increased pulmonary blood volume. Later, slower, changes were
assumed to be due to the development of pulmonary oedema. Early weight increases within
the first 30 seconds were associated with a reduction in Cdyn(L). Frankly oedematous lungs
have long been recognised to have reduced compliance (96, 204, 313).
The changes in pulmonary blood volume occurring at altitude are also poorly defined. Doyle
et al (130) found no increase in pulmonary blood volume in human subjects during acute
hypoxia using a modified Stewart-Hamilton dye dilution technique. Coates et al (92), using
radio labelled iron, found no evidence of an increase in pulmonary blood volume up to 19
hours after decompression to an equivalent altitude of 4268 m.
The diffusing capacity for CO (DLCO) has been used in a number of studies as a measure of
pulmonary capillary blood volume and the results interpreted as showing a decrease in
pulmonary capillary blood volume with hypobaric hypoxia (176, 449, 457). There are,
however, difficulties in interpreting this data as PO2 per se can affect DLCO. Capen and
Wagner (81) tried to circumvent this problem by measuring DLCO during hypoxia with and
27
without an infusion of the pulmonary vasodilator prostaglandin E1 so that they could reverse
hypoxic pulmonary vasoconstriction while maintaining hypoxia. They concluded that
pulmonary capillary volume was increased in hypoxia. During Operation Everest II, Welsh et
al (451) calculated the theoretical change in blood volume occurring upon descent from
4267m to sea level based upon the work of Roy et al (365) and found that it correlated well
with the immediate increase in FVC observed upon return to sea level.
In an intact dog model Wagner et al observed recruitment of apical subpleural capillaries
during alveolar hypoxia (444). It is unknown however whether capillary flow in subpleural
regions corresponds to what occurs in other regions of the lung (183). In rat (3), cat (113) and
dog (114, 308) isolated lung models, hypoxia has been shown to reduce pulmonary blood
volume. In a radiological study of isolated perfused dog lungs Clough et al (91) also
demonstrated that hypoxia reduces pulmonary microvascular volume. Cogo et al (93) claim
that the changes in spirometry that they demonstrated, discussed above, are partly or wholly
explained by an increase in pulmonary blood volume although pulmonary blood volume was
not measured as part of this study.
CONCLUSIONS ON PULMONARY BLOOD VOLUME CHANGES AT ALTITUDE
Although there are a few contradictory studies, the weight of evidence is that hypoxia either
reduces or has no effect on pulmonary blood volume and can be excluded as a cause for the
reduction in VC seen at altitude. It will not be discussed further.
c) Respiratory muscle function
Vital capacity is influenced not only by the static elastic properties of the lung but also by the
strength of the respiratory muscles. It can be limited by weakness in both the inspiratory and
expiratory muscles with inspiratory weakness preventing full inflation and expiratory
weakness limiting expiration. The effect of respiratory muscle weakness on the changes in the
other lung volumes is less clear. Residual volume is usually normal, or increased, especially
with expiratory muscle weakness which causes a reduction in the muscular strength which
counters the elastic recoil of the chest wall (2, 99). Total lung capacity is usually moderately
reduced. In chronic respiratory muscle weakness a reduction in the compliance of both the
28
chest wall and the lung also occur. In severe muscle weakness VC depends more on lung
compliance than respiratory muscle strength. The mechanism of reduction in lung compliance
in chronic respiratory muscle weakness is not fully understood (2).
The maximum static inspiratory pressure (MIP) and maximum static expiratory pressure
(MEP) generated at the mouth are simple, easily tolerated, estimates of the pressure generated
by the inspiratory and expiratory muscles plus the elastic recoil of the respiratory system. The
curvilinear relation between VC and MIP means that while in normal individuals a large
reduction in MIP must occur to produce even a small reduction in VC, with more severe
inspiratory muscle weakness only a small change in MIP can result in a significant reduction
in VC (Figure 4). Because elastic recoil varies greatly between RV and TLC, estimates of
MIP and MEP can show a variability of up to 25%. The sniff nasal inspiratory pressure
(SNIP) is a more reproducible measure of diaphragmatic or global inspiratory function and
shows less variability (2).
Figure 4: The curvilinear relationship between the maximum inspiratory pressure (MIP: a measure of inspiratory muscle strength) and vital
capacity (VC) in 25 patients with chronic muscle weakness of varying severity. The greater fall in VC than predicted is due to the, not fully
understood, reduction in lung as well as chest wall compliance. The curvilinear relation between VC and MIP means that while in normal
individuals a large reduction in MIP must occur to produce even a small reduction in VC, with more severe inspiratory muscle weakness
only a small change in MIP can result in a significant reduction in VC.
Source: The ATS/ERS Statement on respiratory muscle testing. Am J Respir Crit Care Med 166: 518-624, 2002. (Ref 2)
Information on respiratory muscle function at altitude is limited. Rahn and Hammond (350)
measured MEP in 5 subjects acutely exposed for 15 minutes to inspired O2 concentrations of
14.4, 12.3 and 10.4%, approximately equivalent to altitudes of 3050, 4300 and 5500 m
respectively. At these altitude equivalents they demonstrated a reduction in MEP of 8.2, 8.7
29
and 12.3% respectively. Restoration of normoxia returned MEP towards normal although it
was still reduced by 6.5% compared to the pre-hypoxia control. There is no statistical analysis
of this data in the original paper, but re-analysis of the published results reveals the above
changes not to be statistically significant and equally to be underpowered16.
As part of the study of Forte et al (149) discussed above that was designed to investigate
ventilatory endurance, MIP and MEP were measured at sea level and at 4300 m actual and
simulated altitude. Measurements were made before and after a maximal sustainable
ventilation (MSV) lasting up to 15 minutes. During the 3 week stay at 4300 m on Pikes Peak
MIP did not differ from sea level values. The post-MSV MEP was reduced by 6.3 %
compared with sea level although the pre-MSV value was unchanged. During the hypobaric
chamber part of the study the post-MSV MIP was reduced compared with sea level but the
pre-MSV MIP and the MEPs did not differ from control. It should be noted that in this study
the values for MIP and MEP were obtained using instantaneous measures of Pmax whereas in
the studies of Fasano et al (145) and Deboeck (118), considered below, pressures were
required to be maintained for at least 1.5 seconds. Instantaneous Pmax measurements may
produce higher values than sustained measurements and this may explain the absence of
consistent changes in this study (2).
Deboeck et al (118) demonstrated a fall in MIP, MEP and the SNIP during 12 hours exposure
to acute hypobaric hypoxia equivalent to an altitude of 4267 m. The reductions in all 3 values
were statistically significant at 1 hour (percentage reduction compared with control: MIP: 7%;
MEP: 10%; SNIP: 14%) and were reduced further at 6 hours (reduction c.f. control: MIP:
11%; MEP: 12%; SNIP: 15%) and 12 hours (percentage reduction compared with control:
MIP: 12%; MEP: 15%; SNIP: 22%). There was a strong curvilinear correlation between FVC
and MIP (R2 = 0.86), MEP (R2 = 0.69) and SNIP (R2 = 0.59) in hypoxia with those subjects
having the lowest FVC having the lowest MIP, MEP and SNIP. Unfortunately no attempt was
made to correct for body size by using the percentage change from control or the percentage
of predicted FVC so it is not possible to exclude mathematical coupling to account for these
correlations. In addition only absolute values at altitude are used in the correlation rather than
the change from control.
Fasano et al (145) studied 8 subjects ascending over 4 days to an altitude of 4559 m where
they stayed for 3 days. Mean inspiratory pressure; peak expiratory and peak inspiratory flows
16: Using repeat measure ANOVA, the differences in the mean values among the treatment groups do not quite reach
statistical significance (P = 0.057). However, not surprisingly in view of the small sample size, the power for α = 0.05 is
0.438. It should also be noted that the published mean changes differ from those quoted above which were calculated from
the raw published data.
30
(PEF and PIF); FVC and forced expiratory flow after exhalation of 75% of FVC (FEF75) were
measured. The MIP fell significantly at 3200, 3600 and at 4559 m compared with control. The
MIP increased but remained significantly less than control throughout the stay at 4559 m. The
FVC fell at 3600 m and at 4559 m compared with control. Both PEF and PIF increased on
ascent to altitude as would be predicted by the fall in air density with decreasing barometric
pressure (281) but PIF increased less than PEF. There was a correlation between MIP and
SpO2. Accurate interpretation of this study is difficult because the results are only reported
graphically with an absence of raw values or percentage changes. In addition multiple
measurements were made in a small number of subjects at relatively low altitudes where
changes will be less apparent. Despite these shortcomings the study does demonstrate a fall in
MIP at altitude which was maximal on the first day at 4559 m (-18.1 ± 4 % of control). This
would be more than sufficient to bring about what appears to be a fall of around 5% in the
FVC. However it should be noted that while MIP increased towards control values on
subsequent days at 4559 m, FVC did not change.
In the study of Sharma and Brown (395) MIP and MAP were measured over at least 1
second. As discussed above no sea level recordings were made and control measurements
were made at an altitude of 3450 m. The MIP and MAP fell on Day 3 at 3450 m by over 30%
without any change in altitude. Compared with the 3450 m control, MIP and MAP remained
reduced on arrival at Everest Base Camp (5350 m) after a 9 day walk in but then returned
towards the control values over the next 27 days despite ascents to 6300 and 7200 m. As
discussed above, because of the small number of subjects involved and lack of a sea level
control it is impossible to draw any meaningful conclusions from this paper.
Dehnert et al (120) in their study at the Regina Margherita Hut report a significant fall in
inspiratory muscle strength from low altitude values of 8.5% on day 1 and 6% on day 2 at
4559 m. No information is given on the methodology in the paper. It is likely that they used
maximal sniff oesophageal pressures. Of note, the fall in inspiratory muscle strength is
considerably less than that demonstrated by Fasano et al (145) at the same altitude and under
almost identical conditions.
Diaphragmatic fatigue under normoxic conditions is known to occur at around 85% of VO2
max (223). Under hypoxic conditions (FiO2 0.15) exercise time at 85% of VO2 max was
significantly reduced compared with normoxia, although the transdiaphragmatic (118)
pressure produced by supramaximal bilateral phrenic nerve stimulation after exercise was
similarly reduced in both normoxic and hypoxic conditions compared with the pre-exercise
pressures. However recovery to pre-exercise levels was more rapid under normoxic
31
conditions (13). Kayser et al (230) reported that diaphragmatic fatigue contributes to exercise
limitation at high altitude although such changes were only seen during heavy prolonged
whole body exercise, rather than during a vital capacity manoeuvre. Gudjonsdottir et al
(173) also demonstrated diaphragmatic fatigue during maximal exercise both at sea level and
at 3325 m. The reduction in maximal transdiaphragm pressure was greater at altitude than at
sea level and took longer to recover to control values.
SUMMARY OF RESPIRATORY MUSCLE CHANGES
Information on changes in respiratory muscle function is limited. If the study of Forte et al
(149) is excluded because of the method of measuring MIP and MEP and that of Sharma and
Brown (395) because of the lack of a sea level control, to date only Deboeck et al (118);
Fasano et al (145) and Dehnert et al (120) have demonstrated a statistically significant fall in
inspiratory muscle strength at altitude. In the study of Deboeck et al (118), there was a
significant fall in MIP, MEP and importantly in the reproducible SNIP. Interestingly these
changes occurred after only 1 hour at a simulated altitude of 4267 m and correlated well with
the fall in VC. In the study of Fasano et al (145) and Dehnert et al (120) a significant
reduction in MIP was found on the first day at 4559 m which remained reduced throughout
the subsequent 3 or 2 day stay at 4559 m.
While a reduction in inspiratory muscle strength would be predicted to bring about a
reduction in FVC it should also bring about a lesser reduction in TLC; an increase in RV and
a reduction in FRC. As discussed above data on lung volume changes other than VC are
limited and there is too little data to draw any meaningful conclusion about what happens to
TLC, RV and FRC. Traditionally it has always been argued that because of the sigmoid shape
of the P-V curve, a considerable reduction in muscle strength would be required to bring
about even a small reduction in lung volume. However the curvilinear shape of the curve
shown in Figure 4 would suggest that the 7 to 12% reduction in MIP in the study of
Deboecket al (118) and the 18% decrease in MIP in the study of Fasano et al (145) are
sufficient to explain at least some of the 3-5% reductions in FVC seen in these two studies.
The study of Dehnert et al (120) demonstrated a fall in inspiratory muscle strength but was
underpowered to detect a fall in FVC of the magnitude likely to be seen at 4559 m.
32
d) Intra-abdominal distension
Rahn and Hammond (350) suggested that hypobaria per se could bring about a fall in FVC
by increased abdominal distension producing cephalad displacement of the diaphragm.
However even with abdominal distension of between 700 to 800 ml there was no significant
change in FVC. Gilroy et al (159) and Kimball et al (235) demonstrated that abdominal
distension by a gastric balloon or by blood produced a cranial displacement of the diaphragm
but that any effects upon FRC were largely negated by an outward movement of the rib cage.
e) Oxygen absorption
Klocke and Rahn (236) calculated that the theoretical fall in lung volume due to oxygen
absorption from the lungs during 4 to 7 seconds of breath holding would account for 1.6% in
a 7.6% fall of FVC at 10000m. This clearly cannot explain all of the fall in FVC at lower
altitudes, although it might contribute if the cause of the fall in FVC is multi-factorial.
f) Pulmonary oedema
Even small clinically undetectable amounts of pulmonary oedema can affect lung function
(185, 188, 212). The fall in FVC which occurs after strenuous exercise at sea level, such as
long distance running, is thought to occur as a result of early airways closure secondary to
pulmonary oedema (277, 296, 297, 469) and is accompanied by an increase in residual
volume (RV) (277). Similar changes have been demonstrated radiologically at altitude (9,
136) where hypoxia appears to exacerbate the effect of strenuous exercise (136).
There is considerable indirect evidence that sub-clinical pulmonary oedema occurs in most
subjects exposed to high altitude. However interpretation of the available data is made
difficult because the distinction is rarely made between sub-clinical pulmonary oedema and
frank high altitude pulmonary oedema (HAPE). It is likely that a spectrum of increased extravascular lung water exists at altitude. Florid HAPE will obviously bring about major changes
in lung function. The majority of studies discussed here present data that suggest the presence
of sub-clinical pulmonary oedema in otherwise healthy subjects:
33
i) Clinical Evidence
Basal crackles were heard by Houston in a number of climbers reaching the summit of Mt.
Rainier (4392m). The exact incidence is unclear as this observation does not appear to have
been formally published. Gray et al (167) quote an incidence of 22% citing as a source a
personal communication from Houston, while Welsh et al (451) quote an incidence of 15% in
141 asymptomatic climbers. They reference the introductory paper to Operation Everest II
(201) as their source but this figure, or indeed any mention of climbers on Mount Rainier,
does not appear anywhere in the paper. In contrast, Jaeger et al (218) found no clinical
evidence of pulmonary oedema in up to 25 soldiers on manoeuvres at altitudes of up to
4300m on Pike’s Peak, Colorado. In some ways this is less surprising than the high incidence
detected by Houston, as clinical examination is known to be a poorly sensitive method of
detecting pulmonary oedema (14, 275). Singh et al (398) reported their observations on 1925
military patients who developed altitude related illness in the Indian Himalaya at altitudes of
between 3350 and 5500 m. It is difficult to extract any meaningful or accurate figures from
this paper but in a sample of patients with undefined, mild, AMS, 28.8% demonstrated
clinical signs of “lung congestion.”
!
A-aDO
2
ii) Alterations in Gas Exchange:
The alveolar-arterial oxygen difference, or A-aDO2, is the difference between the PAO2,
which may be measured directly from end tidal expired gas or calculated using one of several
forms of the alveolar gas equation17, and the measured PaO2, i.e.:
! O = PAO ! PaO
A ! aD
2
2
2
,
where, for example,
#
PACO 2 #
1! R & &
PAO 2 = % PIO 2 !
+ % PACO 2 " FiO 2 "
R
R (' ('
$
$
17 : The exact form of the alveolar gas equation used in the papers described in this section varies widely from author to
author. For example, Grissom et al (172) used a simplified form of the equation: PAO 2
PIO 2
PACO 2
and estimated R to
R
equal 0.85, while Bartsch et al (41) used a more complete form of the equation to cross check the measured A-aDO2:
PETO 2
=
PIO 2 ! PETCO 2
"
#$ FIO 2 +
=
(1 ! F )%&
IO
2
R
where R was measured directly and PETCO2 assumed to be equal to arterialised capillary PCO2.
!
34
PACO2 is assumed to be equal to the end tidal CO2 (PETCO2) providing all alveoli empty at
an approximately equal rate during expiration. In diseases such as chronic obstructive
pulmonary disease and pulmonary fibrosis this is not the case and the technique is not valid.
The normal sea level value is between 1 and 2 kPa (7.5 to 15 mmHg). The three major
!
A ! aDO
2
components contributing to the
gradient are left to right shunts; ventilation-
perfusion inequality and diffusion limitation (441, 456).
Reeves et al (353) measured
!
A ! aDO
2
in 7 volunteers during an acute simulated ascent to
4572 m in a hypobaric chamber. Resting, supine
!
A ! aDO
2
increased from a mean of 6 mm
Hg at low altitude to 12 mm Hg after between 2 and 3 hours exposure to 4572 m. With
increasing exercise, quantified by O2 consumption (
at a
!O
V
2
! O A ! aDO
!
V
2 ),
2 increased to 20 mm Hg
of 1.6 litres / min. No statistical analysis was performed.
Kronenberg et al (243) measured
!
A ! aDO
2
in 4 subjects during 72 hours acclimatisation to
3800 m at the Barcroft Laboratory on White Mountain. Although the authors quote a mean
increase of 3 mmHg in the
!
A ! aDO
2
during the stay at Barcroft there was no statistical
analysis performed and individual values are not given. No sea level control measurements
were made.
Sutton et al (412) measured
!
A ! aDO
2
in in 7 subjects after ascent to 5360 m with 3 days
spent at 2990 m for acclimatisation. The subjects with the most severe AMS, assessed using a
5 point scoring system similar to the Lake Louise Scoring System (which was not defined
until 1993) had the greatest increase in
!
A ! aDO
2
on ascent to altitude and the lowest PaO2.
The authors state that no clinical pulmonary abnormality was detectable.
Grissom et al (172) measured
!
A ! aDO
2
in 12 climbers with AMS at 4200 m on Mount
Denali in Alaska. Half were treated with 250 mg of acetazolamide and half with placebo.
Both the acetazolamide and control groups had similar
!
A ! aDO
2
at the start of the treatment
period and although the acetazolamide group showed a small, non-significant, reduction in
!
A ! aDO
2
over the following 24 hours, the placebo group showed a significant increase in
!
A ! aDO
2
during the same period (+2.9 mm Hg, p =0.045).
35
Wagner et al (442) studied pulmonary gas exchange in 8 subjects exercising at sea level and
at simulated altitudes of 3050 and 4572 m. Resting
!
A ! aDO
2
fell from a mean value of 8.3
mm Hg at sea level to 3.1 mm Hg at 3050 m and 3.6 mm Hg at 4572 m. During exercise the
!
A ! aDO
2
increased to between 22 and 26 mm Hg at both sea level and at simulated altitude.
! !
Measurement of ventilation-perfusion ( V Q ) inequality using the multiple inert gas technique
enabled a comparison to be made between the measured
!
A ! aDO
2
and the
!
A ! aDO
2
!O
V
2 greater than 35 ml
predicted from the measured V! Q! ratio. During sea level exercise at a
/ min / kg, the measured
!
A ! aDO
2
! !
was significantly greater than that predicted due to V Q
mismatch, the difference being due to diffusion limitation of O2 transfer. At both 3050 and
4572 m even light exercise at a
!O
V
2
above 15 ml / min / kg resulted in diffusion limitation of
O2 transfer. Similar findings were obtained during Operation Everest II (414, 443) although
!
A ! aDO
2
was higher in acute exposure (442) compared to the prolonged exposure seen in
Operation Everest II.
Bartsch et al (41) in a study examining the relationship between the sea level hypoxic and
hypercapnic ventilatory responses (HVR and HCVR respectively) and susceptibility to AMS,
also studied gas exchange in 23 subjects at Regina Margherita Hut 4559 m. Eleven subjects
had significant AMS, as estimated by the environmental symptom questionnaire (374) and the
Lake Louise score (360), while 12 were symptom free. The alveolar-arterial difference was
measured directly from end tidal gas and arterialised capillary blood and also calculated from
the alveolar gas equation. There was good correlation between the measured and calculated
!
A ! aDO
2
at high altitude (r = 0.74 to 0.88) although the measured
!
A ! aDO
2
consistently lower than the calculated value. There was a significant decrease in
was
!
A ! aDO
2
on
ascent to 4559 m compared to low altitude while the A-aDO2 was significantly higher in the
subjects with AMS compared with the AMS free controls. There was a significant correlation
between
!
A ! aDO
2
and the Lake Louise AMS score (r = 0.58 on day 1 and 0.81 on day 2 at
4559m).
In contrast further work from the same group, also at the Regina Margherita Hut at 4559 m
(120), did not demonstrate a significant difference in
whom 14 developed documented AMS.
!
A ! aDO
2
, calculated in 30 subjects, of
36
SUMMARY OF CHANGES IN A-ADO2 AT ALTITUDE
!
The A ! aDO
2 decreases with ascent to altitude and there appears to be a further decrease in
!
!
A ! aDO
2 with long term exposure. The A ! aDO 2 increases with exercise both at sea level
and at altitude and appears to increase in the presence of clinically significant AMS. The
mechanisms behind these changes are complex: alterations in intrapulmonary gas distribution
could result from reduced air density or altered gas flow rates secondary to increased
ventilation. Hypoxic pulmonary vasoconstriction, especially if, as suggested, it occurs
unevenly could result in redistribution of pulmonary blood flow as could the increase in
cardiac output for a given workload seen on acute exposure to altitude. In addition, any degree
of overt or sub-clinical high altitude pulmonary oedema (HAPE) would result in
inequality and diffusion limitation (441).
iii) Changes in the nitrogen washout curve and closing volume
Gray et al (168) measured phase IV18 of the single breath nitrogen washout curve in a
hypobaric chamber decompressed to an altitude equivalent of 4900m and at laboratory on
Mount Logan at 5366 m. In a complicated protocol the 12 subjects who ascended Mount
Logan were divided into 3 groups:
-
4 subjects were given prophylactic acetazolamide prior to being airlifted directly to the
Mount Logan laboratory. Measurements were made after 7 days at the summit.
Satisfactory readings could not be obtained in one of these subjects.
-
5 subjects were airlifted to an intermediate altitude of 3050 m where they stayed for 10
days prior to being airlifted to the laboratory on Mount Logan. Measurements were again
made after 7 days at the summit.
-
3 subjects climbed to the laboratory where they remained for 6 weeks prior to
measurements being made.
The 5 subjects in the hypobaric chamber had measurements made approximately hourly
throughout their 4 hour stay in the chamber.
18: Because RV was not measured results were expressed as the Phase IV volume from TLC. Phase IV is generally taken to
equate with CV.
37
No change in Phase IV volume was found during the hypobaric chamber part of the study but
with only 5 subjects reanalysis of the raw data reveals it to be markedly underpowered19. All 3
subject groups in the Mount Logan part of the study were analysed together despite their very
different ascent profiles. There was no significant change in Phase IV volume at 5366 m
compared with control. Reanalysis of the raw data shows it also to be underpowered20,
although there appear to be trends in the three groups which because of the sample size do not
reach statistical significance: 2 out of the 3 subjects who were flown directly to the Mount
Logan laboratory, albeit on prophylactic acetazolamide, showed a large fall in Phase IV
volume on day 7 of over 50% compared with sea level. Most of the subjects who flew up in 2
stages with an intermediate stay at 3048 m show either no change of a slight fall in Phase IV
volume (mean fall of 17%). Two out of the 3 subjects who walked to the laboratory and were
measured after 6 weeks of acclimatisation show around a 30% increase in Phase IV volume.
Gray et al (167) in a further study, measured single breath nitrogen washout curves in 19
subjects who were initially airlifted to 3050 m where they remained for between 5 and 10
days before being airlifted to the laboratory on Mount Logan (5366 m). Measurements were
then made throughout a 7 day stay at the laboratory. As in their previous study there was no
change in the phase IV volume but as the raw data are not included in the paper it is not
possible to assess the power of the study. There was a significant increase in the phase III
slope (the alveolar plateau) on day 2 at 3050m which increased further on ascent to 5366 m
(87% above control on days 2 and 3). There was a significant decrease in the slope of the
alveolar plateau between days 2 and 7 at 5366 m but at all times it remained greater than
control values. These changes are suggestive of an increase in the heterogeneity of gas mixing
or emptying of lung units (98) which improved with time at altitude. Such changes could be
produced by an increase in extravascular lung water heterogeneously altering resistance and
compliance of lung units.
In a follow up to the above 2 studies, Coates et al (92) studied 4 subjects who were
decompressed to an altitude equivalent of 4268 m for 24 hours. Measurements were made of
lung volume by helium dilution; single breath nitrogen washout; lung density using a
Compton scatter technique and pulmonary blood volume using radio labelled iron. Lung
volumes and pulmonary blood volume have already been discussed above. The phase III
slope of the alveolar plateau was significantly increased after 20 hours by 36% compared with
control. Closing volume was unchanged but as RV was increased closing capacity was
19: Power for α = 0.05 is 0.05
20 : Power for α = 0.05 is 0.39
38
significantly increased at 5 hours compared with control. This means that gas trapping was
occurring at much higher lung volumes than under control conditions. Lung density showed a
trend to increase under hypobaria but this did not reach statistical significance although as
with CV was markedly underpowered21. In summary this study demonstrated increased gas
trapping at higher lung volumes and increased heterogeneity of gas mixing.
Sutton et al (413) as part of the same study discussed above (A-aDO2 (412)) measured both
the phase IV volume and the slope of phase III of the nitrogen washout curve in 7 subjects
after ascent to 5360 m with 3 days spent at 2990 m for acclimatisation. There was no
significant change in the phase IV volume but the phase III slope was significantly increased
after 48 hours at 2990 m and in all subjects, presumably on all days, at 5360 m suggestive of
more heterogeneous emptying of lung units. The authors also state that there was a significant
relationship between the change in the phase III slope, 48 hours after arrival at 5360 m, and
the severity of AMS, with the greatest change in the slope occurring in the sickest subjects. It
is not known if this study was adequately powered.
Jaeger et al (218) measured closing capacity using a single breath nitrogen washout
technique in 25 soldiers during 2 consecutive 72 hour field exercises, first at low altitude and
then at between 3000 and 4300 m. Closing capacity increased significantly with time at both
low and high altitude, although the increase was greatest in the high altitude group. The slope
of phase III of the nitrogen washout curve increased with time equally at both low and high
altitude.
In a large study of 262 alpinists ascending to the summit of the Monte Rosa (4559 m)
Cremona et al (104) found radiological or physical signs of increased extravascular lung
water in 40 (15%) of subjects all of whom were symptom free. AMS scores were significantly
higher in those asymptomatic patients with signs of increased extravascular lung water than
those without signs. Closing volume, unusually, was calculated from the change in the
respiratory quotient (R) obtained from exhaled O2 and CO2, increased significantly in ascent
from 1200 m to 4559 m in all subjects whether they developed oedema or not. However the
increase in the group who developed oedema was significantly larger than in the group who
did not develop oedema. In addition 146 subjects without clinical or radiological evidence of
extravascular lung water showed an increase in CV on ascent to 4559 m. The authors’ claim
that the majority of people (77%) ascending to altitude develop sub-clinical oedema rests
21: Change in CV: control vs 5hrs hypoxia, p= 0.51 but power for α = 0.05 is 0.052. Repeat measure ANOVA for lung
density changes, p= 0.133 but power for α = 0.05 is 0.257.
39
solely on the changes in CV. As noted above FVC was unchanged on ascent from 1200 to
4559 m.
Senn et al (393) in the previously described study at the Regina Margherita Hut at 4559 m
studied the single breath nitrogen washout curve in 26 subjects. They found a 26% increase in
CV, and a 21% increase in the slope of phase III of the nitrogen washout curve, on arrival at
4559 m compared with low altitude. The next day the slope in phase III remained elevated,
but changes in CV were variable.
Despite the significant increase in CV demonstrated by Cremona et al (104) and Senn et al
(393) at the Regina Margherita Hut at 4559 m, Dehnert et al (120) could demonstrate no
change in CV on ascent to altitude using the same single breath nitrogen washout test as
utilised by Senn et al. The cause of the discrepancy between these studies is not known.
40
SUMMARY OF NITROGEN WASHOUT AND CLOSING VOLUME STUDIES
At least 3, and possibly 4, out of the above 8 studies were underpowered. In 3 out of the
remaining 4 studies (104, 218, 393) a significant increase in closing volume or closing
capacity was found at altitude. In each of these studies it is not possible to remove the
confounding effect of intense exercise which can cause a reduction in CV probably due to an
increase in extravascular lung water (277, 296, 297, 469) although in the study of Jaeger et al
(218) a greater increase in closing capacity was found with exercise at altitude than at sea
level. The studies of Cremona et al (104) and Senn et al (393) differ from the other field
studies in that CV was measured within 24 hours of arrival at the summit of the Monte Rosa.
In the other 3 studies with a field component (167, 168, 413), subjects either were allowed to
acclimatise at an intermediate altitude for up to a week prior to ascent or remained at altitude
for between 7 to 42 days prior to the first measurements being made. If, as is speculated, the
changes in CV are thought to be due to sub-clinical pulmonary oedema it is possible that it
may have resolved with acclimatisation before measurements were made. The cause of the
discrepancy between the studies of Cremona et al (104) and Senn et al (393) that
demonstrated an increase in CV at 4559 m and that of Dehnert et al (120) that did not
demonstrate an increase, under almost identical conditions, is not known.
While sub-clinical pulmonary oedema could be responsible for the changes in closing volume
(185, 188) it is not the only possible explanation at altitude. Both hypoxia and hypocapnia can
bring about bronchoconstriction which may increase CV (49). The inhalation of cold, dry air
can also precipitate bronchospasm (8, 128) and this may be exacerbated by exercise (318).
However, as discussed below (see: Altitude-Related Cough) there is no evidence that
bronchospasm occurs in non-asthmatic subjects at altitude.
In 5 out of the 7 studies (92, 167, 218, 393, 413), even when no change was detected in
closing volume, the phase III slope (alveolar plateau) of the nitrogen washout curve was
increased at altitude suggestive of an increase in the heterogeneity of gas mixing or emptying
of lung units. This could be due to an increase in extravascular lung water. Sutton et al (413)
state that there was a positive correlation between the change in the phase III slope and the
severity of AMS.
41
iv) Imaging studies
A number of studies have investigated the radiological changes associated with high altitude
pulmonary oedema (HAPE) (240, 241, 438, 439). This condition is discussed below. A
characteristic of three of these studies (240, 241, 438) is the highly selected group of patients
studied who had severe HAPE sufficient to require evacuation and hospitalisation. This is
very different to the generally well subjects in the studies discussed up to this point. In the
fourth study, Vock et al (439) performed serial chest x-rays on 25 subjects at low altitude
(550 m) and then at the Cappana Regina Margherita hut on Mount Rosa, 6, 18 and 42 hours
after arrival at 4559 m. Nine subjects had a previously radiologically proven diagnosis of
HAPE. At 18 and 42 hours, 6 out of these 9 subjects and 2 of the 16 subjects with no previous
history had developed radiographic signs of HAPE. The details given in the paper regarding
the findings on physical examination are confusing. However it would appear that on several
occasions clinical examination revealed crackles or wheezes in the absence of radiographic
changes (although a number of these subjects later went on to develop radiographic changes)
while in a number of subjects no clinical signs were detectable despite radiographic changes.
During Operation Everest II, Welsh et al (451) obtained chest x-rays 2 hours after descent
from a simulated altitude of 8848m which showed radiological evidence of interstitial oedema
in 6 asymptomatic subjects. These findings however appear to be contradicted by the
introductory paper to Operation Everest II (201) in which the authors, who include 2 of the
authors from the paper by Welsh et al (451), write, “Scattered rales were occasionally heard
in several individuals, and one subject was suspected of early pulmonary edema at 7010 m
(307 Torr) but spontaneously diuresed and finished the study successfully. Three readers
familiar with high-altitude pulmonary edema considered that one subject showed mild
pulmonary edema by x-ray taken at sea level 2 h after the project was completed. Three other
readers did not agree.” Cremona et al (104), as discussed above, carried out chest
radiographs at 4559 m on climbers arriving at the Regina Margherita Hut. They do not report
how many of the 262 asymptomatic subjects had radiological evidence of pulmonary oedema
but rather group together radiological and physical evidence suggestive of increased
extravascular lung water. Forty (15%) of subjects had either one or the other or both but no
further details are given.
It is difficult to draw any meaningful conclusions from these limited studies other than to
confirm that chest radiography, like clinical examination, is an insensitive and unreliable
technique to detect early and small changes in extra-vascular lung water (EVLW) (201, 299).
42
Both high resolution computerised tomography (389, 390) and magnetic resonance imaging
(106) are highly sensitive techniques to assess EVLW but highly impractical for studies in the
field or hypobaric chambers. The thermal dye double indicator dilution technique remains one
of the more accurate methods in intact humans and can determine small changes in EVLW
but is invasive, expensive and impractical to perform routinely (361). Single transpulmonary
indicator techniques produce comparable results to the double indicator technique (369, 387).
Relatively portable systems are commercially available such as the PICCO® system (Pulsion
Systems, Munich, Germany) but require the insertion of both a central venous catheter in the
internal jugular or subclavian vein and a central arterial line (femoral or axillary).
Transthoracic electrical impedance has been used to estimate EVLW at altitude (199, 218,
364) and although simple to use and non-invasive, the results have been disappointing and
changes in lung volume with ventilation; changes in blood volume and a lack of spatial
resolution all complicate the interpretation of the results (146). Electrical impedance
tomography (EIT), by creating a two-dimensional image of the region being studied and
within which image a zone of interest can be delineated, has the potential to overcome many
of these problems (70). Although initial publications have concentrated on measuring regional
lung volumes during mechanical ventilation (247-249) the Sheffield Applied Potential
Tomograph system has been used to monitor changes in lung water in healthy volunteers in
response to fluid (77) or diuretic (311) therapy and to monitor the response to treatment in
patients with left ventricular failure (312) and in non-cardiogenic respiratory failure (250).
There is good correlation between EVLW measured using EIT and the double indicator
technique in ventilated patients (250) and in an oleic acid animal model of the acute
respiratory distress syndrome (ARDS) (71). Mason et al (282) used EIT to estimate EVLW
in 20 subjects during a 2 week stay at 3800 m. The change in impedance from RV to TLC,
normalised for lung volume by dividing by FVC, fell compared with control on the first day
at 3800 m consistent with an increase in EVLW. There was a good correlation between the
normalised change in EIT and FVC (r2 = 0.63 p<0.001).
Before discussing the possible causes of sub-clinical pulmonary oedema at altitude it is
important to fully understand the mechanisms that are responsible for the formation and
resolution of pulmonary oedema.
43
PULMONARY OEDEMA
1) Functional anatomy of the distal airspaces and the alveolar-capillary barrier
The tracheobronchial tree consists of a series of dividing tubes which decrease in diameter
and length and become more numerous as they move distally into the lung. Generations 0 to
14 are the conducting airways which take no part in gas exchange and within which gas
moves by bulk flow (264). Generations 15 to 23 consist of the terminal bronchioles,
respiratory bronchioles, alveolar ducts and alveoli and within which diffusion is the
predominant means of gas movement. Airways distal to the terminal bronchioles form
functional units of gas exchange known as acini. A human lung contains approximately 30
000 acini, each with a diameter of around 3.5 mm. Each acinus contains approximately 10
000 alveoli with a mean diameter of 0.2 mm (264). The alveolar walls, or septa, are held
under tension by a combination of elastic fibres and the air/fluid interface on the luminal side.
This gives rise to flat walls resulting in individual alveoli having a polyhedral rather than
spherical structure.
From the nasal cavity to around generations 11 to 12 the respiratory tract is lined with
pseudostratified columnar ciliated epithelium and is rich in mucus-secreting goblet cells.
Distal to this point the epithelium begins to flatten, being initially cuboidal, before flattening
completely and merging with the alveolar epithelial cells. Up to generations 11 to 12 the
respiratory epithelium also contains basal cells which are probably stem cells and give the
proximal epithelium its pseudo-stratified appearance. The epithelium also contains mast cells;
non-ciliated bronchiolar Clara cells, and APUD (amine precursor uptake and decarboxylation)
cells (68).
Alveoli contain two types of epithelial cells. The predominant alveolar epithelial cells, type I,
form an exceptionally thin lining layer, approximately 0.1µm thick which forms one side of
the alveolar-capillary membrane (Figure 5). Each epithelial cell covers several capillaries.
Type I cells are devoid of intracellular organelles, except for small vacuoles and are
connected into a continuous sheet by tight junctions located near the apical surface of the cell.
Tight junctions are essential for maintaining the integrity of the epithelial barrier and thus
maintaining transepithelial polarity (382, 383). The junctions themselves may contain ion
selective pores (177).
44
Figure 5: Electron micrograph of a capillary and alveoli showing the alveolar-capillary barrier.
Alv: alveolus; Ep: alveolar epithelium; IS : interstitial space ; BM: basement membrane; EN:
endothelial nucleus; End: capillary endothelium; FB : fibroblast ; RBC : red blood cell.
Source: Lumb, AB. Nunn's applied respiratory physiology,
Butterworth-Heinemann, Edinburgh 2000, 5th ed. p29. (ref 264)
Type II alveolar epithelial cells which make up 67% of the alveolar epithelial cells but
constitute only 3% of the alveolar surface are found at the septal junctions of the alveoli
(102). Morphologically they are distinguished from type I cells by being much larger and
rounder, possessing microvilli, large nuclei and characteristic cytoplasmic lamellar bodies.
Type II cells synthesise and store surfactant; are the stem cells from which type I cells arise
and may participate in pulmonary defence by secreting inflammatory cytokines.
Understanding of the relative contribution of Type I and Type II cells to active vectorial ion
transport, and thus the lung’s ability to clear water from the alveolar lumen, has evolved
considerably in recent years and is discussed below (see: Lung epithelial fluid transport).
The alveolar epithelial cells form one side of the alveolar capillary barrier across which gas
exchange takes place. The other side of the barrier is made up by the capillary endothelial
cell. In continuity with the endothelial cells of the pulmonary circulation pulmonary capillary
endothelial cells have a thickness of only 0.1µm, except where expanded to contain the cell
nucleus. In addition to the nucleus, the cytoplasm contains only small vacuoles, or caveolae,
which acts as an extension of the cell membrane, further increasing its already vast surface
45
area. A number of enzymes are found on the surface of the caveolae and capillaries and are
responsible for the very high metabolic activity of the pulmonary endothelium which
approaches that of the liver (18, 367, 368). The junctions between capillary endothelial cells
are loose and approximately 6 nm wide permitting the passage of large molecules such as
albumin.
Between the capillary endothelium and the alveolar epithelium lies the interstitial space. This
is of minimal thickness such that the total distance from alveolar gas to capillary blood is
around 0.3µm. The alveolar interstitium does not contain lymphatics but is in continuity with
the interstitium of the peribronchiolar and perivascular tissue permitting passage of liquid in
the interstitium to the peribronchiolar lymphatic vessels from where it drains towards the hilar
lymphatics (257, 403). The structure of the pulmonary interstitium and the changes occurring
with pulmonary oedema are discussed below.
2) Pathophysiology of oedema formation
Pulmonary oedema may be defined as an increase in extravascular lung water. Traditionally it
has been thought to occur when transudation or exudation of fluid from the pulmonary
capillaries exceeds the capacity of the lungs’ lymphatic drainage. It can be considered to
occur in 3 stages (Figure 6) (254, 265, 403, 405):
Figure 6: The stages of development of pulmonary oedema. On the left the appearance of the alveoli on light microscopy are represented.
On the right is shown the development of excess fluid in the interstitial space surrounding a pulmonary arteriole (PA) and bronchiole (Bron).
See text for full details.
Adapted from: Lumb, AB. Nunn's applied respiratory physiology, Butterworth-Heinemann, Edinburgh 2000, 5th ed. P542 (ref 265)
and Staub NC, Nagano H, and Pearce ML. Pulmonary edema in dogs, especially the sequence of fluid accumulation in lungs.
J Appl Physiol 22: 227-240, 1967 (ref 405).
46
a) Interstitial oedema.
This is the earliest manifestation of pulmonary oedema, and is characterised on light
microscopy by the appearance of fluid in the interstitial tissue surrounding the extra-alveolar
airways and by cuffs of lymphatics around the bronchi and pulmonary arterioles. Electron
microscopy reveals small amounts of fluid in the alveolar septa. The development of
interstitial oedema appears to act as a reservoir for extravascular lung water and by receiving
water from the alveolar space protects gas exchange. As a result, physical signs are minimal,
except for dyspnoea on exercise. The alveolar-arterial oxygen gradient is usually normal.
Septal Kerley B lines may be visible on chest x-ray.
b) Alveolar wall oedema.
As oedema becomes more severe and extravascular lung water increases, fluid in the alveoli
septa increases and becomes visible on light microscopy as crescents in the angles between
adjacent septa. The alveolar-arterial oxygen gradient may be mildly increased and dyspnoea at
rest is common.
c) Alveolar flooding.
This occurs in an all-or-none fashion with alveoli being either flooded or empty. Flooded
alveoli have a smaller volume than their gas filled neighbours. Such a pattern of filling
suggests that as fluid accumulation progresses and alveolar dimensions decrease, a critical
radius of curvature is reached which, according to Laplace’s Law, results in an increase in the
tension at the air-fluid interface and increases fluid transudation into the alveoli. Because of
the gravitational effects on pulmonary blood distribution alveolar flooding shows a
predilection for the dependent parts of the lungs. Alveolar flooding presents clinically with
severe dyspnoea at rest and inspiratory crackles on auscultation. Flooded alveoli cannot
participate in gas exchange and venous admixture results. At its most extreme alveolar
flooding is marked by the airways filling with froth which seriously impairs gas exchange and
is rapidly fatal if not treated.
Movement of fluid from the pulmonary capillaries into the alveoli occurs in three phases:
i) Trans-endothelial movement from the capillaries into the interstitium.
ii) Interstitial fluid dynamics.
iii) Trans-epithelial movement from the interstitium into the alveoli.
47
i) Trans-endothelial movement (403):
Net movement of fluid from the pulmonary capillaries into the interstitium, as in other
capillary beds, depends upon the balance between the capillary and interstitial hydrostatic
pressure gradient and the capillary colloid osmotic pressure which may be represented by
Starling’s equation:
! = K $ Pmv ! Ppmv ! " #mv ! #pmv &
Q
%
'
(
!
Q
)
(
)
is the net trans-endothelial fluid flow. Under normal conditions this will be
equal to the lymphatic drainage.
K
is the hydraulic conductance coefficient (i.e. the flow of fluid per unit
pressure gradient across the endothelium).
Pmv
is the hydrostatic pressure in the microvasculature (i.e. the capillaries).
Ppmv
is the hydrostatic pressure in the perimicrovascular tissue (i.e. the
interstitium). This is normally subatmospheric favouring filtration from the
capillaries into the interstitial space.
σ
is the reflection coefficient (equal to 1 if the barrier is totally impermeable
to proteins; equal to 0 if it is totally permeable. Normally it has a value of
between 0.5 and 0.75).
Πmv
is the solute osmotic pressure within the microvasculature.
Πpmv
is the solute osmotic pressure within the perimicrovascular tissue (i.e.
interstitium).
ii) Interstitial fluid dynamics
Traditionally the alveolar interstitial space was regarded simply as a passive channel
permitting fluid to move from the alveolar interstitium to the pulmonary lymphatics (403,
405). It is now known that this extracellular matrix undergoes structural changes during the
development of interstitial oedema which increase the compliance of the interstitial space,
further exacerbating oedema development.
The
interstitial
connective
tissue
consists
of
fibrillar
collagens,
elastic
fibres,
glycosaminoglycans and proteoglycans. Collagen and elastic fibres provide tensile strength
and elastic recoil. The glycosaminoglycans, and in particular hyaluronic acid, and
48
proteoglycans fill up the fibrous interstitial network. Proteoglycans are a genetically unrelated
family of proteins which link to glycosaminoglycan chains. In models of both hydrostatic and
lesional pulmonary oedema there is weakening of the noncovalent bonds linking
proteoglycans to the structural components of the extracellular matrix and fragmentation of
the proteoglycan chains (306, 307, 334).
Under normal conditions the pulmonary interstitial pressure is sub-atmospheric favouring
capillary filtration into the interstitium from where the liquid is removed via the pulmonary
lymphatics. With the development of early oedema there is a rapid increase in pulmonary
interstitial pressure for very little increase in lung extravascular water. The low compliance of
the normal interstitium which results in this rapid pressure increase protects against early
interstitial oedema development by countering the filtration gradient driving fluid from the
pulmonary capillaries into the interstitial space. With increasing oedema interstitial
compliance increases and pressure decreases removing the protective hydrostatic buffer from
the interstitial space and allowing further fluid transfer into the interstitium. These changes in
the mechanical properties of the interstitium are related to the biochemical degradation of the
extracellular matrix (Figure 7) (306, 307, 334).
Figure 7: Pulmonary interstitial pressure (Pip: measured by micropuncture) plotted as a function of the wet:dry lung weight ratio of rabbit
lungs that had been infused with saline to produce pulmonary oedema. The numbers below each data point indicate the number of averaged
measurements. As early oedema develops there is a rapid increase in Pip for a very small increase in lung wet:dry weight ratio (proportional
to extravascular water). The initial low compliance of the interstitium produces a rapid pressure increase which protects against early
interstitial oedema development by countering the filtration gradient driving fluid from the pulmonary capillaries into the interstitial space.
As oedema increases interstitial compliance also increases as a result of changes in the mechanical properties of the extracellular matrix. This
results in a decrease in Pip which removes the protective hydrostatic buffer from the interstitial space, allowing further fluid transfer into the
interstitium.
Source: Negrini D, Passi A, de Luca G, and Miserocchi G.
Pulmonary interstitial pressure and proteoglycans during development of pulmonary edema.
Am J Physiol 270: H2000-2007, 1996.
49
iii) Trans-epithelial fluid movement
Active sodium transport is the primary mechanism for the clearance of fluid from the alveoli.
When this mechanism is overwhelmed, alveolar oedema results. This is discussed in detail
below.
4) Aetiology of pulmonary oedema
Using the Starling Equation, discussed above, it is traditional to categorise the aetiology of
pulmonary oedema into three groups according to the effects on different factors in the
equation. However it should be noted that this classification is not rigid and that there is
considerable heterogeneity between the groups (103). Other causes of pulmonary oedema, and
in particular high altitude pulmonary oedema, will be discussed.
a) Hydrostatic pulmonary oedema (405)
This is due to an increase in capillary hydrostatic pressure (Pmv). When it exceeds the
osmotic pressure of the plasma proteins (Πmv) fluid exudes into the interstitial space and
once it has overwhelmed the transport capacity of the pulmonary lymphatics, oedema results.
The protein content of the oedema fluid is significantly less than that of the plasma. The
causes of hydrostatic pulmonary oedema include left ventricular failure; mitral valve disease;
fluid overload; post-pneumonectomy pulmonary oedema and high altitude pulmonary oedema
(455). In addition to producing a fluid transudate, severe elevation of Pmv may result in stress
failure of the capillary endothelium with free leak of protein and erythrocytes into the
interstitium and alveoli (459, 460).
b) Increased permeability oedema (404)
This is due to the loss of the integrity of the pulmonary capillary membrane allowing albumin
and other macromolecules to leak from the capillaries into the interstitium and alveoli. The
reflection coefficient (σ) is effectively reduced. The osmotic pressure gradient which opposes
fluid leaving the capillaries (Πpm-Πpmv) is lost and fluid is free to pass into the interstitium
and alveoli. The protein content is equal to that of plasma. Increased permeability oedema is
typical of acute lung injury and the acute respiratory distress syndrome (50, 447).
50
c) Decreased osmotic pressure oedema (455)
Although rarely the sole cause of pulmonary oedema, any reduction in the plasma colloid
osmotic pressure (Πmv) as is frequently seen in the critically ill, will contribute to oedema
formation by reducing the Pmv at which fluid transudation begins to occur. There is evidence
that increasing plasma oncotic pressure with albumin in hypoproteinaemic patients with acute
lung injury improves oxygenation (278).
d) Lymphatic obstruction (255, 257)
As the pulmonary lymphatics drain into the great veins, raised central venous pressure, as
occurs in right heart failure or fluid overload, will reduce pulmonary lymph flow and
precipitate pulmonary oedema at a lower Pmv than normal.
51
LUNG EPITHELIAL FLUID TRANSPORT
1) Evidence for epithelial fluid transport
Although alterations in Starlings Forces may account for the generation of pulmonary oedema
it cannot satisfactorily explain the mechanism by which liquid is removed from the distal
airspaces of the lung once oedema has formed. The first evidence for active transport of liquid
out of the distal airspaces was provided by Matthay et al in a study of anaesthetised,
ventilated sheep (288) in which iso-osmolar liquid clearance occurred from the distal
airspaces against a rising luminal colloid osmotic pressure gradient which would inhibit
simple osmotic fluid reabsorption. Ιn addition the protein concentration declined in the
lymphatic fluid draining the interstitium of the distal airspaces, consistent with the
reabsorption of protein free fluid. Similar results were found in unanaesthetised,
spontaneously breathing sheep (394). Further evidence for active transport was provided by
the temperature dependence of liquid clearance in a perfused goat lung model. The rate of
liquid clearance from the distal airspaces progressively declined as the temperature was
lowered from 37° to 18° C (289). Similar results were obtained in a rat lung model (366) and
in resected human lung (371).
Additional understanding of the mechanism of alveolar liquid clearance has come from
pharmacological inhibition studies. Amiloride, an inhibitor of the principal apical sodium
channel, the epithelial sodium channel (ENaC), inhibits between 30 to 90% of baseline fluid
clearance depending on the species (12, 52, 150, 210, 219, 227, 256, 309, 316, 371, 400, 434)
providing evidence that alveolar liquid clearance is driven by sodium reabsorption. In an
isolated human lung model amiloride inhibited 40% of reabsorption (371). Further proof of
the role of sodium transport came from inhibition of the basolateral Na+-K+-ATPase pump
using the cardiac glycoside ouabain (12, 171, 210, 219, 371) that resulted in a fall in alveolar
liquid clearance of between 30 and 75% depending upon the species. The results with both
amiloride and ouabain are consistent with work using
22
Na labelled sodium flux
measurements (45, 46, 101, 163).
Cultures of isolated rat alveolar type II (ATII) cells enabled further study of the mechanism of
alveolar liquid clearance. When cultured on a non-porous surface they formed, over 2-3 days,
a continuous confluent layer that developed domes of fluid after 3-5 days (125, 161, 284).
Dome formation, which is due to fluid transport from the apical to basolateral surface of the
52
cells, could be inhibited by amiloride or ouabain (86) or by the replacement of sodium with
another cation (162).
The culture of alveolar type II cells on porous supports in Ussing chambers has become a
mainstay of the study of sodium transport in the alveolar epithelium. Culture of isolated type
II cells under precisely controlled conditions can produce an electrically tight cell monolayer
with a transepithelial resistance of around 2 KΩ cm-2 (86, 125). The cell monolayer has a
spontaneous transepithelial potential difference which is proportional to the ion flux across it.
By applying a voltage across the epithelium and “clamping” it to a value of zero, the current
measured across the epithelium, known as the short circuit current (Isc), is a direct measure of
ionic movement across the epithelium (196). In the case of type II cell monolayers it is a
measure of transepithelial sodium transport and substitution of choline for sodium, or the
addition of ouabain reduces Isc to zero (86).
Evidence for lung epithelial fluid transport comes from intact animal studies; isolated lung
preparations and alveolar cell monolayers. Cell studies have predominantly used alveolar type
II cells because of the difficulty in obtaining purified preparations of type I cells. However,
there is increasing evidence that type I cells play a significant role in epithelial fluid transport:
•
Type I cells contain significantly more Na+ channels than type II cells (225). In rat
alveolar epithelium it has been estimated that the cation channel ratio for type I:type II
cells is greater than 40:1 (127).
•
Type I cells demonstrate a high permeability to water and the presence of aquaporins
(126, 151).
•
Freshly isolated type I cells demonstrate significant amounts of inhibition of Rb+ (K+)
uptake by ouabain (225).
•
Based on the relative contributions to epithelial fluid transport of the Na+-K+-ATPase
isoenzymes (discussed below - 3) Na+-K+-ATPase Pump) it has been estimated that
type I cells are responsible for approximately 60% of fluid transport (359).
It is therefore possible that the numerically greater type I cells may actually be responsible for
the majority of basal fluid transport. There is growing evidence that Cl- transport may also
have an important role in the upregulation of lung water reabsorption. This is discussed
below.
53
2) Apical sodium channels – biophysical properties
Sodium channels are found in the apical membrane of many mammalian sodium transporting
epithelia such as the renal tubules; colon; sweat and salivary ducts. They mediate the
controlled entry of Na+ ions into the epithelial cells from luminal fluids, thus controlling the
composition of the absorbed fluid (155). Epithelial sodium transport occurs via amiloridesensitive and amiloride-insensitive apical channels.
a) Amiloride-sensitive sodium channels
Four types of amiloride-sensitive Na+ channels have been characterised according to their
conductance; open probability; cation selectivity and amiloride sensitivity (286):
i)
Highly selective cation (HSC) channel.
ii)
Poorly selective cation (PSC) channel.
iii)
A second poorly selective cation channel.
iv)
Non-selective cation (NSC) channel.
The principal amiloride sensitive sodium channel identified in mammalian salt transporting
epithelia appears to be the HSC channel which has been named the epithelial sodium channel
or ENaC. This HSC channel belongs to the ENaC/degenerin super family of ion channels. In
addition to ENaC, this large group of functionally heterogeneous channels includes acid
sensitive ion channels found in central and peripheral nerves; ion channels associated with
mechanoreceptors and proprioceptors and ligand gated ion channels. ENaC/degenerin ion
channels are found in mammals, molluscs, nematodes and insects and are characterised by
substantial sequence homology (231).
The structure of ENaC was described by cloning and expression of mRNA in Xenopus
oocytes using mRNA extracted from rat colon. Three homologous subunits have been
identified, α, β and γ. They are characterised by two hydrophobic membrane spanning
regions; a large extracellular loop makes up more than half of the total protein and which is
rich in cysteine residues and contains multiple glycosylation sites, and intracellular aminoand carboxy- terminals (78, 79, 263). All three subunits are needed to form the HSC channel
ENaC. The PSC channels would appear to consist of a combination of the α and β or γ
subunits while the NSC channels appear to be made up solely of α subunits (79, 286).
54
Functional studies have demonstrated the importance of ENaC for alveolar fluid absorption.
Knockout mice deficient in the α subunit of ENaC lost amiloride-sensitive sodium transport
from respiratory epithelia, developed respiratory distress and died within 40 hours of birth.
Autopsies revealed no macroscopic changes in organs other than the lungs which were
waterlogged and poorly inflated (209). Knockout mice deficient in the β and γ subunits
develop acute pseudohypoaldosteronism with salt wasting and hyperkalaemia and die
between 2 and 4 days after birth while knockout mice lacking only the γ subunit do not die
although they do clear lung water more slowly (25). Messenger RNA for αENaC is almost
undetectable in pre-term human (440) and rat lung (320). Perinatal changes in lung water
transport are discussed further below.
Further evidence for the functional importance of ENaC in human lung comes from adult
patients with Type-1 pseudohypoaldosteronism, a condition characterised by loss of function
mutations in the genes for the three subunits of ENaC. These patients have no sodium
absorption from their airway surfaces, and a markedly increased volume of airway surface
liquid resulting in increased symptoms of coughing and wheeze (232). The phenotypic
difference between perinatal and adult deficiencies of ENaC subunits highlights that the role
of ENaC in alveolar fluid clearance, particularly under basal conditions, is poorly understood.
b) Amiloride-Insensitive Sodium Channels
Despite the importance of the amiloride-sensitive Na+ channels, and ENaC in particular, in
epithelial Na+ transport, a significant proportion of the alveolar transepithelial Na+ current is
not inhibited by amiloride. This proportion varies from 10% in mice (150) to 40-50% in
humans (371) and also appears to be dependant on lung maturity: as sheep lung matures
epithelial fluid transport becomes increasingly amiloride insensitive (227). There is evidence
in a rat trachea preparation that this amiloride insensitive current can be inhibited by
dichlorobenzamil or L-cis-diltiazem both of which are inhibitors of cyclic nucleotide gated
(CNG) ion channels (388). A large number of CNG types have been described (229) and
mRNA for CNG1 has been detected in rat lung tissue (124), while L-cis-diltiazem blocks a
proportion of the terbutaline stimulated alveolar liquid clearance in rats (317). However it had
no effect on baseline, unstimulated, absorption, while in foetal sheep it has been suggested
that the action of dichlorobenzamil is due to inhibition of an amiloride sensitive sodium
channel (226). It may be that a significant proportion of the amiloride insensitive current in
alveolar epithelium results from sodium transport via CNG channels but this remains to be
55
demonstrated convincingly. In addition it has been suggested that a proportion of alveolar
liquid clearance occurs via the Na+-glucose co-transporter though the evidence is conflicting
(47, 210, 233). The role of other Na+ co-transporters identified in alveolar type II cells
remains to be identified (73, 90).
3) Na+-K+-ATPase Pump
The central role of the Na+-K+-ATPase pump in cellular function has been known for over 30
years since its discovery by the Nobel Laureate Skou (399). The pump, which is confined to
the basolateral surface of cells, consists of α and β subunits in a 1:1 ratio. In some tissues
there is also a γ subunit that modifies pump activity, but its role in lung tissue remains
unknown. There are multiple isoforms of the α- and β-subunits which are expressed in a tissue
specific manner (56). The α-subunit catalyses the exchange of intracellular Na+ for
extracellular K+ in a 3:2 ratio; is phosphorylated by ATP and contains the binding site for the
specific inhibitor ouabain. It has multiple transmembrane domains. The role of the β-subunit,
which consists of a single transmembrane domain, is the assembly and targeting of the Na+K+-ATPase pump into the plasma membrane of the cell. Both α- and β-subunits are required
for the correct insertion and functioning of the pump in the cell membrane.
It was initially thought that the Na+-K+-ATPase pump was predominantly confined to alveolar
type II cells but the mRNA and protein for both the α1, α2 and β1 subunits have been detected
in type I cells (64, 224, 225, 359); ouabain-inhibitable Rb+ uptake, an estimate of K+ uptake,
is approximately 2½ times higher in type I than type II cells (225) and it has been estimated
that the α2-Na+-K+-ATPase is responsible for around 60% of basal alveolar fluid transport in
the rat lung (359). It is therefore likely that type I cells are responsible for the majority of
basal alveolar fluid transport.
Basal activity of the Na+-K+-ATPase pump is around one third of its maximal activity
permitting significant upregulation of pump activity. Short term increases in activity in
alveolar epithelial cells can be stimulated by β-adrenergic agonists and dopamine. Long term
regulation is mediated via transcriptional and post-transcriptional changes such as alterations
in the number of pumps due to changes in pump trafficking from intracellular pools and
changes in protein degradation (424). These mechanisms are mediated by glucocorticoids,
aldosterone, insulin and certain growth factors - see below: Regulation of lung epithelial fluid
transport.
56
4) Chloride channels
It is known that the Cl- secretion, against its electrochemical gradient, plays a crucial role in
the secretion of liquid into the foetal lung (319, 338). Secretion begins between the second
and fourth month of gestation and continues throughout pregnancy. The liquid secreted by the
lung accounts for approximately 50% of the amniotic fluid but also results in a positive
intrapulmonary pressure which is important for the structural development of the foetal lung.
Increasing the amount of fluid or obstructing the outflow from the lungs, as occurs in
laryngeal atresia, results in lung hyperplasia (6, 466). If the amount of lung fluid is decreased,
hypoplasia results (6).
The identity of the apical ion channel or channels responsible for Cl- secretion in the foetal
lung is not yet fully certain, but the cystic fibrosis transmembrane conductance regulator
(CFTR) would appear to play a central role (324). CFTR is a member of the ATP binding
cassette (ABC) transporter super-family. These proteins are characterised structurally by 2
motifs consisting of a membrane spanning domain and a nucleotide binding domain, with
each motif joined together by a regulatory domain. Although sharing their characteristic
structure, CFTR is the only member of the ABC family which is an ion channel rather than a
pump. The membrane spanning domains form the channel pore of CFTR. Hydrolysis of ATP
by the nucleotide binding domains controls channel gating while phosphorylation of the
regulatory domain determines channel activity (220, 396). CFTR is found throughout the
human respiratory tract from the nose to the distal alveoli (67, 68, 139, 428). Mutations in the
gene encoding for CFTR result in the autosomal recessive disorder cystic fibrosis. To date
over 700 mutations have been identified of which the most common is ΔF508 (170, 336).
The cystic fibrosis transmembrane conductance regulator has been implicated in Cl- secretion
in the foetal lung, and is discussed below (see: Regulation of lung epithelial fluid transport),
but it is clear that it is not the only pathway by which Cl- efflux can occur. Experiments using
CFTR knockout mice demonstrated that Cl- and liquid secretion could still occur despite the
absence of the CFTR channel (22). Other possible Cl- channels that may offer alternative
pathways for Cl- secretion are the calcium regulated Cl- channel (CaCC); the outwardly
rectifying Cl- channel (ORCC) and a member of the voltage gated CLC Cl- channel group
(220, 319, 338). In the absence of a normally functioning CFTR it may be that one or more of
these channels is activated (319, 324, 338). This is consistent with CFTR knockout mice not
developing lung disease and patients with cystic fibrosis being born with intact lungs (338).
While the apical route for Cl- secretion remains to be exactly identified it is known that the
basolateral entry of Cl- is in part dependent upon the Na+-K+-2Cl-, although in its absence in
57
knockout mice an alternative pathway or pathways for basolateral entry of Cl- exist (158,
319).
While it is apparent that Cl- secretion plays a crucial role in the development of the foetal lung
the direction in which, and the mechanism by which, Cl- transport occurs in the adult lung
remains debated. CFTR has been detected in both type I and type II alveolar cells (224). In
proximal ciliated airways Na+ absorption via ENaC and Cl- secretion via CFTR appear to be
intricately linked to control airway surface liquid in proximal ciliated airways (67, 68, 202,
251, 252) and in the alveoli (262). In cystic fibrosis, defective CFTR phenotypes result in
unopposed Na+ absorption via ENaC resulting in changes in the volume and composition of
airway surface liquid, depressing mucociliary clearance and leading to chronic infection (67,
68, 202). In distal lung epithelium there is a growing body of evidence that CFTR is also
important in the cAMP mediated upregulation of lung liquid reabsorption. This is discussed
below: Regulation of lung epithelial fluid transport.
5) Aquaporins
Aquaporins are a family of small monomer membrane proteins that act as water transporters.
As a result of their presence the water permeability of the alveolar epithelium is enhanced up
to 50 times that of other tissues. The water permeability of alveolar type I cells is the highest
of any mammalian cell line (126). There are at least 11 mammalian aquaporins characterised.
Of these 4 (AQP1, 3, 4 and 5) are expressed in the respiratory tract where they are found from
the nose to the distal alveoli (65, 435). This widespread presence of aquaporins throughout the
respiratory tract and the resultant high water permeability in the distal lung epithelium would
suggest that they might play a role in trans-epithelial fluid balance. No non-toxic inhibitors of
aquaporins exist so this hypothesis required testing by means of knockout mice. The results of
these knockout studies indicate that despite playing a major role in the kindeys and nervous
system, aquaporins are not required for the physiological clearance of lung water in the
neonatal or adult lung, or for the accumulation of extravascular lung water in the injured lung
(15, 65, 402, 432, 435).
58
SUMMARY OF PULMONARY TRANSEPITHELIAL ION TRANSPORT
Evidence from animal studies and pharmacological inhibition studies in cell cultures has
shown that fluid absorption from the distal airspaces of mammalian lung is dependent upon
active Na+ transport across the distal lung epithelium. This occurs in two stages: an electrochemical gradient is generated by the active extrusion of Na+ from the basolateral surface by
the Na+-K+-ATPase pump. Sodium ions then enter the apical surface by a number of ion
channels. Water travels passively with the osmotic gradient either via aquaporin channels or
paracellular pathways. While initially this mechanism was thought to be confined to alveolar
type II cells there is increasing evidence that type I cells may be responsible for the bulk of
alveolar epithelial fluid transport.
The principle apical Na+ channel is the amiloride inhibitable highly selective cation channel,
ENaC, which accounts for up to 50-60% of apical Na+ transport in humans and 90% in the
mouse. The remaining Na+ transport occurs via amiloride-insensitive CNG channels although
the exact proportion appears to depend on both the species and the maturity of the lung. The
number and structure of the α-, β-, and γ- subunits which make up ENaC remains debated,
although the majority of workers propose that it consists of 4 subunits (2 α, 1 β and 1 γ).
The basolateral Na+-K+-ATPase pump, inhibited by the cardiac glycoside ouabain, consists
of α- and β- subunits expressed in a 1:1 ratio. Basal pump activity is around 1/3 of maximal
activity that permits significant upregulation by a number of catecholamine dependent and
independent mechanisms.
Chloride secretion is crucial for secretion of liquid into the air spaces of the foetal lung. This
occurs by a number of channels of which CFTR is thought to be the most important.
Mutations in the gene encoding for CFTR result in cystic fibrosis. In the adult lung the exact
role of Cl- transport remains debated. Sodium absorption and Cl- secretion are intricately
linked in the proximal airways to control airway surface liquid. In the distal airways it would
appear that CFTR also has a role in secreting airway surface liquid and is important in the
upregulation of lung liquid reabsorption.
Aquaporins are water channels found in many mammalian tissues. Although present in both
alveolar type I and II cells, and despite alveolar type I cells having the highest known water
permeability of any mammalian cell line, knockout studies suggest that aquaporins are not
required for physiological clearance of lung water or for the development of oedema in lung
injury.
59
This model of alveolar epithelial ion and water transport is illustrated in Figure 8.
Figure 8a: Standard paradigm for ion and fluid transport across the alveolar epithelium: an electrochemical
gradient is generated by the active extrusion of Na+ from the basolateral surface of alveolar type II (ATII) cells
by the Na+-K+-ATPase pump. Sodium ions then enter the apical surface by a number of ion channels of which
the principal is the epithelial sodium channel (ENaC). Other channels include cyclic nucleotide gated (CNG)
cation channels. Electrical neutrality is maintained by Cl- transport, either paracellularly or via the cystic fibrosis
transmembrane conductance regulator (CFTR) in ATII cells. Water moves across both ATI and ATII cells via
aquaporins (AQP).
Figure 8b: Evolving paradigm for ion and fluid transport across the avleolar epithelium. Under conditions of net
Na+ absorption, Na+ is extruded from the basolateral surface of both alveolar type I (ATI) and type II (ATII)
cells by the Na+-K+-ATPase pump. Na+ is absorbed from the apical surface of both ATI and ATII cells via the
epithelial sodium channel (ENaC) and via cyclic nucleotide gated (CNG) cation channels in ATI cells. Electrical
neutrality is maintained by Cl- transport via the cystic fibrosis transmembrane conductance regulator (CFTR) in
ATI and ATII cells and/or via paracellular tight junctions. Under these conditions water moves across both ATI
and ATII cells via aquaporins (AQP) or by diffusion. In the formation of alveolar surface liquid, net Clsecretion occurs via CFTR, and possibly by other channels. Net Na+ secretion would then occur via as yet
unidentified pathways. HSC: highly selective Na+ channel. NSC: non-selective.
Source: Dobbs, L.G. and M.D. Johnson, Alveolar epithelial transport in the adult lung. Respir Physiol Neurobiol 33: 283-300, 1997
60
6) Regulation of lung epithelial fluid transport
a) Perinatal changes
Profound changes take place in the foetal lung around the time of birth to enable air breathing
to begin. During late gestation the lung acquires the ability to increase Na+ and lung liquid
absorption in response to catecholamine stimulation (26, 72, 329, 338). This catecholamine
dependent increase in lung liquid absorption can be markedly reduced by amiloride
suggesting transepithelial Na+ transport occurs via ENaC (329) although this may not be the
only pathway for Na+ absorption (26, 338). Installation of intra-alveolar amiloride, or its
analogues, in guinea pigs immediately prior to birth resulted in severe respiratory distress and
severely impaired lung water clearance (321). As discussed above, knockout mice deficient in
the α-subunit of ENaC lose amiloride-sensitive respiratory Na+ transport, develop respiratory
distress and die soon after birth from pulmonary oedema (135, 209).
Further evidence for the ability of lung tissue to increase Na+ transport during the perinatal
period is the large increase in ouabain sensitive Rb+ transport, a measure of Na+-K+-ATPase
activity, which has been shown to occur in rabbit foetal distal lung epithelium (FDLE)
immediately around the time of birth (57). In the foetal rat lung mRNA for the α- and βsubunits of the Na+-K+-ATPase pump appears around the 18th day of gestation (term = 22
days) and increases in the final days towards birth (211). Likewise expression of the α-subunit
of ENaC only occurs during the last 3 days of gestation in rat lung (211).
The catecholamine stimulated increase in Na+ transport at the end of gestation corresponds
with the concurrent increase in glucocorticoid and thyroid hormones, and the ability to
increase Na+ transport is lost in animals that have undergone adrenalectomy or thyroidectomy
(28). This loss of lung Na+ and liquid transport in thyroidectomised fetal lambs was reversed
by infusion of T3 (27). Hydrocortisone therapy increases terbutaline stimulated ion transport
and amiloride sensitivity in late gestational rate FDLE cells These findings may represent the
ability of these hormones to stimulate β-adrenergic receptor formation or the maturation of
downstream cAMP dependant pathways (338).
The increase in PAO2 which occurs at delivery (foetal PAO2 ~25 mmg Hg; neonatal PAO2 ~
100 mm Hg) appears to be crucial for the maintenance of the ion and liquid transport changes
which occur around the time of birth. Exposure of lung explants from aborted midtrimester
human foetuses to increased concentrations of O2 was associated with maturational changes in
lung structure and surfactant although it should be noted that the concentrations of O2 utilised
were as high as 95% (4). Explanted lungs from 14, 20 and 22 (term) day foetal rats were
61
exposed to foetal O2 concentrations (3%) or 21% O2. Exposure to 3% O2 resulted in the
formation of fluid filled cysts in the 20 and 22 day explants while exposure to 21% O2
reduced both the number of fluid filled cysts and the lung:water ratio. Importantly it was
shown that the unresponsiveness of the early lung explants to increased O2 concentrations
could be reversed by the addition of thyroid and glucocorticoids hormones (23). In a study
using monolayers of FDLE cells in Ussing Chambers, Pitkanen et al, demonstrated an
increase in amiloride sensitive Na+ transport and mRNA coding for ENaC subunits on
exposure to 21% O2 (337). Thome et al (425) grew monolayers from FDLE cells isolated
from 19 day old rat foetuses under a variety of O2 concentrations. The FDLE monolayers
grown in 20% O2 had higher short circuit currents and amiloride- and ouabain- sensitive
components of the short circuit current than monolayers grown under hypoxic conditions. In
addition the FDLE cells grown at 5% O2 had less protein for the α-, β- and γ- subunits of
ENaC and the α1- subunit of the Na+-K+-ATPase pump than cells grown in 20% O2.
Gowen et al (166) studied the nasal potential difference (NPD), as a surrogate for distal
respiratory epithelial ion transport, in term neonates with and without transient tachypnoea of
the newborn or acute respiratory. Basal NPDs were higher (i.e. less negative) in neonates with
transient tachypnoea of the newborn and in those delivered by caesarean section without prior
labour compared to those delivered vaginally or by caesarean section but with prior labour.
The percentage of the NPD inhibited by amiloride was also lower in neonates with transient
tachypnoea of the newborn and those delivered by caesarean section without prior labour.
Over 72 hours the NPD hyperpolarised (i.e. become more negative) in the transient
tachypnoea group to values similar to the other groups. The decline in respiratory rate was
paralleled by the hyperpolarisation of the NPD. Gaillard et al (152) studied 20 near term
infants (gestation 38 to 41 weeks) born by normal vaginal delivery and 21 born by caesarean
section (gestation 37 to 40 weeks). They could find no difference in NPD or the amiloride
inhibited portion of the NPD between the 2 groups seemingly refuting the argument that
absence of normal labour resulted in a reduction in ENaC mediated sodium transport.
Barker et al (24) studied 31 very premature infants (gestation at birth <30 weeks) with and
without respiratory distress. Basal NPD was less hyperpolarised (i.e. less negative) in the
respiratory distress group than the non-respiratory distress group. The NPD became
increasingly hyperpolarised with increasing gestational age at birth and with increasing
weight. The less hyperpolarised NPD in the respiratory distress group was also associated
with a smaller proportion of the NPD which was inhibited by amiloride. Gaillard et al (153)
studied 38 infants born at between 29 and 36 weeks gestation. A multiple stepwise regression
62
model confirmed the findings of Barker and colleagues (24) showing a more hyperpolarised
basal NPD with increasing gestational maturity. However there was no relationship between
gestational age and the proportion of the NPD inhibited by amiloride or the capacity for Cl
secretion.
These studies using NPD are confirmed by the work of Helve et al (194) who studied the
expression of the ENaC subunits in nasal epithelium taken from 7 healthy term infants and 5
pre-term infants (gestational age 27 weeks) with respiratory distress syndrome. The pre-term
infants with respiratory distress had reduced expression of all ENaC subunits compared with
the healthy term infants. In addition dexamethasone was found to increase the expression of
α-ENaC in 4 pre-term infants who required prolonged mechanical ventilation. In a further
study from the same group, Helve et al (193) demonstrated a weak correlation (r2 = 0.4, p <
0.003) between NPD measured between 1 and 4 hours after birth and lung compliance
measured between 21 to 48 hours after birth in 20 healthy term infants, suggesting that the
alterations in sodium transport which take place around the time of birth play a role in
pulmonary adaptation. These findings however were not reproduced by a later study from the
same group (192).
The mechanism by which the changes in O2 tension occurring at birth bring about changes in
epithelial ion and fluid transport are not fully understood, but a promoter region of the gene
coding for rat α-ENaC contains a binding side for the transcription factor nuclear factor-κB
(NF-κB). Nuclear factor-κB is thought to be one of the transcription factors, along with the
hypoxia inducible factor-1 (HIF-1), which are important in the regulation of gene expression
in response to changes in oxygen tension (295). Under normoxic conditions NF-κB is bound
to one of a number of inhibitory proteins. Hyperoxia or increased concentrations of reactive
oxygen species (ROS) cause the ubiquitination of the inhibitory proteins allowing NF-κB to
bind to target genes (107, 295). Experiments with FDLE cells have shown that increasing the
PO2 from 25 to 150 mm Hg induces NF-κB and that blocking NF-κB reduces the increase in
Na+ conductance associated with the rise in PO2 [252]. Further experiments demonstrated that
while there is an immediate increase in NF-κB expression in FDLE cells on exposure to
increased PO2 and an increase in Na+-K+-ATPase activity within 6 hours, activation of the αENAC promoter was not seen for 24 hours (16, 351).
63
b) Adult lung: catecholamine-dependent upregulation
As discussed above the release of catecholamines around the time of birth stimulates the
switch from Cl- and liquid secretion to Na+ liquid absorption in mammalian FDLE. In adult
mammals, including humans, stimulation of β2-receptors by adrenergic agonists such as
terbutaline and epinephrine increases luminal liquid clearance in intact and isolated lung
models of rats (85, 101, 219), mice, guinea pigs (316), sheep (52), dogs (276) and humans
(370, 371). This increased liquid clearance can be prevented by β-receptor antagonists such as
propranolol (52, 101, 316). It can also be blocked by amiloride and by oubain consistent with
the catecholamine stimulated increase in alveolar liquid clearance occurring via transepithelial
Na+ transport (52, 171, 316). Interestingly in guinea pigs the more specific β1-receptor
antagonist atenolol was as efficacious in inhibiting the catecholamine mediated increase in
alveolar liquid absorption as the non-specific β-receptor antagonist propranolol although
neither drugs had any effect on basal clearance. In addition while the non-specific βadrenergic receptor agonists epinephrine and isoprenaline augmented alveolar liquid clearance
the β2-adrenergic receptor agonist terbutaline had no effect on liquid clearance (316). In
rabbits catecholamines do not increase alveolar liquid clearance demonstrating an important
species difference (400).
The catecholamine stimulated increase in alveolar liquid clearance is mediated by cAMP as a
second messenger although the mechanism by which this occurs is debated (258, 323, 462).
Proposed
mechanisms
include
increased
+
open
probability
of
ENaC;
increased
+
phosphorylation of α-subunits of the Na -K -ATPase pump; increased delivery of ENaC or
Na+-K+-ATPase subunits to the apical or basolateral membranes respectively (258, 323, 462).
It has however become apparent that Cl- transport may also play an important role in the
cAMP mediated increase in alveolar liquid clearance. Initial work on cAMP mediated Cltransport in alveolar cells suggested that basal Cl- transport took place via a paracellular route
but that with cAMP stimulation it occurred via Cl- channels (234). Jiang et al (222) suggested
that cAMP mediated Na+ transport might require initial uptake of Cl-. Subsequently the same
group (221) demonstrated that terbutaline produced activation of apical Cl- channels and
increased Na+ transport. This work was criticised by some authors partly because of the
uncertainty of the phenotype of the cells studied but also because a cell line may not
reproduce what happens in the intact lung (258, 462).
Fang et al (142) studied liquid clearance in mice with the ΔF508 mutation in CFTR and in
wild type mice. Mice with the ΔF508 mutation could not increase lung liquid clearance in
response to forskolin (a naturally occurring, cell permeable, stimulator of adenyl cyclase) or
64
isoprenaline whereas wild type mice could. Isoprenaline stimulation was also inhibited by the
Cl- channel blocker glibenclamide in both wild type mice and normal human lung. In addition
in a model of acute volume overload pulmonary oedema resulting from the infusion of 40%
body weight of saline, the wet:dry lung ratio increased by 64% in the ΔF508 mice but only
28% in the wild type. Using a new CFTR inhibitor, CFTRinh-172, Fang et al (143) were able
to block cAMP stimulated transport but not basal liquid transport in cultured human alveolar
type II cells. In summary therefore it would appear that CFTR does not play a role in basal
clearance of liquid from the alveoli, but is essential for cAMP mediated upregulation of fluid
transport.
Dopamine increases lung liquid clearance in both alveolar cell cultures (29) and in isolated
perfused lung models (30, 373). Dopamine stimulates both ENaC and non-selective cation
sodium channels in both type I (191) and type II cells (190) via D1 receptors. While the short
term exposure of type II cells to both dopamine and β-adrenergic agonists increases cAMP via
G-protein activation of adenylate cyclase, D1 receptors appear to increases the open
probability (Po) of ENaC (190), whereas β2-adrenergic stimulation increases the number of
channels (87).
Short term exposure of alveolar type II cells to dopamine for up to 15 minutes increases the
Na+-K+-ATPase pump’s affinity for Na+ and causes recruitment of α- and β-subunits from
intracellular pools into the basolateral plasma membrane through a mechanism mediated via
D1 receptors (358). Interestingly this is the opposite to that which occurs in the kidney where
dopamine decreases Na+-K+-ATPase activity through the removal of pumps from the plasma
membrane by endocytosis (53). Longer term exposure to dopamine of cultured alveolar type
II cells for up to 24 hours has been shown to increase both mRNA for the β1-subunit and β1subunit protein abundance through a mechanism mediated via D2 receptors (174, 203).
c) Adult lung: catecholamine-independent upregulation
A number of catecholamine-independent mechanisms exist which can upregulate liquid
transport across the distal lung and in alveolar type II cell cultures:
i.
Adenosine: The purine nucleotide adenosine can influence lung liquid transport and
has been shown in mice to attenuate ventilator induce lung injury (134). Adenosine
can either increase or decrease alveolar liquid clearance and regulates airway surface
liquid by countering Na+ absorption through Cl- secretion via CFTR. Its effects appear
to depend upon the concentration of adenosine and which of the 4 adenosine
65
receptors, all of which have been identified in lung tissue, are stimulated. At
physiological concentrations of adenosine alveolar liquid clearance is decreased, most
likely via A1-receptors producing Cl- secretion via CFTR, while at lower
concentrations adenosine has been shown to increase alveolar liquid clearance via the
A2a and possibly A3 receptors stimulating Na+ absorption. (94, 140).
ii.
Hormonal: Glucocorticoids have been shown to regulate Na+ transport in adult lungs.
In rat ATII cells dexamethasone upregulated mRNA for the β-subunit of the Na+-K+ATPase pump, although the mRNA for the α-subunit was unchanged. The protein
expression and function of the α- and β-subunits of the Na+-K+-ATPase pump were
both increased (31). Lazrak et al (259) studied the effects of dexamethasone on the
human alveolar epithelium tumour cell line, A549. They found little increase in the
expression of α-subunits of ENaC but β- and γ- subunit mRNA increased by 1.6 and
17 fold respectively. Dagenais et al (109) studied the effects of dexamethasone and
cAMP on the expression of mRNA for the α-, β- and γ- subunits of ENaC and the αand β-subunits of the Na+-K+-ATPase pump in rat alveolar epithelial cells.
Dexamethasone upregulated mRNA expression for the 3 ENaC subunits. In contrast
to the findings in A549 cells α-subunit expression increased markedly (4.4 fold after
24 hours). In keeping with the findings of Barquin et al (31), dexamethasone only
upregulated expression of the β-subunit of the Na+-K+-ATPase pump. Treatment with
cAMP produced different results suggesting that glucocorticoids and cAMP
upregulate Na+ transport by different pathways.
Folkesson et al (148) studied the effect of dexamethasone and T3 thyroid hormone on
intact adult rats. They demonstrated that dexamethasone increased alveolar fluid
clearance by 80% compared with control, while T3 treatment increased alveolar fluid
clearance by 65%. Dexamethasone and T3 together had an additive effect, increasing
clearance by 132% over control. The addition of terbutaline to the dexamethasone and
dexamethasone + T3 treated rats did not increase fluid clearance further and neither
was fluid clearance inhibited by propranolol. The stimulated clearance could be
inhibited by amiloride to a greater extent than in controls (62 vs 48%). In a study in
adult rats, Noda et al (314) showed that a single does of intraperitoneal
dexamethasone increased alveolar liquid clearance and increased β- and γ-ENaC
subunit mRNA expression, although α-subunit mRNA expression remained
unchanged. There was no change in mRNA for either the α- or β- subunits of the Na+K+-ATPase pump although function was upregulated. It is known that endogenous
66
cortisol is important for maintaining normal alveolar liquid balance in the adult
guinea-pig and that this is mediated by regulation of ENaC subunit synthesis (315).
Aldosterone is known to play an important role in the regulation of Na+ transport in
epithelia (61) and the lung is known to express mineralocorticoid receptors (244). In
cultured rat ATII cells Olivera et al (328) demonstrated an increase in mRNA for the
β-subunit of the Na+-K+-ATPase pump and a 4 fold increase in hydrolytic activity in
those cells treated with aldosterone. In an isolated perfused rat lung model alveolar
liquid clearance was increased by 53% in those lungs treated with aerosolised
aldosterone compared with controls. Suzuki et al (415) treated rats with a chronic
low Na+ diet so that they developed hyperaldosteronism associated with hypokalaemia
although serum corticosterone and epinephrine were normal. Hyperaldosteronism was
associated with higher fluid clearance than in controls. The increased clearance was
amiloride dependent and could also be inhibited by the mineralocorticoid receptor
antagonist spironolactone.
Stokes and Sigmund (409) demonstrated that there is significant variability in the
response to the different stimuli that regulate ENaC between the lung, kidney and
colon. The mRNA for each of the three ENaC subunits was also influenced by the
type of steroid (gluco– vs mineralocorticoid) and, in the kidney, varied between
different tissue regions - cortex, inner and outer medulla. It is likely that a similar
heterogeneity also exists in the lung between different tissues.
iii.
Growth factors: a number of growth factors have been shown to upregulate lung Na+
transport. Keratinocyte growth factor (KGF), a member of the fibroblast growth factor
family and also known as FGF7, acts as a mitogen to alveolar epithelial cells and
appears to increase fluid clearance by increasing the number of ATII cells (445) as
well as increasing the number of α1 and β1 subunits of the Na+-K+-ATPase pump (62).
The effect of KGF on alveolar fluid clearance is additive with β2 agonists (445).
Keratinocyte growth factor appears to protect normal lungs against injury as well as
promoting recovery of fluid clearance in injured lungs (175, 452, 453).
Epidermal growth factor which increases both Na+ and fluid transport across both rat
ATII cells (63) and intact lungs (420) increases Na+-K+-ATPase pump α1 and β1
subunit mRNA and protein (112). Transforming growth factor-α (TGF-α) increases
sodium transport in ATII cells, although the mechanism by which this occurs remains
debated (89).
67
iv.
Inhibition of lung epithelial fluid transport: A number of factors may inhibit
epithelial fluid transport. Hypoxia is discussed separately below (see: High Altitude
Pulmonary Oedema: Ion and Fluid Transport in Hypoxia). Malnutrition inhibits
alveolar fluid clearance (primarily amiloride-insensitive clearance) in rat lungs (372).
Atrial natriuretic peptide (ANP) has been shown to decrease
22
Na influx in cultured
ATII cells (423) and decrease sodium transport and liquid clearance in isolated rat
lung (327). There is evidence in sheep anaesthetised with the halogenated volatile
anaesthetic agent, halothane, to suggest that ANP inhibits the catecholamine mediated
increase in alveolar liquid clearance in response to left atrial hypertension (76).
However halogenated volatile anaesthetic agents have been shown to reversibly
reduce
22
Na influx in cultured rat ATII cells (300) and amiloride-sensitive alveolar
liquid clearance in rats (355). There is also evidence in humans that volatile
anaesthetic agents damage the alveolar-capillary barrier and increase fluid leak (84). It
is therefore possible that the inhibition of alveolar liquid clearance seen in sheep (76)
may in part be due to the effect of halothane and not simply ANP. The local
anaesthetic lidocaine has also been shown to reduce alveolar liquid clearance in rats
by an ENaC independent mechanism. This effect is reversed by β2 agonists (253). The
inflammatory cytokines interleukin-1β (IL-1β), IL-4 and tumour necrosis factor
(TNF) have all been demonstrated to inhibit ENaC expression and activity in cultured
ATII cells (111, 154, 363). Dexamethasone has been shown to inhibit the effect of
TNF on ENaC (110).
68
SUMMARY OF REGULATION OF LUNG EPITHELIAL FLUID TRANSPORT
It would appear that the increase in sodium and liquid absorption which occurs around the
time of birth in response to an increase in PO2 stems primarily from an increase in Na+-K+ATPase activity which may be mediated via NF-κB. The subsequent increase in Na+
conduction may occur as a result of this increase in Na+-K+-ATPase activity rather than cause
the increase in Na+-K+-ATPase activity. Both the apical and basolateral elements of the Na+
transport pathway require both glucocorticoid and thyroid hormones for maturation. Reduced
gestational age is associated with reduced Na+ transport as assessed by NPD measurement
and the expression of ENaC subunits in the nasal epithelium. Benign transient tachypnoea of
the newborn in term babies is associated with a reduced capacity for Na+ transport. In
premature infants infant respiratory distress syndrome is also associated with defective
transepithelial Na+ transport.
In the majority of adult mammalian species respiratory lumen liquid clearance may be
upregulated by both catecholamine-dependant and -independent mechanisms. The
catecholamine-dependent β2-receptor mediated increase in liquid clearance can be blocked by
β-receptor antagonists, but also by amiloride and ouabain, consistent with it occurring via
transepithelial Na+ transport mechanisms. Upregulation of catecholamine-dependant liquid
clearance is mediated via cAMP. Although the precise mechanism remains debated, Cltransport appears to play an important role most likely via CFTR Cl- channels.
Catecholamine-independent upregulation of luminal liquid clearance occurs via both
mineralocorticoids and glucocorticoids as well as via dopamine and a number of growth
factors including TGF-α and KGF. Dependent upon the species, dexamethasone has been
shown to increase the expression of differing subunits of ENaC and the β-subunit of the Na+K+-ATPase pump as well as increasing its activity. This occurs via a non-cAMP dependent
mechanism. Aldosterone is well known to play an important role in the regulation of
epithelial Na+ transport. A number of factors inhibit epithelial fluid transport including
hypoxia, which is discussed in more detail below, ANP, inflammatory cytokines and both
volatile and local anaesthetic agents.
69
HIGH ALTITUDE PULMONARY OEDEMA (HAPE)
1) Pathophysiology and the role of hypoxic pulmonary vasoconstriction
High altitude pulmonary oedema (HAPE) is a non-cardiogenic, hydrostatic pulmonary
oedema that occurs in unacclimatised individuals between 24 and 72 hours after ascent to a
new altitude. It can occur at altitudes as low as 2500m. It can also occur in high altitude
natives returning to altitude after a stay at lower elevations (206, 391). The symptoms of acute
mountain sickness may or may not be present. The clinical features of HAPE are shown in
Table 3. The prevalence depends upon the altitude and the rate of ascent and individual
susceptibility with certain individuals developing the condition repeatedly on ascent to high
altitude.
Breathlessness:
Chest pain:
Headache:
Nocturnal dyspnoea:
Dry cough:
Haemoptysis:
Nausea :
Insomnia:
Dizziness:
Pyrexia (≥ 36.8°C ≤ 39°C):
Heart rate > 120/min:
Respiratory rate > 30/min:
Cyanosis:
83.2%
65.3%
62.4%
58.4%
50.5%
38.6%
25.7%
22.8%
17.8%
70.0%
69.3%
68.3%
51.5%
Table 3: Signs and symptoms of high altitude pulmonary oedema (HAPE).
Source: Menon ND, High-Altitude Pulmonary Edema: A Clinical Study.
N Engl J Med 273: 66-73 1965.
A defining characteristic of patients with HAPE is an abnormally elevated pulmonary artery
pressor response to hypoxia. During episodes of HAPE systolic pulmonary artery pressure
(PAP) is elevated to around 60-80 mm Hg, but values as high as 144 mm Hg have been
reported (208). Drugs which lower PAP are effective in both the prevention and treatment of
HAPE (38, 267, 325, 381). Individuals susceptible to HAPE also have an exaggerated
pulmonary pressor response to exercise in normoxia (137); a tendency towards a lower
hypoxic ventilatory response (HVR) (182, 198, 287) and smaller lung volumes (137, 407)
compared with non-susceptible controls which may exacerbate alveolar hypoxia.
The mechanism responsible for the increased hypoxic pulmonary vasoconstriction is probably
multi-factorial. There is evidence for impaired endothelial function in HAPE-susceptible
individuals. Decreased exhaled nitric oxide (NO) concentrations were found in HAPE-
70
susceptible individuals during both acute hypoxia at sea level and during the development of
HAPE at 4559 m (75, 132). Nitrate and nitrite concentrations in broncho-alveolar lavage fluid
were found to be reduced in HAPE-susceptible individuals and increased in resistant subjects
at 4559 m (419). In addition in the Japanese population HAPE is associated with 2 endothelial
nitric oxide synthase (eNOS) polymorphisms associated with vascular disease (131) although
this is not the case in the Caucasian population (450). Sartori et al (378) found elevated
systemic venous plasma endothelin concentrations in HAPE-susceptible individuals compared
with normal controls at an altitude of 4559 m. Endothelin was reduced irrespective of whether
the subject developed HAPE and correlated well with the PAP. There is evidence of increased
sympathetic activity in HAPE-susceptible individuals both during HAPE (242) and, in a small
number of cases, preceding HAPE (40). In a study comparing a number of vasodilators the αblocker phentolamine was shown to be the most efficacious in reducing hypoxic pulmonary
vasoconstriction (181).
A high PAP is clearly necessary, but not sufficient alone for the development of HAPE.
Scherrer et al (381) found no difference in the systolic PAP between those susceptible
individuals who developed HAPE and those who did not. Maggiorini et al (269)
demonstrated an elevated pulmonary capillary pressure in individuals with HAPE. How can
pulmonary capillary pressure be elevated and lead to oedema if pulmonary arteries and
arterioles are constricted? A number of mechanisms have been proposed:
•
Patchy vasoconstriction with over perfusion of non-vasoconstricted areas. The
concept of increased capillary pressure due to high flow was proposed by Maurice
Visscher (436) and popularised by Hultgren (205, 207). Further evidence supporting this
theory has come from a study using fluorescent microspheres in a pig model which
suggests that hypoxic pulmonary vasoconstriction is inherently uneven (197) and from
functional magnetic resonance imaging in HAPE-susceptible and HAPE-resistant subjects
(200). During acute hypoxia the HAPE-susceptible individuals had increased
heterogeneity of pulmonary blood flow consistent with patchy hypoxic pulmonary
vasoconstriction.
•
Hypoxic venoconstriction. The venous side of the pulmonary circulation constricts in
response to hypoxia and animal studies have demonstrated that this can account for up to
20% of the total increase in pulmonary vascular resistance in hypoxia (11, 470). Thus
hypoxic pulmonary venosconstriction might explain the reason for the increase in
pulmonary capillary pressure seen in HAPE.
71
•
Pulmonary arteriolar leak. There is evidence from micropipette studies that the
capillary-veonus segment of the pulmonary circulation includes small arterioles up to 150
µm in diameter (184). Studies in the rat suggest that in the presence of high PAPs these
arterioles can leak (461).
Thus heterogeneous hypoxic pulmonary vasoconstriction could result in over perfusion of
capillary beds which would lead to increased capillary pressure because of pulmonary
venoconstriction. In addition abnormally elevated PAP could result in direct leakage from the
pulmonary arterioles. This increased hydrostatic pressure leads to leakage of high protein
fluid consistent with an increased permeability oedema.
West used the term “stress failure,” to describe the structural damage to the alveolar-capillary
barrier which could occur as a result of the increase in hydrostatic pressure (458-460). While
this may be a useful concept in severe or developed HAPE many of the markers associated
with stress failure are absent in early HAPE and it may be that the high permeability oedema
seen in early HAPE is due to more dynamic, non-traumatic opening of pores or transcellular
pathways (326, 468). This is supported by the finding that HAPE developed at a pulmonary
capillary pressure > 19 mm Hg which is less than the 24 mm Hg at which West and
colleagues reported the first signs of capillary stress failure and considerably less than the 39
mm Hg at which stress failure was commonly found (429, 458). West’s group demonstrated
rapid reversibility, within minutes, of stress failure, but hypothesised that this rapid reversal
can only occur when the basement membrane remains intact – i.e. genuine stress failure had
not occurred (138, 458).
2) Inflammation in HAPE
A role for inflammation in the pathogenesis of HAPE remains controversial and debated.
Schoene et al (384, 385) performed broncho-alveolar lavage (BAL) on subjects with HAPE
at 4400 m on Mount Denali. The BAL fluid from those subjects with developed HAPE had a
markedly elevated leucocyte content (predominantly alveolar macrophages) compared with
both normal controls and subjects with AMS. Levels of leukotriene (LTE) B4 and
complement were also elevated in those subjects with HAPE compared with the other 2
groups. Very high quantities of both large and small molecule proteins were present in the
HAPE BAL fluid. Kubo et al (245, 246) found elevated leucocytes (alveolar macrophages,
neutrophils and lymphocytes) and inflammatory cytokines (IL-1β, IL-6, IL-8, TNF-α and IL1ra) in patients evacuated to a low altitude hospital with HAPE compared with normal
72
controls. Kaminsky et al (228) found elevated urinary LTE-E4 in 38 patients with HAPE at 5
clinics in Colorado during the ski season. All of these patient groups however had established
HAPE and so it was not possible to say if the increase in inflammatory markers could be
implicated in the pathogenesis of HAPE or was merely a response to the increased protein
leak.
Swenson et al (419) performed a prospective BAL study in known HAPE-susceptible
subjects and in HAPE-resistant controls both prior to exposure to altitude and either
immediately before, or at, the onset of HAPE at an altitude of 4559 m. At 4559 m the mean
systolic PAP was 66 mm Hg in the HAPE-susceptible subjects and 37 mm Hg in the HAPEresistant controls. Out of 10 HAPE-susceptible subjects on whom control and altitude BALs
were performed, 3 developed HAPE on their second day at altitude before bronchoscopy and
6 developed HAPE on day 3 after bronchoscopy. The BAL fluid from the 7 HAPEsusceptibles who were well when they were bronchoscoped at 4559 m had significantly
higher erythrocyte counts, albumin and IgG than the HAPE-resistant subjects, while those 3
subjects who had recently developed HAPE when they were bronchoscoped had erythrocyte
counts, albumin and IgG that were even higher than the HAPE-susceptible subjects without
HAPE. Plotting the erythrocyte count against systolic PAP it would appear that erythrocytes
begin to leak into the alveoli at systolic PAPs of around 60 mm Hg although albumin appears
at around 35 mm Hg. There was no increase in inflammatory markers in the BAL (IL-1β; IL8; TNF-α; LTE-B4; PGE2; thromboxane) in the HAPE-susceptible subjects between low and
high altitude. Neither was there a significant difference in these markers between HAPEsusceptible or –resistant subjects at 4559m. Nitrate-nitrite concentrations were decreased in
the HAPE-susceptible subjects and increased in the –resistant subjects as discussed above.
This prospective study is important because it demonstrated evidence of a high permeability
oedema (high protein concentrations in the BAL fluid) at the onset of HAPE but in the
absence of inflammatory markers, excluding inflammation as a factor in the pathogenesis of
HAPE.
These findings are supported by studies of exhaled nitric oxide (NO) in subjects exposed to
acute hypoxia and during a stay at 4559 m. Exhaled NO is a non-invasive marker of
inflammation in the upper and lower airways and increases with increasing airway
inflammation (380). Duplain et al (132) measured oral exhaled NO in 28 HAPE-susceptible
and 24 control subjects before and during 48 hours at 4559 m. There was no difference
between the 2 groups at baseline but throughout the 48 hours at 4559 m, exhaled NO levels
were significantly lower in the HAPE-susceptible subjects compared to the control subjects.
73
Of the 28 HAPE-susceptible subjects, 13 developed radiological evidence of HAPE. Exhaled
NO was significantly lower in these 13 HAPE-susceptible subjects who developed HAPE
compared with the remaining HAPE-susceptible subjects who did not develop HAPE. There
was an inverse correlation between the systolic pulmonary artery pressure (PAP) and exhaled
NO level in all subjects. Busch et al (75) measured oral and nasal exhaled NO in 9 HAPEsusceptible and 9 control subjects during acute exposure to an inspired oxygen concentration
of 12%. In the control group there was no change from baseline in either oral or nasal exhaled
NO during 2 hours exposure to 12% O2. Nasal exhaled NO was also unchanged from baseline
in the HAPE-susceptible subjects during hypoxia while oral exhaled NO fell significantly.
There was an inverse correlation between the change in systolic PAP on exposure to hypoxia
and the fall in oral exhaled NO.
Inflammatory change could still occur as a secondary phenomenon in response to hypoxia; the
protein leak or damage to the capillary endothelium or alveolar epithelium and this could
further exacerbate the leak across the alveolar-capillary barrier (82, 408) or precipitate HAPE
in otherwise resistant individuals (82, 133).
3) Ion and fluid transport in hypoxia and HAPE
The importance of epithelial fluid transport in the resolution of pulmonary oedema and the
mechanisms underlying it have been discussed above. In both cultured cells and intact lungs,
hypoxia has been shown to down regulate Na+ and liquid transport.
Planes et al (340) studied primary cultures of rat ATII cells exposed to 3% O2 for 24 hours.
Hypoxia decreased the transepithelial Na+ current and the amiloride-sensitive portion of the
current by 37 and 45% respectively. There was no reduction in the mRNA coding for the
ENaC subunits or of subunit protein. There were however a decreased number of β- and γsubunits in the apical plasma membrane. Terbutaline reversed all effects of hypoxia on the
function and membrane insertion of the ENaC subunits. In a study of rat SV40-transformed
alveolar cells the same group (342) demonstrated a time and O2 concentration dependent
reduction in Na+-K+-ATPase pump activity after up to 24 hours exposure to 0% and 5% O2.
The effects of hypoxia could be completely inhibited by the Ca++ channel blocker, nifedipine.
Planes et al (341) demonstrated a reduction in
22
Na uptake in primary cultures of rat ATII
cells exposed to 3% and 0% hypoxia for 18 hours. At 0% the inhibition was time dependent.
It had become significant at 3 hours and peaked at 12 to 18 hours. There was also a decrease
in the mRNA coding for all 3 subunits of ENaC and a 42% decrease in α-ENaC protein
74
synthesis. There was a time dependent decrease in the ouabain-sensitive Rb current as well as
in mRNA coding for both the α- and β- subunits of the Na+-K+-ATPase pump. Although these
O2 concentrations might seem very low it should be remembered that they represent the
concentration within the incubator and do not equate with the O2 tension in the culture
medium. After 18 hours of 0%; 24 hours of 5% and 24 hours of 21% O2, the PO2 in the
culture medium of SV40-transformed rat ATII cells was 30, 50 and 140 mm Hg respectively
(342).
Mairbaurl et al (273) studied the effects of hypoxia on both A549 cells and primary cultures
of rat ATII cells. Hypoxia produced a PO2 dependent inhibition of the total and amiloride
sensitive
22
Na uptake and of Na+-K+-ATPase pump activity. These changes occurred within
30 minutes of exposure of the cells to hypoxia; were stable for 20 hours and were fully
reversed after 2 hours of normoxia. These changes could not be blocked by nifedipine. There
was a reduction in the number of Na+-K+-ATPase pumps and also Na+-K+-2Cl- cotransporters in the plasma membrane. Wodopia et al (467) studied A549 cells exposed to 3%
O2; primary cultures of ATII cells and whole lung preparations from rats exposed to 10% O2
for between 1 and 24 hours. In A549 cells there was a time dependent decrease in the protein
of all 3 ENaC subunits; in the α- and β- subunits of the Na+-K+-ATPase pump and the Na+K+-2Cl- co-transporter. In both cultured rat ATII cells and in the whole lung preparations
there was a 25% decrease in the α-subunit of the Na+-K+-ATPase pump. The changes in the βsubunit did not reach statistical significance. There was a decrease in mRNA coding for the αand β- subunits of ENaC (γ- could not be detected) after 24 hours exposure to 10% O2. The
amount of protein of the Na+-K+-2Cl- co-transporter initially increased after 1 hour exposure
to 10% O2, but it was decreased by nearly 40% after 24 hours. Mairbaurl et al (270)
extended this work further, studying the functional changes in the bioelectrical properties of
primary cultures of rat ATII cells exposed to 1.5% and 5% O2 for up to 24 hours in Ussing
Chambers. The short circuit current (Isc) decreased with the duration of hypoxia while little
change was seen in the transepithelial resistance (Rt). The amiloride sensitive portion of Isc
decreased proportionally with the time exposed to hypoxia but the amiloride insensitive
proportion of Isc did not. Isolation of the apical Na+ channels and basolateral Na+-K+-ATPase
pumps by permeabilisation of the opposing part of the plasma membrane demonstrated a
reduction in the capacity of the Na+-K+-ATPase pump and in apical amiloride-sensitive
transport with hypoxia.
Vivona et al (437) investigated the effects of hypoxia on alveolar Na+ and liquid transport in
an in vivo rat model. The animals were exposed to 8% O2 for up to 24 hours. Alveolar liquid
75
clearance fell in a time dependent manner. It was significantly reduced compared with
normoxia after 6 hours, reaching 50% of control conditions after 24 hours. After 24 hours of
re-exposure to normoxia alveolar liquid clearance had returned to control levels. Steady state
levels of mRNA were increased for the α-subunit of ENaC and the β-subunit of the Na+-K+ATPase pump while levels of mRNA for the γ-subunit of ENaC and the α-subunit of the Na+K+-ATPase pump were unchanged. Plasma membrane protein levels for the α-subunit of
ENaC and the α-and β-subunits of the Na+-K+-ATPase pump were unchanged in hypoxia. As
in primary cultures of rat ATII cells (340), the effect of hypoxia on Na+ transport was fully
reversed in the rats by instillation of terbutaline into the lungs.
Carpenter et al (83) measured Na+-K+-ATPase pump activity and the expression of mRNA
and proteins involved in sodium transport in rats exposed to normobaric hypoxia for 24 hours
at an FiO2 of 0.1. The Na+-K+-ATPase pump activity was decreased by 40% in hypoxia,
returning to control values on re-exposure to normoxia. Lung expression of mRNA and
protein for the α- β- and γ- subunits of ENaC and of the α-1 unit of the Na+-K+-ATPase pump
were unchanged after exposure to hypoxia. From these results, and those of Vivona et al
(437) and Planes et al (340), it would therefore seem that the reduction of Na+ transport seen
in hypoxia is, at least in part, due to a reduction in activity of the Na+ transport system as
opposed to a downregulation of Na+ transport protein expression.
Atrial natriuretic peptide (ANP) which, as discussed above, may inhibit alveolar liquid
clearance, is released by atrial myocytes primarily in response to atrial stretch. In hypoxia its
release may be mediated by a number of other stimuli including pulmonary hypertension,
tachycardia and via a neural reflex and may be important in reducing pulmonary artery
pressure. Subjects susceptible to HAPE have significantly increased ANP levels at altitude
compared with healthy controls in whom ANP levels are unchanged from sea level (356).
Increased levels of ANP, associated with hypoxia-induced pulmonary hypertension, could
thus inhibit alveolar liquid clearance and either precipitate or exacerbate HAPE.
Endogenous digitalis-like compounds secreted by the adrenal glands inhibit the Na+-K+ATPase pump and have been implicated in the pathophysiology of hypertension (386). A
digitalis-like compound was found to be elevated in both plasma and urine in healthy human
subjects after 20 day’s exposure to 4930 m (115) and the levels of a digitalis-like compound
were found to be inversely related to the degree of arterial hypoxaemia in patients with
chronic obstructive pulmonary disease (147). Release of a soluble digitalis-like compound
might explain the reduction in Na+-K+-ATPase pump activity seen in hypoxia (83, 342, 427).
76
SUMMARY OF ION AND FLUID TRANSPORT IN HYPOXIA
These results in various cultured alveolar cell lines and in an intact rat model demonstrate a
clear O2 dependence of transepithelial Na+ transport. Allowing for the differences between
cell lines, which raise important questions as to the most appropriate model in which to study
the effects of hypoxia (273), it would appear that the decrease in Na+ transport and alveolar
liquid clearance with hypoxia does not depend upon downregulation of Na+ transport proteins
but rather upon a downregulation in their functional activity. The mechanism by which this
occurs is unclear. Likewise the signalling pathway by which these changes occur is also
unclear although there is growing evidence of a role for reactive O2 species (ROS) and
reactive nitrogen species in the regulation of ion channel function (95, 108, 285). Endogenous
digitalis-like compounds and ANP may also play a role in hypoxic inhibition of
transepithelial Na+ transport and alveolar liquid clearance.
4) Problems of measuring alveolar liquid clearance in vivo in humans
The above cellular and animal studies shed light on the control of Na+ transport and alveolar
liquid clearance under hypoxic conditions. However it is difficult to translate these studies
and investigate the effects of hypoxia in vivo in humans. Precise measurement of alveolar
liquid clearance would require serial bronchoscopies. While this is feasible in intubated and
ventilated patients (290, 448) repeated measurements on awake subjects in the high altitude
environment would be poorly tolerated and not be without risk. In addition lidocaine, a local
anaesthetic, which is instilled into the upper airways during bronchoscopy is known to reduce
alveolar liquid clearance (253). Investigation of the effect of hypoxia on alveolar liquid
clearance in humans must at present therefore rest on indirect measurement. These include
assessment of extra-vascular lung water; surrogate measurements of respiratory epithelial ion
transport and pharmacological interventions.
5) Nasal potential difference measurements
While measurement of alveolar ion transport in vivo in awake humans is not feasible,
measurement of the potential difference (PD) generated by ion transport in the nasal mucosa
can be used as a surrogate marker of ion transport in the more distal respiratory epithelium.
The nasal mucosa is also a polarised epithelium which contains the same transport proteins
77
(ENaC; Na+-K+-ATPase pump; Na+-K+-2Cl- co-transporter) as more distal respiratory
epithelium although the relative proportions of the individual transporters and their subunits
varies throughout the length of the respiratory tract (74, 144, 330, 339). In addition, the
alveolar epithelium which consists predominantly of squamous ATI cells with some cuboidal
ATII cells is primarily absorptive apart from the secretion of surfactant. The nasal epithelium
is a pseudostratified columnar, ciliated epithelium rich in goblet cells and which secretes
water, Cl- and mucus (68).
Despite these differences Knowles et al (238) described the measurement of transepithelial
PD in the nasal mucosa of normal subjects based on the work of Melon (294). Knowles et al
demonstrated a wide variation in the measured nasal potential difference (NPD) within the
nasal cavity due to the range of epithelial cell types present and observed that the magnitude
of the PD at a given site seemed to correlate with the amount of ciliated cells present. The
maximal PD was obtained at the inferior surface of the inferior turbinate although this could
easily be abolished by abrasion of the mucosa with a cotton bud demonstrating the importance
of a physically intact mucosa for meaningful measurements. Nasal PD measurements are
abnormally hyperpolarised in cystic fibrosis due to the increase in Na+ reabsorption (237).
Nasal PD has become a useful test in the diagnosis of cystic fibrosis, especially in those
patients with equivocal sweat tests (7, 239, 362). In patients with cystic fibrosis the NPD has
been shown to correlate with respiratory function (141).
Tomlinson et al (427) studied NPD in rats exposed to 24 hours of hypobaric hypoxia at a
barometric pressure of 0.5 atmospheres, equivalent to an approximate altitude of 5800 m. The
basal NPD depolarised after exposure to hypoxia (mean (± SEM) NPD pre-hypoxia -23.7 (±
0.8) mV; NPD after 24 hours of hypoxia: -18.8 (± 0.8) mV; p = 0.002). The portion of the
NPD inhibited by nebulised amiloride was significantly reduced by hypoxia (NPD after
amiloride pre-hypoxia: -11.8 mV (Δamiloride: 11.9 mV); NPD after amiloride after 24 hours
hypoxia: 13.3 mV (Δamiloride: 5.5 mV); p <0.001). Treatment with intraperitoneal ouabain
alone under normoxic conditions depolarised the NPD to a similar extent to amiloride alone
(16.8 ± 2.1 mV vs 16.1 ± 1.7 mV). The addition of amiloride to ouabain was additive and
produced a significantly greater depolarisation of NPD than either product alone (-12.8 ± 1.0
mV; p =0.045). The effect of ouabain under hypoxic conditions was no different to the effect
of hypoxia alone. These findings are consistent with the results from studies in cultures of rat
ATII cells and from intact lung preparations discussed in section 3 above and hypoxia appears
to inhibit nasal transepithelial Na+ transport in the rat via ENaC and the Na+-K+-ATPase
pump in a similar manner. Nasal PD measurement in the rat appears to correlate well with the
78
changes occurring in the more distal alveolar epithelium. In a subsequent study the same
group
(83) again demonstrated a similar depolarisation in the NPD in rats exposed to
normobaric hypoxia, with an FiO2 of 0.1, for 24 hours and which returned to normal on reexposure to normoxia. The changes in the expression of mRNA and proteins for sodium
transport, and in the activity of the Na+-K+-ATPase pump in this paper are discussed above.
Sartori et al (376) compared NPDs at sea level in HAPE-sensitive and HAPE-resistant
subjects as part of a study designed to look at the efficacy of the inhaled β2 agonist salmeterol
in the prevention of HAPE at high altitude. The basal NPD was 32% lower in the HAPEsensitive subjects compared with the HAPE-resistant subjects (17.2 ± 5.8 mV vs 25.4 ± 9.6
mV, p<0.001) and the HAPE-sensitive subjects also had a smaller proportion of the NPD
which was inhibited by amiloride (10 ± 4.6 mV vs 15.3 ± 7.3 mV, p<0.001). After
superperfusion with amiloride there was no longer any significant difference between the
HAPE-sensitive and HAPE-resistant subjects. These results are summarised in Table 4.
Low
altitude
Day 1 altitude
Day 2 altitude
Day 5 altitude
HAPE-R
HAPE-S
HAPE-R
HAPE-S
HAPE-R
HAPE-S
HAPE-R
HAPE-S
basal PD
-25.4 ± 9.6
-17.2 ± 5.8**
--
--
--
--
--
--
amiloride PD
-10.1 ± 6.8
-7.3 ± 5.2
--
--
--
--
--
--
Δ amiloride
15.3 ± 7.3
10.0 ± 4.6**
--
--
--
--
--
--
basal PD
-15.1 ± 6.7
--
-19.8 ± 6.6*
--
-19.0 ± 7.3*
--
-13.9 ± 4.0
--
amiloride PD
-12.2 ± 6.4
--
-15.7 ± 6.0
--
-16.0 ± 6.5
--
-11.9 ± 3.7
--
2.9 ± 1.7
--
4.2 ± 3.7
--
3.0 ± 1.5
--
2.0 ± 1.0
--
isoprenaline PD
-18.0 ± 7.3
--
-25.1 ± 7.5*
--
-23.4 ± 7.3
--
-21.9 ± 5.9
--
Δ isoprenaline
-5.8 ± 3.3
--
-9.4 ± 4.3
--
-7.4 ± 5.7
--
-10.0 ± 6.1
--
basal PD
21
15**
31
27
67*
46*
--
--
amiloride PD
12
9**
29
24
67*
44*
--
--
Δ amiloride
9
6
2*
3
0*
2
--
--
Δ isoprenaline
15
14
32*
40*
25*
22
--
--
basal PD
-25.6 ± 9.4
-18.0 ± 6.2**
--
--
-22.9 ± 9.2
-12.5 ± 6.8** *
--
--
amiloride PD
-12.0 ± 7.0
-8.3 ± 2.8**
--
--
-8.2 ± 5.5*
-5.3 ± 3.1** *
--
--
Δ amiloride
14.8 ± 7.7
10.0 ± 5.1**
--
--
12.8 ± 6.6
10.3 ± 7.3
--
--
Sartori (2002)
Mason (2003)
Δ amiloride
Mairbaurl
(2003)†
Sartori (2004)
Table 4: Summary of results from studies investigating nasal potential difference in human subjects at altitude.
All results, except Mairbaurl (2003), mean ± SD
†: approximate results for Mairbaurl (2003) estimated from graphs
HAPE-R: HAPE resistant or healthy subjects HAPE-S: HAPE susceptible subjects
* Significant difference from low altitude
** Significant difference between HAPE-S and HAPE-R
79
The effects of inhaled salmeterol on NPD were not assessed. The effects on the incidence of
HAPE are discussed below (see: Pharmacological interventions).
Mason et al (282) studied NPD in 20 healthy subjects during a 2 week stay at 3800 m.
Satisfactory data were obtained in 18 out of the 20 subjects. In one subject no stable trace was
ever obtained while another subject could not tolerate the nasal catheter which caused
repeated gagging. In view of the problems reported by Mairbaurl et al (272), subjects lavaged
their nostrils morning and evening with 0.9% saline. As a result no problems were
encountered with nasal drying or crusting. Basal NPD hyperpolarised from a control value of
-15.1 ± 1.6 mV (mean ± SEM) to -19.8 ± 1.6 mV on the first day at 3800 m. The basal NPD
had returned to control values after 5 days at altitude. The amiloride-sensitive portion of the
NPD did not change on ascent to altitude. Although isoprenaline-stimulated Cl- secretion
increased from a control value of -5.8 ± 0.8 mV to -9.4 ± 1.0 mV on day 1 at altitude, this did
not reach statistical significance. These results are summarised in Table 4. There was a
statistically significant relationship between NPD and the normalised change in electrical
impedance tomography (EIT) between RV and TLC (r2= 0.42 p<0.001) with less
hyperpolarised (i.e. closer to zero) NPDs associated with greater normalised changes in EIT
(suggestive of increased extravascular lung water).
Mairbaurl et al (272) measured NPD in HAPE-sensitive and HAPE-resistant subjects on
ascent to the Margherita Hut at 4559 m. In the 12 HAPE resistant subjects the basal NPD
showed marked hyperpolarisation on ascent to altitude (the estimated NPD values are
summarised in Table 4. Absolute values are not quoted in the article and so have been
estimated from the graphs in the paper). This was associated with a significant fall in the
amiloride-sensitive portion of the NPD, such that on the second day at altitude it was
undetectable, and a significant increase in isoprenaline-stimulated Cl- secretion during both
days at altitude.
Similar to Sartori’s findings (376), the basal NPD was around 20% less negative at sea level
in the HAPE-susceptible subjects (approximately -15 mV vs -21 mV in HAPE-resistant
subjects p<0.05). The NPD hyperpolarised on ascent to 4559m. Although less than the
HAPE-resistant subjects the difference was not statistically significant. Unlike the HAPEresistant subjects there was no change in the amiloride-sensitive portion of the NPD at
altitude. Isoprenaline-stimulated Cl- secretion also increased on day 1 at altitude but had
returned to sea level values by the second day. In neither group was there any demonstrable
relationship between the PaO2 or SaO2 and the NPD.
80
Because of technical difficulties in obtaining NPD measurements at altitude where drying and
crusting of the nose made obtaining results difficult, an acute normobaric hypoxia study was
performed in which 17 subjects were exposed to 12% O2 for 6 hours but with humidity
controlled at around 50%. Surprisingly there was no change in basal NPD, the amiloridesensitive portion or in isoprenaline-stimulated Cl- secretion.
Sartori et al (377) effectively repeated the work of Mairbaurl et al (272) by studying NPD on
ascent to the Margherita Hut at 4559 m in 21 HAPE-susceptible and 29 HAPE–resistant
subjects. The basal NPD at low altitude was significantly less hyperpolarised in the HAPEsusceptible subjects than the –resistant subjects (18.0 ± 6.2 mV vs 25.6 ± 9.4 mV, p<0.05).
On ascent to 4559 m there was no change in the basal NPD in the HAPE-resistant subjects but
in the HAPE-susceptible subjects the basal NPD fell to 12.5 ± 6.8 mV (p<0.05 cf with both
low altitude and –resistant subjects at 4559 m). There was no change in the amiloridesensitive portion of the NPD in either the HAPE-resistant or HAPE-susceptible subjects
between low altitude and 4559 m, but at low altitude the amiloride-sensitive portion of the
NPD was significantly lower in the HAPE–susceptible subjects compared with the HAPEresistant subjects (10.0 ± 5.1 mV cf 14.8 ± 7.7 mV, p<0.05). The amiloride-insensitive
portion of the NPD fell in both groups on ascent to altitude. It was significantly lower at both
low and high altitude in the HAPE-susceptible subjects compared with the HAPE-resistant
subjects. These results are summarised in Table 4.
Maggiorini et al (267) measured NPD as part of study investigating whether dexamethasone
and the phosphodiesterase-5 inhibitor, tadalafil, could be used to prevent HAPE on ascent to
4559 m in HAPE-susceptible individuals. The placebo group consisted of only 9 subjects and
results, presented as changes in NPD from baseline rather than absolute values, are difficult to
interpret. Although they appear to be in keeping with the findings of Mason et al (282) and
Mairbaurl et al (272) they are not discussed further here.
81
SUMMARY OF STUDIES OF NASAL POTENTIAL DIFFERENCE AT ALTITUDE
The results from human studies of NPD at high altitude are conflicting. In normal subjects
(i.e. those not susceptible to HAPE), Mason et al (282) and Mairbaurl et al (272) both
demonstrated a hyperpolarisation in the basal PD on ascent to altitude although their results
differed in a number of ways. Mason et al demonstrated hyperpolarisation of the basal PD on
day 1 and day 2 at 3800 m and Mairbaurl et al on day 2 at 4559 m. Although Mason et als’
low altitude basal PDs were only slightly lower than those of Mairbaurl et al, ascent to 3800
m did not produce such a large hyperpolarisation as seen by Mairbaurl et al. In addition
Mairbaurl et al demonstrated both a reduction in the amiloride sensitive PD and also an
increase in isoprenaline stimulated Cl- secretion (Δamiloride and Δisoprenaline in Table 4)
which were unchanged in the study of Mason et al. In contrast Sartori et al (377) could
demonstrate no change in basal PD on ascent to the same altitude as Mairbaurl et al while the
amiloride-sensitive portion of the PD fell at altitude.
It is difficult to reconcile the differences between the above studies. The findings of Sartori et
al (377) are consistent with cellular and animal data on the effects of hypoxia on Na+ transport
in respiratory epithelia in that they demonstrate a decrease in amiloride sensitive transport.
However both Mason et al (282) and Mairbaurl et al (272) demonstrated hyperpolarisation on
ascent to high altitude in very different circumstances and in different ethnic populations.
Mairbaurl et al also demonstrated a reduction in the amiloride-sensitive PD at altitude in
keeping with Sartori et als’ findings and with the cellular and animal data. Any
hyperpolarisation must be due to either an increase in Na+ absorption, the opposite of what is
seen in cell culture and animal models in hypoxia, or Cl- secretion as occurs in the foetal lung
or in the adult nose in response to environmental stimuli such as nasal dryness (36). While
Mairbaurl et al reported problems with nasal drying and crusting this was avoided by Mason
et al by regular nasal lavage with 0.9% saline solution.
Mason et als’ study took place at an altitude 750 m lower than that of Mairbaurl et al and
Sartori et al and subjects had almost no AMS. A prevalence of AMS of 53% has been
reported at the Margherita hut used by Mairbaurl et al and Sartori et al for the high altitude
part of their studies (268). The lower altitude with the resultant higher partial pressure of
oxygen or the lower incidence of AMS could explain the difference in the amiloride-sensitive
82
portion of the PD and in isoprenaline stimulated Cl- secretion. As discussed above the
reduction in Na+ transport increases with increasing hypoxia. In addition Mairbaurl et als’
study was on white Caucasians whereas the subjects in Mason et als’ study were Altai
subtype Mongolians. A difference in NPD exists between blacks and whites (213) but little is
known about other racial differences in NPD or respiratory epithelial ion transport. It is
possible that racial differences in ion transport may explain the disparity in the results.
Variations in technique may also explain some of the differences seen (5) as may differences
in temperature of the nasal solutions, especially the results for Cl- secretion (69). It will not be
possible to resolve these apparent contradictions without further studies in healthy subjects,
carefully controlling these different variables.
Results for HAPE-susceptible individuals seem clearer. In all three studies in which NPDs
have been compared, HAPE-susceptible individuals have a lower basal PD (i.e. less negative)
at low altitude than HAPE-resistant controls (272, 376, 377). In addition in both studies from
Sartori’s group (376, 377) the amiloride-sensitive portion of the PD was reduced in the
HAPE-susceptible subjects compared with –resistant controls. Although it was also lower in
Mairbaurl et als’ study it did not reach statistical significance. These findings at low altitude
suggest that HAPE-susceptible individuals have an underlying defect in Na+ transport which
if paralleled in the distal lung epithelium would be accompanied by decreased lung liquid
clearance. A similar abnormal basal NPD and reduced amiloride-sensitive PD has been shown
in α-ENaC knockout mice which are susceptible to developing pulmonary oedema (135).
In view of these findings it is possible to speculate that in HAPE-susceptible individuals an
underlying defect in alveolar Na+ and liquid transport results in reduced clearance of lung
water which is worsened with increasing altitude and hypoxia. This will worsen alveolar
hypoxia, decreasing further alveolar Na+ transport and increasing PAP and pulmonary
capillary pressure resulting in an increased hydrostatic leak of fluid into the interstitial space
and alveoli which the Na+ transport system cannot clear. A vicious circle is thus set up in
which worsening hypoxia results in higher vascular pressures; an increased hydrostatic leak
and further reductions in an already impaired alveolar liquid transport mechanism. HAPE
ensues.
In normal, HAPE-resistant, subjects the situation appears more complex and speculation is
hampered by the lack of coherent data. If alveolar Na+ transport which is normal at sea level
is inhibited by hypoxia any resulting reduction in lung liquid clearance could easily
contribute to the formation of a sub-clinical pulmonary oedema. Alternatively if the hyperpolarisation reported by Mason et al and Mairbaurl et al is reflected in the distal respiratory
83
epithelium, is sub-clinical pulmonary oedema the result of hypoxia stimulated Cl-, and
therefore water, secretion as occurs in the foetal lung?
6) Pharmacological interventions
Sartori et al (376) compared the effect of the β2 agonist salmeterol (125 mcg twice daily)
with placebo in 37 HAPE-susceptible subjects exposed to 4559 m in a double-blind controlled
trial to test that hypothesis that β-receptor stimulation increases lung water clearance and
would act to protect against the development of HAPE. The incidence of HAPE was
dramatically reduced in the treatment group who commenced salmeterol on the day before
ascent and continued it throughout the study. Seventy-four percent of the control group and
only 33% of the treatment group developed clinical or radiological evidence of HAPE without
any difference in pulmonary artery pressure between the 2 groups. While at first sight this
result is impressive with an absolute risk reduction of 0.40 (number needed to treat to prevent
1 case of HAPE = 2.5) which compares very favourably with nifedipine, the standard
treatment for and prophylaxis against HAPE, with an absolute risk reduction of 0.54 [309]
(number needed to treat = 1.8), the study has a number of significant weaknesses. No
description is given in the paper as to how blinding or randomisation took place. The choice
of salmeterol and the dose used, which is nearly 3 times the normal asthma dose (1), are
controversial. Salmeterol in addition to its β-receptor agonist action could have reduced
oedema formation by other mechanisms including a reduction in PAP; an anti-inflammatory
action or a direct effect on pulmonary capillary integrity. These shortcomings make it
impossible to draw any meaningful conclusions from this paper and, to date, these findings
have not been replicated in another study.
Maggiorini et al (267) compared the phosphodiesterase-5 inhibitor tadalafil with
dexamethasone and placebo in 29 HAPE-susceptible subjects ascending to 4559 m. Seven out
of 9 subjects taking placebo developed HAPE at 4559 m. Only 1 out of 8 subjects on tadalafil
developed HAPE ( p = 0.001 vs placebo) while no subjects on dexamethasone developed
HAPE (p < 0.001 vs. placebo and p = 0.26 vs tadalafil). From these results it would appear
that the efficacy of both drugs in preventing HAPE is similar to the standard therapy of
nifedipine (38). The efficacy of tadalafil is unsurprising, in view of its mechanisms of action
(302) and the pathophysiology of HAPE, and will not be discussed further here.
The results of this study and case reports of HAPE developing in patients given
dexamethasone acutely to treat severe AMS (42, 305) would suggest that dexamethasone
84
needs to be taken prophylactically to be effective in preventing HAPE. This implies an effect
on gene expression. As discussed above, dexamethasone has been shown to upregulate
various components of the Na+ transport pathway in cell lines (31, 109, 259) and to increase
alveolar liquid clearance in rats (148, 314), as discussed above. However in Maggiorini’s
study there was no demonstrable difference difference between the dexamethasone or placebo
groups in nasal potential difference or in leucocyte mRNA expression for the α1-subunit of the
Na+-K+-ATPase pump or the β-subunit of ENaC which, while surrogate markers for alveolar
changes, would argue against the effects of dexamethasone in preventing HAPE being
mediated by increases in alveolar liquid clearance. Surprisingly dexamethasone was as
effective as tadalafil in reducing the increase in pulmonary artery pressure on ascent to 4559
m and also resulted in a comparable increase in urinary cyclic guanosine monophosphate
(cGMP) as seen with tadalafil. The mechanism by which dexamethasone reduced the
incidence of HAPE in this study would therefore appear to be by influencing the eNOS-NOcGMP pathway which is known to be defective in HAPE (75, 132) and not via an increase in
alveolar liquid clearance.
85
ALTITUDE-RELATED COUGH
Published in modified form as:
Mason NP, Barry PW, Altitude-related cough,
Pulm Pharmacol Ther; 20:388-395, 2007
Anecdotal reports of a troublesome cough affecting visitors to high altitude and which can be
severe enough to cause rib fractures are widespread within the mountaineering community
(401, 406, 421). In one survey of 283 high altitude trekkers in the Everest region of Nepal,
42% complained of cough (303) while in another study in the same region the prevalence of
cough was found to be 22% between 4243 and 4937 m (43). In the first formal study of cough
at high altitude, Barry et al (37) studied 10 subjects trekking to Everest Base Camp (5300 m)
using nocturnal cough frequency monitoring and demonstrated a rise in nocturnal cough
frequency with increasing altitude (Figure 9). The 3 climbers in whom recordings were
possible above Base Camp demonstrated a massive increase in nocturnal cough frequency. As
part of the same study citric acid cough threshold was measured in 42 subjects at sea level and
on arrival at Base Camp and in 23 of these same subjects after they had spent at least 9 days at
or above 5000 m. Citric acid cough threshold was unchanged on arrival at 5300 m compared
with sea level but was significantly reduced on the second visit to base camp compared with
both the sea level and first visit to Base Camp measurements. Despite both anecdotal and
observational evidence of an increase in cough with altitude, associated with a change in
cough threshold, the aetiology of the condition is poorly understood and treatment is
symptomatic and unsatisfactory.
Figure 9: Change in nocturnal cough frequency in 10 subjects trekking to Everest Base Camp, Nepal (5300m) and in 3 climbers on Mount
Everest. *: in this subject the recorder suffered battery failure after only 1 hour.
Data plotted from: Barry PW et al. Cough frequency and cough-receptor sensitivity are increased in man at altitude. Clin Sci (Colch) 93:181186, 1997. (Reference 37)
86
1) Aetiology of altitude-related cough
Altitude-related cough has traditionally been attributed to the inspiration of the cold, dry air
which characterises the high altitude environment (421). However, during Operation Everest
II the majority of the subjects developed pain and dryness in the throat and an irritating cough
at altitudes above 7000 m despite the chamber being maintained at a relative humidity of
between 72 and 82% and a temperature of 23° C (201). This argued against the widely held
view that cough at high altitude was due to the inspiration of dry, cold air.
In the next major hypobaric chamber study, Operation Everest III, Mason et al (280) studied
nocturnal cough frequency and citric acid cough threshold during Operation Everest III.
During the experiment the temperature and relative humidity of the chamber were maintained
at between 18 and 24 °C and 30-60% respectively. Nocturnal cough frequency increased in
the 8 subjects in the study with increasing altitude. Of note, when the subjects descended to
5000 m to recuperate before the ascent to the "summit," the cough frequency fell before
increasing again on ascent to 8000m. Cough frequency immediately returned to control values
on descent to sea level. This data is shown in Figure 10.
Figure 10: Change in nocturnal cough frequency in 8 subjects during Operation Everest III.
Box plots indicate the median and 25th and 75th percentile. Whiskers indicate the 10th and 90th percentiles. *: p<0.05 cf sea level control.
Data plotted from: Mason NP, et al. Cough frequency and cough receptor sensitivity to citric acid challenge during a simulated ascent to
extreme altitude. Eur Respir J 13:508-513, 1999 (Reference 280)
87
Citric acid cough threshold was measured at sea level, 5000 m, the return to 5000 m and at
8000m. There was no difference between either 5000 m or the return to 5000 m and the sea
level control, which may have been due to inadequate study power. A maximum of 8 people
could be accommodated in the hypobaric chamber, whereas a priori calculations based on the
results of previous work (37) suggested that at least 9 subjects would be required to detect a
significant fall in cough threshold. Cough threshold was however reduced at 8000 m
compared to both sea level and the first arrival at 5000 m, despite the constant environmental
humidity and temperature. The results of Operation Everest III appear to refute the hypothesis
that altitude-related cough is simply due to the inspiration of dry, cold air.
If altitude-related cough is not simply due to the inspiration of dry, cold air, what might its
aetiology be? There are a number of potential mechanisms:
• Acute mountain sickness (AMS).
• Sub-clinical high altitude pulmonary oedema.
• Changes in the central and peripheral control of cough.
• Respiratory tract infections.
• Loss of water from the respiratory tract.
• Bronchoconstriction and asthma.
• Vasomotor-rhinitis and post-nasal drip.
• Gastro-oesophageal reflux.
a) Acute mountain sickness
Despite both AMS and cough occurring commonly at high altitude, no relationship has been
demonstrated between them in any study of altitude-related cough (37, 280, 283, 426). In 21
studies dealing with AMS cough was not a reported symptom (39). A failure to demonstrate a
relationship between altitude-related cough and AMS may reflect the poor sensitivity of the
Lake Louise Scoring system (360), the standard tool used to measure the presence and
severity of AMS, to detect mild AMS, but overall it seems unlikely that AMS plays a part in
the aetiology of cough at high altitude.
b) Sub-clinical high altitude pulmonary oedema
As discussed above the majority of subjects ascending to high altitude may develop a degree
of sub-clinical pulmonary oedema. Is it possible that altitude-related cough is merely a
88
manifestation of sub-clinical pulmonary oedema? This question requires two further questions
to be answered: firstly can pulmonary oedema per se cause cough and secondly, if it can, is
there a sufficient stimulus in asymptomatic subjects at altitude to activate such a pathway?
The apparently simple question as to whether pulmonary oedema per se can cause cough
illustrates the uncertainty that continues to exist as to the precise identity and interplay of the
afferent pathways responsible for cough (80, 463, 465). However a number of pertinent
observations can be made: in cardiogenic pulmonary oedema cough is not a predominant
clinical feature although dyspnoea and tachypnoea are (310); the tachypnoea and dyspnoea
associated with pulmonary oedema are mediated via bronchopulmonary C-fibres (291, 331)
but whether bronchopulmonary C-fibres can also produce cough remains strongly debated
(463, 465); the major known sites which can produce cough in humans are located above the
segmental airways (division 4) (291, 405) significantly more proximal than the small airways
in which one would expect to see the first evidence of sub-clinical pulmonary oedema (405).
There is however evidence that even small amounts of sub-clinical oedema could be sufficient
to stimulate any cough receptors that might exist in the distal airways.
In a series of experiments in rabbits, Kappagoda and colleagues demonstrated that even
small changes in left atrial pressure of between 2 to 5 mm Hg resulted in sufficient pulmonary
venous congestion to stimulate airway rapidly adapting receptors (59, 178, 179, 187). While it
is not known what these pressure changes equate to in humans, it is easy to imagine that the
systolic pulmonary artery pressures of over 40 mm Hg recorded in healthy control subjects at
4559 m (381) would result in a similar amount of congestion as an increase in left atrial
pressure of 2 to 5 mm Hg. Equally a reduction in respiratory epithelial fluid clearance and the
resultant increase in extra-vascular lung water is another possible mechanism by which
airway cough receptors could be stimulated at high altitude.
If a reduction in respiratory epithelial fluid clearance were an aetiological factor in altituderelated cough then it would be expected that agents that augment the clearance of lung water,
such as β-agonists, might be expected to improve altitude-related cough. Sartori et al (376)
described the effectiveness of inhaled salmeterol in reducing the incidence of HAPE in a
group of HAPE-susceptible subjects but any effect on cough was not described in the paper.
In contrast a randomised, placebo controlled trial investigating the effect of inhaled B2
agonists on altitude-related cough, published in abstract form only (17) did not show any
change in cough receptor sensitivity in individuals inhaling salmeterol compared to controls.
However, the study was small, and the incidence of hypoxia induced change in lung water
clearance may have been reduced by a very conservative ascent profile.
89
The natural history of HAPE, which is clearly associated with a recent gain in altitude and
improves dramatically with descent, is similar to the cough seen in Operation Everest III
(280) but unlike that of other altitude-related cough that may occur during a prolonged period
at one altitude when acclimatisation may be considered to be well advanced and which can
persist for a time after descent even to sea level. Furthermore, there are both functional and
structural differences in ion transport between HAPE-susceptible and control subjects (271,
272, 377). If altitude-related cough was due to pulmonary oedema, one might expect HAPEsusceptible subjects to have more severe cough. This is not described in the literature.
c) The central control of cough
Respiratory control undergoes profound changes with acclimatisation (446). The central
control of cough is complex and poorly understood (58, 60, 80, 322, 332, 465) and while it
may be incorrect to draw too close parallels between the central controls of respiration and
cough, a number of factors which suppress cough also suppress ventilation, such as sleep
(129, 164) and centrally acting anti-tussive agents (180). In addition Banner (19)
demonstrated a relationship between the hypercapnic ventilatory response (HCVR) and the
cough threshold to hypotonic saline. Those subjects who responded to the hypotonic saline
challenge had a higher HCVR than the subjects who did not respond. Post-hoc analysis of
data from the 1994 British Mount Everest Medical Expedition also demonstrated a
relationship between the citric acid cough threshold and the dynamic ventilatory response to
CO2 (34).
As a result of these findings Thompson et al (426) studied the relationship between the citric
acid cough threshold and the hypercapnic ventilatory response (HCVR) in 25 healthy subjects
during a 9 day stay at 5200 m. Citric acid cough threshold fell significantly on ascent to
altitude and the HCVR, measured using the Read re-breathing method and expressed by the
slope S, increased significantly on ascent to 5200 m. There was however no demonstrable
relationship between the citric acid cough threshold and HCVR, or any change in these
parameters on ascent to altitude. It is therefore unlikely that altitude-related cough is mediated
through changes in central control mechanisms.
d) Bradykinin and altitude-related cough
While the neuronal pathways that mediate the citric acid cough threshold remain debated they
include stimulation of airway sensory nerves including the rapidly adapting receptor (RAR)
90
(80). Cough is a well recognised side effect in a proportion of patients taking angiotensin
converting enzyme (ACE) inhibitors and is thought to be due to sensitisation of airway RARs
by increased levels of plasma bradykinin and substance P (216). Bradykinin is degraded by a
number of enzymes collectively known as kininases which include ACE, aminopeptidase P
and neutral endopeptidase and in human serum over 75% of bradykinin metabolism occurs
via ACE (54, 119). The early literature on the response of serum ACE activity to hypoxia in
humans is confusing and contradictory (418). Nothing was known about what happened to
bradykinin at altitude beyond exposure to 1 hour of normobaric hypoxia (10).
Mason et al (283) measured citric acid cough threshold, angiotensin converting enzyme
activity and plasma bradykinin concentration in 20 healthy volunteers before and during a
stay at 3800 m. Serum ACE activity was unchanged on ascent to 3800 m, although plasma
bradykinin fell significantly suggesting that local endothelial ACE activity may have been
responsible for the fall in plasma bradykinin on ascent to altitude, or that it was metabolised
by a kinin other than ACE. It is thus unlikely that bradykinin plays a role in the change in
citric acid cough threshold seen on ascent to altitude.
e) Respiratory tract infections
Respiratory tract infections are the commonest cause of acute cough at sea level (215, 292,
346) and occur commonly in visitors to altitude (44, 303, 349). There is also evidence of
impairment of mucociliary transport, a crucial respiratory defence mechanism, at altitude
(35). While there was no clinical evidence of respiratory infection observed in any of the
subjects during Operation Everest III (357), anecdotally cough associated with the production
of purulent sputum is a not uncommon finding at altitude, particularly following prolonged
vigourous exertion. Although the anecdotally poor response of this productive cough to
antibiotic therapy argues against bacterial infection, prolonged cough following relatively
minor atypical bacterial respiratory infections are well recognised at sea level (55, 186). Viral
infections are also a common cause of cough (217) but little is known about the incidence of
viral infections in visitors to altitude and evidence for immune dysfunction at altitude is
limited (293).
f) Respiratory water loss
It is still possible, despite the increase in nocturnal cough frequency and decrease in citric acid
cough threshold observed under the controlled environmental conditions of Operation Everest
91
III (280) that water loss from the respiratory tract plays a role in the aetiology of altituderelated cough. Banner et al (20) studied subjects susceptible to exercise-induced cough and
found that hyperpnoea for 4 minutes with cold air, at respiratory rates similar to those
occurring with strenuous exercise, was associated with an increase in cough frequency in the
30 minute period post-hyperpnoea. All subjects showed no evidence of bronchial
hyperreactivity to a methacholine challenge and in addition continued to cough despite
blocking the minimal bronchoconstriction that occurred with inhaled β2 agonists.
Extending this work, again in subjects susceptible to exercise induced cough, by using
voluntary isocapnic hyperpnoea to over 90 litres / minute with a variety of inspired air
conditions, Banner et al (21) demonstrated that the cough frequency depended directly upon
respiratory water loss in a linear manner and that in the absence of water loss, even in the
presence of heat loss, cough frequency did not increase. Hyperpnoea with warm, dry air
produced more coughing than hyperpnoea with cold air despite less heat loss. Hyperpnoea
with ambient air produced as great an increase in cough frequency as hyperpnoea with cold
air because it was associated with significant water loss. These findings are illustrated in
Figure 11.
Figure 11: Number of coughs in excess of baseline (Cough); respiratory water loss (RWE) and respiratory heat loss (RHE) in 7 subjects with
exercise-induced cough during a 19 minute period after 4 minutes of hyperpnoea with cold air at -16ºC or warm, dry air at 38.7ºC. Both the
cold air and the warm dry air had a water content of 0 g m-3.
From: Banner et al, Relation of respiratory water loss to coughing after exercise.
N Eng J Med 311: 883-886, 1984.
92
These findings raise the possibility that, at least in a subgroup of sufferers, altitude-related
cough may be caused by water loss from the respiratory tract. How this water loss might
stimulate afferent cough pathways is not known but possible mechanisms include physical
distortion of afferent nerve endings (375) and changes in the ionic composition of airway
surface liquid (195). Increased minute ventilation is an early and prominent feature of the
body’s response to hypobaric hypoxia. It increases with increasing altitude and will increase
further with exercise (279). It is therefore possible that despite the controlled environmental
conditions seen during Operation Everest III (280) respiratory water loss could have
contributed to the increase in cough frequency and sensitivity demonstrated. There is only
limited data available on respiratory water loss at altitude and only during exercise (348)
while in the Operation Everest II and III chamber studies the subjects were mainly
sedentary22.
Other factors may also increase respiratory water loss. There is evidence of subjective nasal
blockage and an increase in nasal resistance at altitude which may result in increased mouth
breathing (35, 36, 464) and it is known that water loss increases during mouth, rather than
nasal breathing (417).
g) Bronchoconstriction
Cough may be the only symptom of asthma (121, 122). Bronchoconstriction occurs after
hyperpnoea with cold air in asthmatics (117, 411) and in healthy subjects (165, 333) although
the mechanism again appears to be related to water loss rather than an effect of temperature
(410). In healthy subjects the citric acid cough threshold is not changed by bronchodilator
therapy (48). None of the subjects in Operation Everest III had any history of exercise related
wheeze and the change in cough threshold was not associated with a change in FEV1 or PEF
during Operation Everest III (280) or at Mount Everest Base Camp (37). Both acute hypoxia
and hypocapnia have been shown to cause bronchoconstriction in animal models (169, 304,
354). However in a study at Mount Everest Base Camp Pollard et al (343) could demonstrate
no evidence of bronchoconstriction in healthy lowland subjects and Dehnert et al (120)
demonstrated no change in either effective or specific airway resistance on ascent to 4559 m.
22: Exercise at high altitude and the resultant high levels of ventilation do seem to be temporally related to a troublesome
cough. It is a personal observation that on a number of occasions when I have been well acclimatised to altitudes of between
4500 and 6000 m prolonged vigorous exercise over several hours at these altitudes has resulted in an almost constant dry,
non-productive cough, which has persisted for several days.
93
It is therefore unlikely that bronchoconstriction plays a major role in altitude-related cough in
non-asthmatic subjects.
h) Vasomotor rhinitis and post-nasal drip
Upper airway cough syndrome (previously known as post-nasal drip syndrome) is reported in
some series to be one of the most common causes of chronic cough (345) and although the
exact nature of the condition remains debated (301) there are a significant number of subjects
with chronic cough at sea level who respond to anti-histamine decongestant therapy (345).
Nasal blockage and symptoms of rhinitis are common complaints at altitude (35, 44) but their
relationship with cough has not been investigated.
i) Gastro-oesophageal reflux
Gastro-oesophageal reflux has been reported in up to 40% of patients with chronic cough at
sea level (214, 301, 347). Nothing is known about the incidence of gastro-oesophageal reflux
at altitude.
2) Changes in nebuliser output at altitude
One possible explanation for the findings in the studies of Barry et al (37) and Mason et al
(280) would be alterations in nebuliser output due to the reduction in barometric pressure
which occurs at altitude resulting in a false change in citric acid cough threshold. Svartengren
et al (416) demonstrated an increase in deposition of 3.6 to 3.8 micron Teflon particles in the
distal airways of the lung in both bronchoconstricted and non-constricted airways when using
a helium-oxygen mixture compared with air, although there was no difference in deposition in
the upper airways. While these results may, on first reading, suggest that the reductions in
citric acid cough sensitivity seen in the studies of Barry et al (37) and Mason et al (280) were
due to increased deposition of citric acid at altitude, it should be remembered that a heliumoxygen mixture is less dense compared to air but marginally more viscous and it is gas
viscosity which dictates the onset of the turbulent flow which predominates in the upper
airways where the majority of airway cough receptors reside and where Teflon deposition was
unchanged (266). At altitude there is a reduction in both gas density and viscosity.
Barry assessed the output of 3 types of nebulisers in a hypobaric chamber at sea level and
altitudes equivalent to 4200 and 5300 m by collecting the nebuliser output of salbutamol onto
94
a filter (32). The output from both a conventional jet nebuliser and a breath enhanced
nebuliser fell by over 50% at the simulated altitudes compared with sea level, while the output
of the ultrasonic nebuliser fell by only 23% at 4200 m and 13% at 5300 m compared with sea
level. In a further follow up study Barry et al (33) measured serum salbutamol levels in
human subjects following nebulised administration of salbutamol via an ultrasonic nebuliser
at both sea level and a simulated altitude of 5000 m in a hypobaric chamber. There was no
difference in serum salbutamol levels between the 2 altitudes. Taken together these results
suggest that providing an ultrasonic nebuliser is used at altitude, as was the case in the studies
of Barry et al (37) and Mason et al (280), changes in cough sensitivity due to alterations in the
delivery of tussive agents should be minimised.
3) Treatment of altitude-related cough
The treatment of cough at altitude is notoriously difficult and there are no controlled trials of
any therapy. Simple over-the-counter cough sweets; the use of face masks to warm and
humidify inspired air; inhaled corticosteroids and β2 agonists; anti-histamines and codeine
phosphate have all been used but their efficacy is unknown. As no treatment has gained
popularity it is likely that none is particularly effective. It is only once the aetiology of
altitude-related cough is understood that appropriate therapies can be developed.
95
CONCLUSIONS: ALTITUDE-RELATED COUGH
The nature and aetiology of cough at high altitude remains unclear and it is possible that the
term covers a number of conditions and aetiologies. While it has traditionally been
attributed to the inspiration of dry, cold air, evidence from Operation Everest III (280)
argues against this as the only aetiology for the condition. Anecdotally however, prolonged
exercise, presumably via the associated increase in ventilation, does seem to precipitate
cough at altitude in some individuals. This may be due to water loss from the respiratory
tract (20, 21) or trauma to the respiratory mucosa that may be complicated by infection. It
seems increasingly likely that altitude-related cough may be a symptom of a number of
unrelated conditions. It is possible to identify at least 2 forms of cough at altitude:
1. A cough which may occur at lower altitudes, which is related to exercise and persists
despite descent. The characteristics of this cough suggest trauma to the respiratory
mucosa possibly complicated by respiratory tract infection.
2. A cough which occurs at altitudes greater than 5000 - 6000m and which improves with
descent to lower altitudes. It does not depend upon the inspiration of cold, dry air and it
is likely to be due to sub-clinical pulmonary oedema.
Future work needs to:
•
Define more clearly the epidemiology of altitude-related cough.
•
Address the role of sub-clinical pulmonary oedema through interventional studies using
pulmonary vasodilators to block the hypoxia-mediated increase in pulmonary artery
pressure.
•
Investigate the role of water loss from the respiratory tract at altitude, particularly during
exercise.
•
Investigate the prevalence of upper respiratory tract infection at altitude and its
relationship to cough.
•
Investigate further the changes in respiratory defence mechanisms at altitude as well as
the prevalence of vasomotor rhinitis and gastro-oesophageal reflux and any relationship
that may exist between these conditions and altitude-related cough.
96
REFERENCES
1.
Adrenoceptor agonists. In: British National Formulary (51, March 2006 ed.): BMJ
Publishing Group, London, 2006, p. 144-149.
2.
ATS/ERS Statement on respiratory muscle testing. Am J Respir Crit Care Med 2002;
166: 518-624.
3.
Aarseth P, Bjertnaes L, and Karlsen J. Changes in blood volume and extravascular
water content in isolated perfused rat lungs during ventilation hypoxia. Acta Physiol
Scand 1980; 109: 61-67.
4.
Acarregui MJ, Brown JJ, and Mallampalli RK. Oxygen modulates surfactant
protein mRNA expression and phospholipid production in human fetal lung in vitro.
Am J Physiol 1995; 268: L818-825.
5.
Ahrens RC, Standaert TA, Launspach J, Han SH, Teresi ME, Aitken ML, Kelley
TJ, Hilliard KA, Milgram LJ, Konstan MW, Weatherly MR, and McCarty NA.
Use of nasal potential difference and sweat chloride as outcome measures in
multicenter clinical trials in subjects with cystic fibrosis. Pediatr Pulmonol 2002; 33:
142-150.
6.
Alcorn D, Adamson TM, Lambert TF, Maloney JE, Ritchie BC, and Robinson
PM. Morphological effects of chronic tracheal ligation and drainage in the fetal lamb
lung. J Anat 1977; 123: 649-660.
7.
Alton EW, Currie D, Logan-Sinclair R, Warner JO, Hodson ME, and Geddes
DM. Nasal potential difference: a clinical diagnostic test for cystic fibrosis. Eur Respir
J 1990; 3: 922-926.
8.
Amirav I and Plit M. Temperature and humidity modify airway response to inhaled
histamine in normal subjects. Am Rev Respir Dis 1989; 140: 1416-1420.
9.
Anholm JD, Milne EN, Stark P, Bourne JC, and Friedman P. Radiographic
evidence of interstitial pulmonary edema after exercise at altitude. J Appl Physiol
1999; 86: 503-509.
10.
Ashack R, Farber MO, Weinberger MH, Robertson GL, Fineberg NS, and
Manfredi F. Renal and hormonal responses to acute hypoxia in normal individuals. J
Lab Clin Med 1985; 106: 12-16.
97
11.
Audi SH, Dawson CA, Rickaby DA, and Linehan JH. Localization of the sites of
pulmonary vasomotion by use of arterial and venous occlusion. J Appl Physiol 1991;
70: 2126-2136.
12.
Azzam ZS, Saldias FJ, Comellas A, Ridge KM, Rutschman DH, and Sznajder JI.
Catecholamines increase lung edema clearance in rats with increased left atrial
pressure. J Appl Physiol 2001; 90: 1088-1094.
13.
Babcock MA, Johnson BD, Pegelow DF, Suman OE, Griffin D, and Dempsey JA.
Hypoxic effects on exercise-induced diaphragmatic fatigue in normal healthy humans.
J Appl Physiol 1995; 78: 82-92.
14.
Badgett RG, Lucey CR, and Mulrow CD. Can the clinical examination diagnose
left-sided heart failure in adults? JAMA 1997; 277: 1712-1719.
15.
Bai C, Fukuda N, Song Y, Ma T, Matthay MA, and Verkman AS. Lung fluid
transport in aquaporin-1 and aquaporin-4 knockout mice. J Clin Invest 1999; 103:
555-561.
16.
Baines DL, Ramminger SJ, Collett A, Haddad JJ, Best OG, Land SC, Olver RE,
and Wilson SM. Oxygen-evoked Na+ transport in rat fetal distal lung epithelial cells.
J Physiol 2001; 532: 105-113.
17.
Bakewell SE, Hart ND, Wilson CM, McMorrow R, Collier D, Williams D, and
PW B. A randomised, double blind, placebo controlled trial of the effect of inhaled
nedocromil sodium or salmeterol xinafoate on the citric acid cough threshold in
subjects travelling to high altitude (abstract). In: Hypoxia: Into the Next Millennium,
edited by Roach RC, Wagner PD and Hackett PH. New York: Plenum/Kluwer
Academic Publishing, 1999, p. 362.
18.
Bakhle YS. Pharmacokinetic and metabolic properties of lung. Br J Anaesth 1990; 65:
79-93.
19.
response to CO2 in normal subjects. Am Rev Respir Dis 1988; 137: 647-650.
20.
Banner AS, Chausow A, and Green J. The tussive effect of hyperpnea with cold air.
Am Rev Respir Dis 1985; 131: 362-367.
21.
Banner AS, Green J, and O'Connor M. Relation of respiratory water loss to
coughing after exercise. N Engl J Med 1984; 311: 883-886.
98
22.
Barker PM, Brigman KK, Paradiso AM, Boucher RC, and Gatzy JT. Clsecretion by trachea of CFTR (+/-) and (-/-) fetal mouse. Am J Respir Cell Mol Biol
1995; 13: 307-313.
23.
Barker PM and Gatzy JT. Effect of gas composition on liquid secretion by explants
of distal lung of fetal rat in submersion culture. Am J Physiol 1993; 265: L512-517.
24.
Barker PM, Gowen CW, Lawson EE, and Knowles MR. Decreased sodium ion
absorption across nasal epithelium of very premature infants with respiratory distress
syndrome. J Pediatr 1997; 130: 373-377.
25.
Barker PM, Nguyen MS, Gatzy JT, Grubb B, Norman H, Hummler E, Rossier B,
Boucher RC, and Koller B. Role of gammaENaC subunit in lung liquid clearance
and electrolyte balance in newborn mice. Insights into perinatal adaptation and
pseudohypoaldosteronism. J Clin Invest 1998; 102: 1634-1640.
26.
Barker PM and Olver RE. Invited Review: Clearance of lung liquid during the
perinatal period. J Appl Physiol 2002; 93: 1542-1548.
27.
Barker PM, Strang LB, and Walters DV. The role of thyroid hormones in
maturation of the adrenaline-sensitive lung liquid reabsorptive mechanism in fetal
sheep. J Physiol 1990; 424: 473-485.
28.
Barker PM, Walters DV, Markiewicz M, and Strang LB. Development of the lung
liquid reabsorptive mechanism in fetal sheep: synergism of triiodothyronine and
hydrocortisone. J Physiol 1991; 433: 435-449.
29.
Barnard ML, Olivera WG, Rutschman DM, Bertorello AM, Katz AI, and
Sznajder JI. Dopamine stimulates sodium transport and liquid clearance in rat lung
epithelium. Am J Respir Crit Care Med 1997; 156: 709-714.
30.
Barnard ML, Ridge KM, Saldias F, Friedman E, Gare M, Guerrero C, Lecuona
E, Bertorello AM, Katz AI, and Sznajder JI. Stimulation of the dopamine 1
receptor increases lung edema clearance. Am J Respir Crit Care Med 1999; 160: 982986.
31.
Barquin N, Ciccolella DE, Ridge KM, and Sznajder JI. Dexamethasone
upregulates the Na-K-ATPase in rat alveolar epithelial cells. Am J Physiol 1997; 273:
L825-830.
32.
Barry P. The output of nebulisers at high altitude (abstract). High Alt Med Biol 2000;
2: 114.
99
33.
Barry P, Hart N, Wilson C, Bakewell S, McMorrow R, Watt S, and Pollard A.
Lung deposition of therapeutic aerosols at simulated altitude (abstract). High Alt Med
Biol 2000; 2: 114.
34.
receptor sensitivity and dynamic ventilatory response to carbon dioxide in man
acclimatised to high altitude (Abstract). J Physiol 1996: 497P: 429-430.
35.
Barry PW, Mason NP, and O'Callaghan C. Nasal mucociliary transport is impaired
at altitude. Eur Respir J 1997; 10: 35-37.
36.
Respir J 2002; 19: 16-19.
37.
Barry PW, Mason NP, Riordan M, and O'Callaghan C. Cough frequency and
cough-receptor sensitivity are increased in man at altitude. Clin Sci (Colch) 1997; 93:
181-186.
38.
Bartsch P, Maggiorini M, Ritter M, Noti C, Vock P, and Oelz O. Prevention of
high-altitude pulmonary edema by nifedipine [see comments]. N Engl J Med 1991;
325: 1284-1289.
39.
Bartsch P and Roach R. Acute Mountain Sickness and High-Altitude Cerebral
Edema. In: High Altitude: an exploration of human adaptation, edited by Hornbein T
and Schoene R. New York: Marcel Dekker, 2001, p. 732-740.
40.
Bartsch P, Shaw S, Franciolli M, Gnadinger MP, and Weidmann P. Atrial
natriuretic peptide in acute mountain sickness. J Appl Physiol 1988; 65: 1929-1937.
41.
Bartsch P, Swenson ER, Paul A, Julg B, and Hohenhaus E. Hypoxic ventilatory
response, ventilation, gas exchange, and fluid balance in acute mountain sickness.
High Alt Med Biol 2002; 3: 361-376.
42.
Bartsch P, Vock P, and Franciolli M. High altitude pulmonary edema after
successful treatment of acute mountain sickness with dexamethasone. J Wilderness
Med 1990; 1: 162-164.
43.
Basnyat B, Gertsch JH, Johnson EW, Castro-Marin F, Inoue Y, and Yeh C.
Efficacy of low-dose acetazolamide (125 mg BID) for the prophylaxis of acute
mountain sickness: a prospective, double-blind, randomized, placebo-controlled trial.
High Alt Med Biol 2003; 4: 45-52.
100
44.
Basnyat B and Litch JA. Medical problems of porters and trekkers in the Nepal
Himalaya. Wilderness Environ Med 1997; 8: 78-81.
45.
Basset G, Crone C, and Saumon G. Fluid absorption by rat lung in situ: pathways
for sodium entry in the luminal membrane of alveolar epithelium. J Physiol 1987; 384:
325-345.
46.
Basset G, Crone C, and Saumon G. Significance of active ion transport in
transalveolar water absorption: a study on isolated rat lung. J Physiol 1987; 384: 311324.
47.
Basset G, Saumon G, Bouchonnet F, and Crone C. Apical sodium-sugar transport
in pulmonary epithelium in situ. Biochim Biophys Acta 1988; 942: 11-18.
48.
Belcher N and Rees PJ. Effects of pholcodine and salbutamol on citric acid induced
cough in normal subjects. Thorax 1986; 41: 74-75.
49.
Benson MK, Newberg LA, and Jones JG. Nitrogen and bolus closing volumes:
differences after histamine-induced bronchoconstriction. J Appl Physiol 1975; 38:
1088-1091.
50.
Bernard GR, Artigas A, Brigham KL, Carlet J, Falke K, Hudson L, Lamy M,
Legall JR, Morris A, and Spragg R. The American-European Consensus
Conference on ARDS. Definitions, mechanisms, relevant outcomes, and clinical trial
coordination. Am J Respir Crit Care Med 1994; 149: 818-824.
51.
Bert P. La Pression Barometrique.
52.
Berthiaume Y, Staub NC, and Matthay MA. Beta-adrenergic agonists increase lung
liquid clearance in anesthetized sheep. J Clin Invest 1987; 79: 335-343.
53.
Bertorello AM and Sznajder JI. The dopamine paradox in lung and kidney epithelia:
sharing the same target but operating different signaling networks. Am J Respir Cell
Mol Biol 2005; 33: 432-437.
54.
Bhoola KD, Figueroa CD, and Worthy K. Bioregulation of kinins: kallikreins,
kininogens, and kininases. Pharmacol Rev 1992; 44: 1-80.
55.
Birkebaek NH, Kristiansen M, Seefeldt T, Degn J, Moller A, Heron I, Andersen
PL, Moller JK, and Ostergard L. Bordetella pertussis and chronic cough in adults.
Clin Infect Dis 1999; 29: 1239-1242.
101
56.
Blanco G and Mercer RW. Isozymes of the Na-K-ATPase: heterogeneity in
structure, diversity in function. Am J Physiol 1998; 275: F633-650.
57.
Bland RD and Boyd CA. Cation transport in lung epithelial cells derived from fetal,
newborn, and adult rabbits. J Appl Physiol 1986; 61: 507-515.
58.
Bolser DC and Davenport PW. Functional organization of the central cough
generation mechanism. Pulm Pharmacol Ther 2002; 15: 221-225.
59.
Bonham AC, Kott KS, Ravi K, Kappagoda CT, and Joad JP. Substance P
contributes to rapidly adapting receptor responses to pulmonary venous congestion in
rabbits. J Physiol 1996; 493: 229-238.
60.
Bonham AC, Sekizawa SI, Chen CY, and Joad JP. Plasticity of brainstem
mechanisms of cough. Respir Physiol Neurobiol 2006.
61.
Booth RE, Johnson JP, and Stockand JD. Aldosterone. Adv Physiol Educ 2002; 26:
8-20.
62.
Borok Z, Danto SI, Dimen LL, Zhang XL, and Lubman RL. Na(+)-K(+)-ATPase
expression in alveolar epithelial cells: upregulation of active ion transport by KGF.
Am J Physiol 1998; 274: L149-158.
63.
Borok Z, Hami A, Danto SI, Lubman RL, Kim KJ, and Crandall ED. Effects of
EGF on alveolar epithelial junctional permeability and active sodium transport. Am J
Physiol 1996; 270: L559-565.
64.
Borok Z, Liebler JM, Lubman RL, Foster MJ, Zhou B, Li X, Zabski SM, Kim
KJ, and Crandall ED. Na transport proteins are expressed by rat alveolar epithelial
type I cells. Am J Physiol Lung Cell Mol Physiol 2002; 282: L599-608.
65.
Borok Z and Verkman AS. Lung edema clearance: 20 years of progress: invited
review: role of aquaporin water channels in fluid transport in lung and airways. J Appl
Physiol 2002; 93: 2199-2206.
66.
Borst HG, Berglund E, Whittenberger JL, Mead J, Mc GM, and Collier C. The
effect of pulmonary vascular pressures on the mechanical properties of the lungs of
anesthetized dogs. J Clin Invest 1957; 36: 1708-1714.
67.
Boucher RC. Human airway ion transport. Part one. Am J Respir Crit Care Med
1994; 150: 271-281.
102
68.
Boucher RC. Human airway ion transport. Part two. Am J Respir Crit Care Med
1994; 150: 581-593.
69.
Boyle MP, Diener-West M, Milgram L, Knowles M, Foy C, Zeitlin P, and
Standaert T. A multicenter study of the effect of solution temperature on nasal
potential difference measurements. Chest 2003; 124: 482-489.
70.
Brown BH. Electrical impedance tomography (EIT): a review. J Med Eng Technol
2003; 27: 97-108.
71.
Brown BH, Flewelling R, Griffiths H, Harris ND, Leathard AD, Lu L, Morice
AH, Neufeld GR, Nopp P, and Wang W. EITS changes following oleic acid induced
lung water. Physiol Meas 1996; 17 Suppl 4A: A117-130.
72.
Brown MJ, Olver RE, Ramsden CA, Strang LB, and Walters DV. Effects of
adrenaline and of spontaneous labour on the secretion and absorption of lung liquid in
the fetal lamb. J Physiol 1983; 344: 137-152.
73.
Brown SE, Kim KJ, Goodman BE, Wells JR, and Crandall ED. Sodium-amino
acid cotransport by type II alveolar epithelial cells. J Appl Physiol 1985; 59: 16161622.
74.
Burch LH, Talbot CR, Knowles MR, Canessa CM, Rossier BC, and Boucher RC.
Relative expression of the human epithelial Na+ channel subunits in normal and cystic
fibrosis airways. Am J Physiol 1995; 269: C511-518.
75.
Busch T, Bartsch P, Pappert D, Grunig E, Hildebrandt W, Elser H, Falke KJ,
and Swenson ER. Hypoxia decreases exhaled nitric oxide in mountaineers
susceptible to high-altitude pulmonary edema. Am J Respir Crit Care Med 2001; 163:
368-373.
76.
Campbell AR, Folkesson HG, Berthiaume Y, Gutkowska J, Suzuki S, and
Matthay MA. Alveolar epithelial fluid clearance persists in the presence of moderate
left atrial hypertension in sheep. J Appl Physiol 1999; 86: 139-151.
77.
Campbell JH, Harris ND, Zhang F, Morice AH, and Brown BH. Detection of
changes in intrathoracic fluid in man using electrical impedance tomography. Clin Sci
(Lond) 1994; 87: 97-101.
78.
Canessa CM, Horisberger JD, and Rossier BC. Epithelial sodium channel related to
proteins involved in neurodegeneration. Nature 1993; 361: 467-470.
103
79.
Canessa CM, Schild L, Buell G, Thorens B, Gautschi I, Horisberger JD, and
Rossier BC. Amiloride-sensitive epithelial Na+ channel is made of three homologous
subunits [see comments]. Nature 1994; 367: 463-467.
80.
Canning BJ. Anatomy and neurophysiology of the cough reflex: ACCP evidencebased clinical practice guidelines. Chest 2006; 129: 33S-47S.
81.
Capen RL and Wagner WW, Jr. Intrapulmonary blood flow redistribution during
hypoxia increases gas exchange surface area. J Appl Physiol 1982; 52: 1575-1580.
82.
Carpenter TC, Reeves JT, and Durmowicz AG. Viral respiratory infection
increases susceptibility of young rats to hypoxia-induced pulmonary edema. J Appl
Physiol 1998; 84: 1048-1054.
83.
Carpenter TC, Schomberg S, Nichols C, Stenmark KR, and Weil JV. Hypoxia
reversibly inhibits epithelial sodium transport but does not inhibit lung ENaC or NaK-ATPase expression. Am J Physiol Lung Cell Mol Physiol 2003; 284: L77-83.
84.
ChangLai SP, Hung WT, and Liao KK. Detecting alveolar epithelial injury
following volatile anesthetics by (99m)Tc DTPA radioaerosol inhalation lung scan.
Respiration 1999; 66: 506-510.
85.
Charron PD, Fawley JP, and Maron MB. Effect of epinephrine on alveolar liquid
clearance in the rat. J Appl Physiol 1999; 87: 611-618.
86.
Cheek JM, Kim KJ, and Crandall ED. Tight monolayers of rat alveolar epithelial
cells: bioelectric properties and active sodium transport. Am J Physiol 1989; 256:
C688-693.
87.
Chen XJ, Eaton DC, and Jain L. Beta-adrenergic regulation of amiloride-sensitive
lung sodium channels. Am J Physiol Lung Cell Mol Physiol 2002; 282: L609-620.
88.
Cibella F, Cuttitta G, Romano S, Grassi B, Bonsignore G, and Milic-Emili J.
Respiratory energetics during exercise at high altitude. J Appl Physiol 1999; 86: 17851792.
89.
Clerici C and Matthay MA. Transforming growth factor-beta 1 regulates lung
epithelial barrier function and fluid transport. Am J Physiol Lung Cell Mol Physiol
2003; 285: L1190-1191.
104
90.
Clerici C, Soler P, and Saumon G. Sodium-dependent phosphate and alanine
transports but sodium-independent hexose transport in type II alveolar epithelial cells
in primary culture. Biochim Biophys Acta 1991; 1063: 27-35.
91.
Clough AV, Haworth ST, Ma W, and Dawson CA. Effects of hypoxia on
pulmonary microvascular volume. Am J Physiol Heart Circ Physiol 2000; 279:
H1274-1282.
92.
Coates G, Gray C, Mansell A, Nahmias C, Powles A, Sutton J, and Webber C.
Changes in lung volume, lung density, and distribution of ventilation during hypobaric
decompression. J Appl Physiol 1979; 46: 752-755.
93.
Cogo A, Legnani D, and Allegra L. Respiratory function at different altitudes.
Respiration 1997; 64: 416-421.
94.
Com G and Clancy JP. Adenosine receptors, cystic fibrosis, and airway hydration.
Handb Exp Pharmacol 2009: 363-381.
95.
Comellas AP, Dada LA, Lecuona E, Pesce LM, Chandel NS, Quesada N,
Budinger GR, Strous GJ, Ciechanover A, and Sznajder JI. Hypoxia-mediated
degradation of Na,K-ATPase via mitochondrial reactive oxygen species and the
ubiquitin-conjugating system. Circ Res 2006; 98: 1314-1322.
96.
Cook CD, Mead J, Schreiner GL, Frank NR, and Craig JM. Pulmonary mechanics
during induced pulmonary edema in anesthetized dogs. J Appl Physiol 1959; 14: 177186.
97.
Cotes J. Assessment of bellows and mechanical attributes of the lung. In: Lung
Function: Assessment and Application in Medicine (5th ed.). Oxford: Blackwell
Scientfic Publications, 1993, p. 65-81.
98.
Cotes J. Assessment of distribution of ventilation and of blood flow through the lung.
In: Lung Function: Assessment and Application in Medicine (5th ed.). Oxford:
Blackwell Scientfic Publications, 1993, p. 213-225.
99.
Cotes J. Lung function in disease. In: Lung Function: Assessment and Application in
Medicine (5th ed.). Oxford: Blackwell Scientfic Publications, 1993, p. 514-609.
100.
Cotes J. Structure, expansion and movement of the lung. In: Lung Function:
Assessment and Application in Medicine (5th ed.). Oxford: Blackwell Scientfic
Publications, 1993, p. 82-130.
105
101.
Crandall ED, Heming TA, Palombo RL, and Goodman BE. Effects of terbutaline
on sodium transport in isolated perfused rat lung. J Appl Physiol 1986; 60: 289-294.
102.
Crandall ED and Matthay MA. Alveolar epithelial transport. Basic science to
clinical medicine. Am J Respir Crit Care Med 2001; 163: 1021-1029.
103.
Crapo JD. New concepts in the formation of pulmonary edema [editorial; comment].
Am Rev Respir Dis 1993; 147: 790-792.
104.
Cremona G, Asnaghi R, Baderna P, Brunetto A, Brutsaert T, Cavallaro C, Clark
TM, Cogo A, Donis R, Lanfranchi P, Luks A, Novello N, Panzetta S, Perini L,
Putnam M, Spagnolatti L, Wagner H, and Wagner PD. Pulmonary extravascular
fluid accumulation in recreational climbers: a prospective study. Lancet 2002; 359:
303-309.
105.
Cruz JC. Mechanics of breathing in high altitude and sea level subjects. Respir
Physiol 1973; 17: 146-161.
106.
Cutillo AG, Morris AH, Blatter DD, Case TA, Ailion DC, Durney CH, and
Johnson SA. Determination of lung water content and distribution by nuclear
magnetic resonance. J Appl Physiol 1984; 57: 583-588.
107.
D'Angio CT and Finkelstein JN. Oxygen regulation of gene expression: a study in
opposites. Mol Genet Metab 2000; 71: 371-380.
108.
Dada LA, Chandel NS, Ridge KM, Pedemonte C, Bertorello AM, and Sznajder
JI. Hypoxia-induced endocytosis of Na,K-ATPase in alveolar epithelial cells is
mediated by mitochondrial reactive oxygen species and PKC-zeta. J Clin Invest 2003;
111: 1057-1064.
109.
Dagenais A, Denis C, Vives MF, Girouard S, Masse C, Nguyen T, Yamagata T,
Grygorczyk C, Kothary R, and Berthiaume Y. Modulation of alpha-ENaC and
alpha1-Na+-K+-ATPase by cAMP and dexamethasone in alveolar epithelial cells. Am
J Physiol Lung Cell Mol Physiol 2001; 281: L217-230.
110.
Dagenais A, Frechette R, Clermont ME, Masse C, Prive A, Brochiero E, and
Berthiaume Y. Dexamethasone inhibits the action of TNF on ENaC expression and
activity. Am J Physiol Lung Cell Mol Physiol 2006; 291: L1220-1231.
111.
Dagenais A, Frechette R, Yamagata Y, Yamagata T, Carmel JF, Clermont ME,
Brochiero E, Masse C, and Berthiaume Y. Downregulation of ENaC activity and
106
expression by TNF-alpha in alveolar epithelial cells. Am J Physiol Lung Cell Mol
Physiol 2004; 286: L301-311.
112.
Danto SI, Borok Z, Zhang XL, Lopez MZ, Patel P, Crandall ED, and Lubman
RL. Mechanisms of EGF-induced stimulation of sodium reabsorption by alveolar
epithelial cells. Am J Physiol 1998; 275: C82-92.
113.
Dawson CA, Jones RL, and Hamilton LH. Hemodynamic responses of isolated cat
lungs during forward and retrograde perfusion. J Appl Physiol 1973; 35: 95-102.
114.
Dawson CA, Linehan JH, Rickaby DA, and Krenz GS. Effect of vasoconstriction
on longitudinal distribution of pulmonary vascular pressure and volume. J Appl
Physiol 1991; 70: 1607-1616.
115.
De Angelis C, Farrace S, Urbani L, Porcu S, Ferri C, D'Amelio R, Santucci A,
and Balsano F. Effects of high altitude exposure on plasma and urinary digoxin-like
immunoreactive substance. Am J Hypertens 1992; 5: 600-607.
116.
De Troyer A, Yernault JC, and Rodenstein D. Influence of beta-2 agonist aerosols
on pressure-volume characteristics of the lungs. Am Rev Respir Dis 1978; 118: 987995.
117.
Deal EC, Jr., McFadden ER, Jr., Ingram RH, Jr., Breslin FJ, and Jaeger JJ.
Airway responsiveness to cold air and hyperpnea in normal subjects and in those with
hay fever and asthma. Am Rev Respir Dis 1980; 121: 621-628.
118.
DeBoeck G, Moraine J-J, and Naeije R. Respiratory muscle strength may explain
hypoxia-induced decrease in vital capacity. Medicine and science in sports and
exercise 2005.
119.
Decarie A, Raymond P, Gervais N, Couture R, and Adam A. Serum interspecies
differences in metabolic pathways of bradykinin and [des-Arg9]BK: influence of
enalaprilat. Am J Physiol 1996; 271: H1340-1347.
120.
Dehnert C, Luks AM, Schendler G, Menold E, Berger MM, Mairbaurl H, Faoro
V, Bailey DM, Castell C, Hahn G, Vock P, Swenson ER, and Bartsch P. No
evidence for interstitial lung oedema by extensive pulmonary function testing at 4,559
m. Eur Respir J 2010; 35: 812-820.
121.
Dicpinigaitis PV. Chronic cough due to asthma: ACCP evidence-based clinical
practice guidelines. Chest 2006; 129: 75S-79S.
107
122.
Dicpinigaitis PV. Cough. 4: Cough in asthma and eosinophilic bronchitis. Thorax
2004; 59: 71-72.
123.
Dillard TA, Rajagopal KR, Slivka WA, Berg BW, Mehm WJ, and Lawless NP.
Lung function during moderate hypobaric hypoxia in normal subjects and patients
with chronic obstructive pulmonary disease. Aviat Space Environ Med 1998; 69: 979985.
124.
Ding C, Potter ED, Qiu W, Coon SL, Levine MA, and Guggino SE. Cloning and
widespread distribution of the rat rod-type cyclic nucleotide-gated cation channel. Am
J Physiol 1997; 272: C1335-1344.
125.
Dobbs LG. Isolation and culture of alveolar type II cells. Am J Physiol 1990; 258:
L134-147.
126.
Dobbs LG, Gonzalez R, Matthay MA, Carter EP, Allen L, and Verkman AS.
Highly water-permeable type I alveolar epithelial cells confer high water permeability
between the airspace and vasculature in rat lung. Proc Natl Acad Sci U S A 1998; 95:
2991-2996.
127.
Dobbs LG and Johnson MD. Alveolar epithelial transport in the adult lung. Respir
Physiol Neurobiol 2007; 159: 283-300.
128.
Dosman JA, Hodgson WC, and Cockcroft DW. Effect of cold air on the bronchial
response to inhaled histamine in patients with asthma. Am Rev Respir Dis 1991; 144:
45-50.
129.
Douglas NJ. Control of breathing during sleep. Clin Sci (Lond) 1984; 67: 465-471.
130.
Doyle JT, Wilson JS, and Warren JV. The pulmonary vascular responses to shortterm hypoxia in human subjects. Circulation 1952; 5: 263-270.
131.
Droma Y, Hanaoka M, Ota M, Katsuyama Y, Koizumi T, Fujimoto K,
Kobayashi T, and Kubo K. Positive association of the endothelial nitric oxide
synthase gene polymorphisms with high-altitude pulmonary edema. Circulation 2002;
106: 826-830.
132.
Duplain H, Sartori C, Lepori M, Egli M, Allemann Y, Nicod P, and Scherrer U.
Exhaled nitric oxide in high-altitude pulmonary edema: role in the regulation of
pulmonary vascular tone and evidence for a role against inflammation. Am J Respir
Crit Care Med 2000; 162: 221-224.
108
133.
Durmowicz AG, Noordeweir E, Nicholas R, and Reeves JT. Inflammatory
processes may predispose children to high-altitude pulmonary edema. J Pediatr 1997;
130: 838-840.
134.
Eckle T, Grenz A, Laucher S, and Eltzschig HK. A2B adenosine receptor signaling
attenuates acute lung injury by enhancing alveolar fluid clearance in mice. J Clin
Invest 2008; 118: 3301-3315.
135.
Egli M, Duplain H, Lepori M, Cook S, Nicod P, Hummler E, Sartori C, and
Scherrer U. Defective respiratory amiloride-sensitive sodium transport predisposes to
pulmonary oedema and delays its resolution in mice. J Physiol 2004; 560: 857-865.
136.
Eldridge MW, Braun RK, Yoneda KY, and Walby WF. Effects of altitude and
exercise on pulmonary capillary integrity: evidence for subclinical high-altitude
pulmonary edema. J Appl Physiol 2006; 100: 972-980.
137.
Eldridge MW, Podolsky A, Richardson RS, Johnson DH, Knight DR, Johnson
EC, Hopkins SR, Michimata H, Grassi B, Feiner J, Kurdak SS, Bickler PE,
Wagner PD, and Severinghaus JW. Pulmonary hemodynamic response to exercise
in subjects with prior high-altitude pulmonary edema. J Appl Physiol 1996; 81: 911921.
138.
Elliott AR, Fu Z, Tsukimoto K, Prediletto R, Mathieu-Costello O, and West JB.
Short-term reversibility of ultrastructural changes in pulmonary capillaries caused by
stress failure. J Appl Physiol 1992; 73: 1150-1158.
139.
Engelhardt JF, Zepeda M, Cohn JA, Yankaskas JR, and Wilson JM. Expression
of the cystic fibrosis gene in adult human lung. J Clin Invest 1994; 93: 737-749.
140.
Factor P, Mutlu GM, Chen L, Mohameed J, Akhmedov AT, Meng FJ, Jilling T,
Lewis ER, Johnson MD, Xu A, Kass D, Martino JM, Bellmeyer A, Albazi JS,
Emala C, Lee HT, Dobbs LG, and Matalon S. Adenosine regulation of alveolar
fluid clearance. Proc Natl Acad Sci U S A 2007; 104: 4083-4088.
141.
Fajac I, Hubert D, Bienvenu T, Richaud-Thiriez B, Matran R, Kaplan JC,
Dall'Ava-Santucci J, and Dusser DJ. Relationships between nasal potential
difference and respiratory function in adults with cystic fibrosis. Eur Respir J 1998;
12: 1295-1300.
109
142.
Fang X, Fukuda N, Barbry P, Sartori C, Verkman AS, and Matthay MA. Novel
Role for CFTR in Fluid Absorption from the Distal Airspaces of the Lung. J Gen
Physiol 2002; 119: 199-208.
143.
Fang X, Song Y, Hirsch J, Galietta LJ, Pedemonte N, Zemans RL, Dolganov G,
Verkman AS, and Matthay MA. Contribution of CFTR to apical-basolateral fluid
transport in cultured human alveolar epithelial type II cells. Am J Physiol Lung Cell
Mol Physiol 2006; 290: L242-249.
144.
Farman N, Talbot CR, Boucher R, Fay M, Canessa C, Rossier B, and Bonvalet
JP. Noncoordinated expression of alpha-, beta-, and gamma-subunit mRNAs of
epithelial Na+ channel along rat respiratory tract. Am J Physiol 1997; 272: C131-141.
145.
Fasano V, Paolucci E, Pomidori L, and Cogo A. High-altitude exposure reduces
inspiratory muscle strength. Int J Sports Med 2007; 28: 426-430.
146.
Fein A, Grossman RF, Jones JG, Goodman PC, and Murray JF. Evaluation of
transthoracic electrical impedance in the diagnosis of pulmonary edema. Circulation
1979; 60: 1156-1160.
147.
Ferri C, Bellini C, Coassin S, De Angelis C, Perrone A, and Santucci A. Plasma
endogenous digoxin-like substance levels are dependent on blood O2 in man. Clin Sci
(Lond) 1994; 87: 447-451.
148.
Folkesson HG, Norlin A, Wang Y, Abedinpour P, and Matthay MA.
Dexamethasone and thyroid hormone pretreatment upregulate alveolar epithelial fluid
clearance in adult rats. J Appl Physiol 2000; 88: 416-424.
149.
Forte VA, Jr., Leith DE, Muza SR, Fulco CS, and Cymerman A. Ventilatory
capacities at sea level and high altitude. Aviat Space Environ Med 1997; 68: 488-493.
150.
Fukuda N, Folkesson HG, and Matthay MA. Relationship of interstitial fluid
volume to alveolar fluid clearance in mice: ventilated vs. in situ studies. J Appl
Physiol 2000; 89: 672-679.
151.
Funaki H, Yamamoto T, Koyama Y, Kondo D, Yaoita E, Kawasaki K, Kobayashi
H, Sawaguchi S, Abe H, and Kihara I. Localization and expression of AQP5 in
cornea, serous salivary glands, and pulmonary epithelial cells. Am J Physiol 1998;
275: C1151-1157.
110
152.
Gaillard EA, Shaw NJ, Wallace HL, Subhedar NV, and Southern KW. Airway
ion transport on the first postnatal day in infants delivered vaginally or by elective
cesarean section. Pediatr Res 2003; 54: 58-63.
153.
Gaillard EA, Shaw NJ, Wallace HL, Subhedar NV, and Southern KW. Nasal
potential difference increases with gestation in moderately preterm neonates on the
first postnatal day. Arch Dis Child Fetal Neonatal Ed 2005; 90: F172-173.
154.
Galietta LJ, Pagesy P, Folli C, Caci E, Romio L, Costes B, Nicolis E, Cabrini G,
Goossens M, Ravazzolo R, and Zegarra-Moran O. IL-4 is a potent modulator of ion
transport in the human bronchial epithelium in vitro. J Immunol 2002; 168: 839-845.
155.
Garty H and Palmer LG. Epithelial sodium channels: function, structure, and
regulation. Physiol Rev 1997; 77: 359-396.
156.
Gautier H, Peslin R, Grassino A, Milic-Emili J, Hannhart B, Powell E,
Miserocchi G, Bonora M, and Fischer JT. Mechanical properties of the lungs
during acclimatization to altitude. J Appl Physiol 1982; 52: 1407-1415.
157.
Giannelli S, Jr., Ayres SM, and Buehler ME. Effect of pulmonary blood flow upon
lung mechanics. J Clin Invest 1967; 46: 1625-1642.
158.
Gillie DJ, Pace AJ, Coakley RJ, Koller BH, and Barker PM. Liquid and ion
transport by fetal airway and lung epithelia of mice deficient in sodium-potassium-2chloride transporter. Am J Respir Cell Mol Biol 2001; 25: 14-20.
159.
Gilroy RJ, Jr., Lavietes MH, Loring SH, Mangura BT, and Mead J. Respiratory
mechanical effects of abdominal distension. J Appl Physiol 1985; 58: 1997-2003.
160.
Goldstein R, Zamel N, and Rebuck A. Absence of effects of hypoxia on small
airway function in humans. J Appl Physiol 1979; 47: 251-256.
161.
Goodman BE and Crandall ED. Dome formation in primary cultured monolayers of
alveolar epithelial cells. Am J Physiol 1982; 243: C96-100.
162.
Goodman BE, Fleischer RS, and Crandall ED. Evidence for active Na+ transport
by cultured monolayers of pulmonary alveolar epithelial cells. Am J Physiol 1983;
245: C78-83.
163.
Goodman BE, Kim KJ, and Crandall ED. Evidence for active sodium transport
across alveolar epithelium of isolated rat lung. J Appl Physiol 1987; 62: 2460-2466.
111
164.
Gothe B, Altose MD, Goldman MD, and Cherniack NS. Effect of quiet sleep on
resting and CO2-stimulated breathing in humans. J Appl Physiol 1981; 50: 724-730.
165.
Gotshall RW. Exercise-induced bronchoconstriction. Drugs 2002; 62: 1725-1739.
166.
Gowen CW, Jr., Lawson EE, Gingras J, Boucher RC, Gatzy JT, and Knowles
MR. Electrical potential difference and ion transport across nasal epithelium of term
neonates: correlation with mode of delivery, transient tachypnea of the newborn, and
respiratory rate. J Pediatr 1988; 113: 121-127.
167.
Gray G, McFadden D, Houston C, and Bryan A. Changes in the single-breath
nitrogen washout curve on exposure to 17,600 ft. J Appl Physiol 1975; 39: 652-656.
168.
Gray G, Rennie I, Houston C, and Bryan A. Phase IV of the single-breath nitrogen
washout curve on exposure to altitude. J Appl Physiol 1973; 35: 227-230.
169.
Green M and Widdicombe JG. The effects of ventilation of dogs with different gas
mixtures on airway calibre and lung mechanics. J Physiol 1966; 186: 363-381.
170.
Greger R, Schreiber R, Mall M, Wissner A, Hopf A, Briel M, Bleich M, Warth R,
and Kunzelmann K. Cystic fibrosis and CFTR. Pflugers Arch 2001; 443: S3-7.
171.
Grimme JD, Lane SM, and Maron MB. Alveolar liquid clearance in multiple
nonperfused canine lung lobes. J Appl Physiol 1997; 82: 348-353.
172.
Grissom CK, Roach RC, Sarnquist FH, and Hackett PH. Acetazolamide in the
treatment of acute mountain sickness: clinical efficacy and effect on gas exchange.
Ann Intern Med 1992; 116: 461-465.
173.
Gudjonsdottir M, Appendini L, Baderna P, Purro A, Patessio A, Vilianis G,
Pastorelli M, Sigurdsson SB, and Donner CF. Diaphragm fatigue during exercise at
high altitude: the role of hypoxia and workload. Eur Respir J 2001; 17: 674-680.
174.
Guerrero C, Pesce L, Lecuona E, Ridge KM, and Sznajder JI. Dopamine activates
ERKs in alveolar epithelial cells via Ras-PKC-dependent and Grb2/Sos-independent
mechanisms. Am J Physiol Lung Cell Mol Physiol 2002; 282: L1099-1107.
175.
Guery BP, Mason CM, Dobard EP, Beaucaire G, Summer WR, and Nelson S.
Keratinocyte growth factor increases transalveolar sodium reabsorption in normal and
injured rat lungs. Am J Respir Crit Care Med 1997; 155: 1777-1784.
176.
Guleria JS, Pande JN, Sethi PK, and Roy SB. Pulmonary diffusing capacity at high
altitude. J Appl Physiol 1971; 31: 536-543.
112
177.
Gumbiner BM. Breaking through the tight junction barrier. J Cell Biol 1993; 123:
1631-1633.
178.
Gunawardena S, Bravo E, and Kappagoda CT. Effect of chronic mitral valve
damage on activity of pulmonary rapidly adapting receptors in the rabbit. J Physiol
1998; 511: 79-88.
179.
Gunawardena S, Bravo E, and Kappagoda CT. Rapidly adapting receptors in a
rabbit model of mitral regurgitation. J Physiol 1999; 521 Pt 3: 739-748.
180.
Gutstein H and Akil H. Opioid analgesics - centrally acting anti-tussive agents. In:
Goodman and Gilman's The pharmacological basis of therapeutics (11th ed.), edited
by Brunton L. New York: McGraw-Hill, 2006, p. 578-579.
181.
Hackett PH, Roach RC, Hartig GS, Greene ER, and Levine BD. The effect of
vasodilators on pulmonary hemodynamics in high altitude pulmonary edema: a
comparison. Int J Sports Med 1992; 13 Suppl 1: S68-71.
182.
Hackett PH, Roach RC, Schoene RB, Harrison GL, and Mills WJ, Jr. Abnormal
control of ventilation in high-altitude pulmonary edema. J Appl Physiol 1988; 64:
1268-1272.
183.
Hakim TS. Is flow in subpleural region typical of the rest of the lung? A study using
laser-Doppler flowmetry. J Appl Physiol 1992; 72: 1860-1867.
184.
Hakim TS and Kelly S. Occlusion pressures vs. micropipette pressures in the
pulmonary circulation. J Appl Physiol 1989; 67: 1277-1285.
185.
Hales CA and Kazemi H. Small-airways function in myocardial infarction. N Engl J
Med 1974; 290: 761-765.
186.
Hallander HO, Gnarpe J, Gnarpe H, and Olin P. Bordetella pertussis, Bordetella
parapertussis, Mycoplasma pneumoniae, Chlamydia pneumoniae and persistent cough
in children. Scand J Infect Dis 1999; 31: 281-286.
187.
Hargreaves M, Ravi K, and Kappagoda CT. Responses of slowly and rapidly
adapting receptors in the airways of rabbits to changes in the Starling forces. J Physiol
1991; 432: 81-97.
188.
Harken AH and O'Connor N E. The influence of clinically undetectable pulmonary
edema on small airway closure in the dog. Ann Surg 1976; 184: 183-188.
113
189.
Hauge A, Bo G, and Waaler BA. Interrelations between pulmonary liquid volumes
and lung compliance. J Appl Physiol 1975; 38: 608-614.
190.
Helms MN, Chen XJ, Ramosevac S, Eaton DC, and Jain L. Dopamine regulation
of amiloride-sensitive sodium channels in lung cells. Am J Physiol Lung Cell Mol
Physiol 2006; 290: L710-L722.
191.
Helms MN, Self J, Bao HF, Job LC, Jain L, and Eaton DC. Dopamine activates
amiloride-sensitive sodium channels in alveolar type I cells in lung slice preparations.
Am J Physiol Lung Cell Mol Physiol 2006; 291: L610-618.
192.
Helve O, Andersson S, Kirjavainen T, and Pitkanen OM. Improvement of lung
compliance during postnatal adaptation correlates with airway sodium transport. Am J
Respir Crit Care Med 2006; 173: 448-452.
193.
Helve O, Pitkanen O, Kirjavainen T, and Andersson S. Sodium transport in airway
epithelium correlates with lung compliance in healthy newborn infants. J Pediatr
2005; 146: 273-276.
194.
Helve O, Pitkanen OM, Andersson S, O'Brodovich H, Kirjavainen T, and
Otulakowski G. Low expression of human epithelial sodium channel in airway
epithelium of preterm infants with respiratory distress. Pediatrics 2004; 113: 12671272.
195.
Higenbottam T. The ionic composition of airway surface liquid and coughing. Bull
Eur Physiopathol Respir 1987; 23 Suppl 10: 25s-27s.
196.
Hille B. Ionic Channels of Excitable Membranes. Sunderland, Massachusetts: Sinauer
Associates Inc, 1992.
197.
Hlastala MP, Lamm WJ, Karp A, Polissar NL, Starr IR, and Glenny RW. Spatial
distribution of hypoxic pulmonary vasoconstriction in the supine pig. J Appl Physiol
2004; 96: 1589-1599.
198.
Hohenhaus E, Paul A, McCullough RE, Kucherer H, and Bartsch P. Ventilatory
and pulmonary vascular response to hypoxia and susceptibility to high altitude
pulmonary oedema. Eur Respir J 1995; 8: 1825-1833.
199.
Hoon RS, Balasubramanian V, Tiwari SC, Mathew OP, Behl A, Sharma SC, and
Chadha KS. Changes in transthoracic electrical impedance at high altitude. Br Heart
J 1977; 39: 61-66.
114
200.
Hopkins SR, Garg J, Bolar DS, Balouch J, and Levin DL. Pulmonary blood flow
heterogeneity during hypoxia and high-altitude pulmonary edema. Am J Respir Crit
Care Med 2005; 171: 83-87.
201.
Houston CS, Sutton JR, Cymerman A, and Reeves JT. Operation Everest II: man
at extreme altitude. J Appl Physiol 1987; 63: 877-882.
202.
Huang P, Gilmore E, Kultgen P, Barnes P, Milgram S, and Stutts MJ. Local
regulation of cystic fibrosis transmembrane regulator and epithelial sodium channel in
airway epithelium. Proc Am Thorac Soc 2004; 1: 33-37.
203.
Huff RM. Signal transduction pathways modulated by the D2 subfamily of dopamine
receptors. Cell Signal 1996; 8: 453-459.
204.
Hughes R, May AJ, and Widdicombe JG. The effect of pulmonary congestion and
oedema on lung compliance. J Physiol 1958; 142: 306-313.
205.
Hultgren H, Robinson M, and Wuerflein R. Over perfusion pulmonary edema.
Circulation 1966; 34: 132-133.
206.
Hultgren H and Spickard W. Medical experiences in Peru. Stanford Med Bull 1960;
263: 478-480.
207.
Hultgren HN. High-altitude pulmonary edema: current concepts. Annu Rev Med
1996; 47: 267-284.
208.
Hultgren HN, Lopez CE, Lundberg E, and Miller H. Physiologic Studies of
Pulmonary Edema at High Altitude. Circulation 1964; 29: 393-408.
209.
Hummler E, Barker P, Gatzy J, Beermann F, Verdumo C, Schmidt A, Boucher
R, and Rossier BC. Early death due to defective neonatal lung liquid clearance in
alpha-ENaC-deficient mice. Nat Genet 1996; 12: 325-328.
210.
Icard P and Saumon G. Alveolar sodium and liquid transport in mice. Am J Physiol
1999; 277: L1232-1238.
211.
Ingbar DH, Weeks CB, Gilmore-Hebert M, Jacobsen E, Duvick S, Dowin R,
Savik SK, and Jamieson JD. Developmental regulation of Na, K-ATPase in rat lung.
Am J Physiol 1996; 270: L619-629.
212.
Interiano B, Hyde RW, Hodges M, and Yu PN. Interrelation between alterations in
pulmonary mechanics and hemodynamics in acute myocardial infarction. J Clin Invest
1973; 52: 1994-2006.
115
213.
Ireson NJ, Tait JS, MacGregor GA, and Baker EH. Comparison of nasal pH values
in black and white individuals with normal and high blood pressure. Clin Sci (Colch)
2001; 100: 327-333.
214.
Irwin RS. Chronic cough due to gastroesophageal reflux disease: ACCP evidencebased clinical practice guidelines. Chest 2006; 129: 80S-94S.
215.
Irwin RS, Rosen MJ, and Braman SS. Cough. A comprehensive review. Arch Intern
Med 1977; 137: 1186-1191.
216.
Israili ZH and Hall WD. Cough and angioneurotic edema associated with
angiotensin-converting enzyme inhibitor therapy. A review of the literature and
pathophysiology. Ann Intern Med 1992; 117: 234-242.
217.
Jacoby DB. Pathophysiology of airway viral infections. Pulm Pharmacol Ther 2004;
17: 333-336.
218.
Jaeger JJ, Sylvester JT, Cymerman A, Berberich JJ, Denniston JC, and Maher
JT. Evidence for increased intrathoracic fluid volume in man at high altitude. J Appl
Physiol 1979; 47: 670-676.
219.
Jayr C, Garat C, Meignan M, Pittet JF, Zelter M, and Matthay MA. Alveolar
liquid and protein clearance in anesthetized ventilated rats. J Appl Physiol 1994; 76:
2636-2642.
220.
Jentsch TJ, Stein V, Weinreich F, and Zdebik AA. Molecular structure and
physiological function of chloride channels. Physiol Rev 2002; 82: 503-568.
221.
Jiang X, Ingbar DH, and O'Grady SM. Adrenergic regulation of ion transport
across adult alveolar epithelial cells: effects on Cl- channel activation and transport
function in cultures with an apical air interface. J Membr Biol 2001; 181: 195-204.
222.
Jiang X, Ingbar DH, and O'Grady SM. Adrenergic stimulation of Na+ transport
across alveolar epithelial cells involves activation of apical Cl- channels. Am J Physiol
1998; 275: C1610-1620.
223.
Johnson BD, Babcock MA, Suman OE, and Dempsey JA. Exercise-induced
diaphragmatic fatigue in healthy humans. J Physiol 1993; 460: 385-405.
224.
Johnson MD, Bao HF, Helms MN, Chen XJ, Tigue Z, Jain L, Dobbs LG, and
Eaton DC. Functional ion channels in pulmonary alveolar type I cells support a role
for type I cells in lung ion transport. Proc Natl Acad Sci U S A 2006; 103: 4964-4969.
116
225.
Johnson MD, Widdicombe JH, Allen L, Barbry P, and Dobbs LG. Alveolar
epithelial type I cells contain transport proteins and transport sodium, supporting an
active role for type I cells in regulation of lung liquid homeostasis. Proc Natl Acad Sci
U S A 2002; 99: 1966-1971.
226.
Junor RW, Benjamin AR, Alexandrou D, Guggino SE, and Walters DV. Lack of a
role for cyclic nucleotide gated cation channels in lung liquid absorption in fetal
sheep. J Physiol 2000; 523 Pt 2: 493-502.
227.
Junor RW, Benjamin AR, Alexandrou D, Guggino SE, and Walters DV. A novel
role for cyclic nucleotide-gated cation channels in lung liquid homeostasis in sheep. J
Physiol 1999; 520 Pt 1: 255-260.
228.
Kaminsky DA, Jones K, Schoene RB, and Voelkel NF. Urinary leukotriene E4
levels in high-altitude pulmonary edema. A possible role for inflammation. Chest
1996; 110: 939-945.
229.
Kaupp UB and Seifert R. Cyclic nucleotide-gated ion channels. Physiol Rev 2002;
82: 769-824.
230.
Kayser B, Narici M, and Cibella F. Fatigue and muscle performance at high altitude.
In: Hypoxia and Molecular Medicine, edited by Sutton J, Coates G and Houston C.
Burlington: Queen City Printers, 1993, p. 222-234.
231.
Kellenberger S and Schild L. Epithelial sodium channel/degenerin family of ion
channels: a variety of functions for a shared structure. Physiol Rev 2002; 82: 735-767.
232.
Kerem E, Bistritzer T, Hanukoglu A, Hofmann T, Zhou Z, Bennett W,
MacLaughlin E, Barker P, Nash M, Quittell L, Boucher R, and Knowles MR.
Pulmonary epithelial sodium-channel dysfunction and excess airway liquid in
pseudohypoaldosteronism. N Engl J Med 1999; 341: 156-162.
233.
Kerr JS, Reicherter J, and Fisher AB. 2-Deoxy-D-glucose uptake by rat granular
pneumocytes in primary culture. Am J Physiol 1982; 243: C14-19.
234.
Kim KJ, Cheek JM, and Crandall ED. Contribution of active Na+ and Cl- fluxes to
net ion transport by alveolar epithelium. Respir Physiol 1991; 85: 245-256.
235.
Kimball WR, Kelly KB, and Mead J. Thoracoabdominal blood volume change and
its effect on lung and chest wall volumes. J Appl Physiol 1986; 61: 953-959.
117
236.
Klocke FJ and Rahn H. Breath holding after breathing of oxygen. J Appl Physiol
1959; 14: 689-693.
237.
Knowles M, Gatzy J, and Boucher R. Increased bioelectric potential difference
across respiratory epithelia in cystic fibrosis. N Engl J Med 1981; 305: 1489-1495.
238.
Knowles MR, Carson JL, Collier AM, Gatzy JT, and Boucher RC. Measurements
of nasal transepithelial electric potential differences in normal human subjects in vivo.
Am Rev Respir Dis 1981; 124: 484-490.
239.
Knowles MR, Paradiso AM, and Boucher RC. In vivo nasal potential difference:
techniques and protocols for assessing efficacy of gene transfer in cystic fibrosis. Hum
Gene Ther 1995; 6: 445-455.
240.
Kobayashi T, Koyama S, Kubo K, Fukushima M, and Kusama S. Clinical features
of patients with high-altitude pulmonary edema in Japan. Chest 1987; 92: 814-821.
241.
Koizumi T, Kawashima A, Kubo K, Kobayashi T, and Sekiguchi M. Radiographic
and hemodynamic changes during recovery from high-altitude pulmonary edema.
Intern Med 1994; 33: 525-528.
242.
Koyama S, Kobayashi T, Kubo K, Fukushima M, Yoshimura K, Shibamoto T,
and Kusama S. The increased sympathoadrenal activity in patients with high altitude
pulmonary edema is centrally mediated. Jpn J Med 1988; 27: 10-16.
243.
Kronenberg R, Safar P, Lee J, Wright F, Noble W, Wahrenbrock R, Hickey R,
Nemoto E, and Severinghaus J. Pulmonary artery pressure and alveolar gas
exchange in man during acclimatization to 12,470 ft. J Clin Invest 1971; 50: 827-837.
244.
Krozowski Z and Funder JW. Mineralocorticoid receptors in the rat lung.
Endocrinology 1981; 109: 1811-1813.
245.
Kubo K, Hanaoka M, Hayano T, Miyahara T, Hachiya T, Hayasaka M, Koizumi
T, Fujimoto K, Kobayashi T, and Honda T. Inflammatory cytokines in BAL fluid
and pulmonary hemodynamics in high-altitude pulmonary edema. Respir Physiol
1998; 111: 301-310.
246.
Kubo K, Hanaoka M, Yamaguchi S, Hayano T, Hayasaka M, Koizumi T,
Fujimoto K, Kobayashi T, and Honda T. Cytokines in bronchoalveolar lavage fluid
in patients with high altitude pulmonary oedema at moderate altitude in Japan. Thorax
1996; 51: 739-742.
118
247.
Kunst PW, Bohm SH, Vazquez de Anda G, Amato MB, Lachmann B, Postmus
PE, and de Vries PM. Regional pressure volume curves by electrical impedance
tomography in a model of acute lung injury. Crit Care Med 2000; 28: 178-183.
248.
Kunst PW, de Vries PM, Postmus PE, and Bakker J. Evaluation of electrical
impedance tomography in the measurement of PEEP-induced changes in lung volume.
Chest 1999; 115: 1102-1106.
249.
Kunst PW, Vonk Noordegraaf A, Hoekstra OS, Postmus PE, and de Vries PM.
Ventilation and perfusion imaging by electrical impedance tomography: a comparison
with radionuclide scanning. Physiol Meas 1998; 19: 481-490.
250.
Kunst PW, Vonk Noordegraaf AV, Raaijmakers E, Bakker J, Groeneveld AB,
Postmus PE, and de Vries PM. Electrical impedance tomography in the assessment
of extravascular lung water in noncardiogenic acute respiratory failure. Chest 1999;
116: 1695-1702.
251.
Kunzelmann K, Schreiber R, and Boucherot A. Mechanisms of the inhibition of
epithelial Na(+) channels by CFTR and purinergic stimulation. Kidney Int 2001; 60:
455-461.
252.
Kunzelmann K, Schreiber R, Nitschke R, and Mall M. Control of epithelial Na+
conductance by the cystic fibrosis transmembrane conductance regulator. Pflugers
Arch 2000; 440: 193-201.
253.
Laffon M, Jayr C, Barbry P, Wang Y, Folkesson HG, Pittet JF, Clerici C, and
Matthay MA. Lidocaine induces a reversible decrease in alveolar epithelial fluid
clearance in rats. Anesthesiology 2002; 96: 392-399.
254.
Lai-Fook SJ. Mechanical factors in lung liquid distribution. Annu Rev Physiol 1993;
55: 155-179.
255.
Laine GA, Allen SJ, Katz J, Gabel JC, and Drake RE. Effect of systemic venous
pressure elevation on lymph flow and lung edema formation. J Appl Physiol 1986; 61:
1634-1638.
256.
Lane SM, Maender KC, Awender NE, and Maron MB. Adrenal epinephrine
increases alveolar liquid clearance in a canine model of neurogenic pulmonary edema.
Am J Respir Crit Care Med 1998; 158: 760-768.
257.
Lauweryns JM and Baert JH. Alveolar clearance and the role of the pulmonary
lymphatics. Am Rev Respir Dis 1977; 115: 625-683.
119
258.
Lazrak A, Nielsen VG, and Matalon S. Mechanisms of increased Na(+) transport in
ATII cells by cAMP: we agree to disagree and do more experiments [see comments].
259.
Lazrak A, Samanta A, Venetsanou K, Barbry P, and Matalon S. Modification of
biophysical properties of lung epithelial Na(+) channels by dexamethasone. Am J
Physiol Cell Physiol 2000; 279: C762-770.
260.
Lefrancois R, Gautier H, and Pasquis P. Mécanique ventilatoire chez l'homme à
haute altitude. C R Soc Biol 1969; 163: 2037-2042.
261.
Leith DE and Brown R. Human lung volumes and the mechanisms that set them. Eur
Respir J 1999; 13: 468-472.
262.
Lindert J, Perlman CE, Parthasarathi K, and Bhattacharya J. Chloride-dependent
secretion of alveolar wall liquid determined by optical-sectioning microscopy. Am J
Respir Cell Mol Biol 2007; 36: 688-696.
263.
Lingueglia E, Voilley N, Waldmann R, Lazdunski M, and Barbry P. Expression
cloning of an epithelial amiloride-sensitive Na+ channel. A new channel type with
homologies to Caenorhabditis elegans degenerins. FEBS Lett 1993; 318: 95-99.
264.
Lumb A. Functional anatomy of the respiratory tract. In: Nunn's applied respiratory
physiology. Edinburgh: Butterworth-Heinemann, 2000, p. 15-36.
265.
Lumb A. Pulmonary vascular disease. In: Nunn's applied respiratory physiology.
Edinburgh: Butterworth-Heinemann, 2000, p. 541-550.
266.
Lumb A. Respiratory system resistance. In: Nunn's applied respiratory physiology.
Edinburgh: Butterworth-Heinemann, 2000, p. 58-81.
267.
Maggiorini M, Brunner-La Rocca HP, Peth S, Fischler M, Bohm T, Bernheim A,
Kiencke S, Bloch KE, Dehnert C, Naeije R, Lehmann T, Bartsch P, and
Mairbaurl H. Both tadalafil and dexamethasone may reduce the incidence of highaltitude pulmonary edema: a randomized trial. Ann Intern Med 2006; 145: 497-506.
268.
Maggiorini M, Buhler B, Walter M, and Oelz O. Prevalence of acute mountain
sickness in the Swiss Alps. Bmj 1990; 301: 853-855.
269.
Maggiorini M, Melot C, Pierre S, Pfeiffer F, Greve I, Sartori C, Lepori M,
Hauser M, Scherrer U, and Naeije R. High-altitude pulmonary edema is initially
caused by an increase in capillary pressure. Circulation 2001; 103: 2078-2083.
120
270.
Mairbaurl H, Mayer K, Kim KJ, Borok Z, Bartsch P, and Crandall ED. Hypoxia
decreases active Na transport across primary rat alveolar epithelial cell monolayers.
271.
Mairbaurl H, Schwobel F, Hoschele S, Maggiorini M, Gibbs S, Swenson ER, and
Bartsch P. Altered ion transporter expression in bronchial epithelium in mountaineers
with high-altitude pulmonary edema. J Appl Physiol 2003; 95: 1843-1850.
272.
Mairbaurl H, Weymann J, Mohrlein A, Swenson ER, Maggiorini M, Gibbs JS,
and Bartsch P. Nasal epithelium potential difference at high altitude (4,559 m):
evidence for secretion. Am J Respir Crit Care Med 2003; 167: 862-867.
273.
Mairbaurl H, Wodopia R, Eckes S, Schulz S, and Bartsch P. Impairment of cation
transport in A549 cells and rat alveolar epithelial cells by hypoxia. Am J Physiol 1997;
273: L797-806.
274.
Mansell A, Powles A, and Sutton J. Changes in pulmonary PV characteristics of
human subjects at an altitude of 5,366 m. J Appl Physiol 1980; 49: 79-83.
275.
Marantz PR, Tobin JN, Wassertheil-Smoller S, Steingart RM, Wexler JP,
Budner N, Lense L, and Wachspress J. The relationship between left ventricular
systolic function and congestive heart failure diagnosed by clinical criteria.
Circulation 1988; 77: 607-612.
276.
Maron MB. Dose-response relationship between plasma epinephrine concentration
and alveolar liquid clearance in dogs. J Appl Physiol 1998; 85: 1702-1707.
277.
Maron MB, Hamilton LH, and Maksud MG. Alterations in pulmonary function
consequent to competitive marathon running. Med Sci Sports 1979; 11: 244-249.
278.
Martin GS, Moss M, Wheeler AP, Mealer M, Morris JA, and Bernard GR. A
randomized, controlled trial of furosemide with or without albumin in
hypoproteinemic patients with acute lung injury. Crit Care Med 2005; 33: 1681-1687.
279.
changes occurring on ascent to altitude. Current anaesthesia and critical care 2000;
11: 34-41.
280.
Mason NP, Barry PW, Despiau G, Gardette B, and Richalet JP. Cough frequency
and cough receptor sensitivity to citric acid challenge during a simulated ascent to
extreme altitude. Eur Respir J 1999; 13: 508-513.
121
281.
Mason NP, Barry PW, Pollard AJ, Collier DJ, Taub NA, Miller MR, and
Milledge JS. Serial changes in spirometry during an ascent to 5,300 m in the Nepalese
Himalayas. High Alt Med Biol 2000; 1: 185-195.
282.
Mason NP, Petersen M, Melot C, Imanow B, Matveykine O, Gautier MT,
Sarybaev A, Aldashev A, Mirrakhimov MM, Brown BH, Leathard AD, and
Naeije R. Serial changes in nasal potential difference and lung electrical impedance
tomography at high altitude. J Appl Physiol 2003; 94: 2043-2050.
283.
Mason NP, Petersen M, Melot C, Kim EV, Aldashev A, Sarybaev A,
Mirrakhimov MM, and Naeije R. Changes in plasma bradykinin concentration and
citric acid cough threshold at altitude. Wilderness and Environmental Medicine 2009;
20: 353-358.
284.
Mason RJ, Williams MC, Widdicombe JH, Sanders MJ, Misfeldt DS, and Berry
LC, Jr. Transepithelial transport by pulmonary alveolar type II cells in primary
culture. Proc Natl Acad Sci U S A 1982; 79: 6033-6037.
285.
Matalon S, Hardiman KM, Jain L, Eaton DC, Kotlikoff M, Eu JP, Sun J,
Meissner G, and Stamler JS. Regulation of ion channel structure and function by
reactive oxygen-nitrogen species. Am J Physiol Lung Cell Mol Physiol 2003; 285:
L1184-1189.
286.
Matalon S, Lazrak A, Jain L, and Eaton DC. Invited review: biophysical properties
of sodium channels in lung alveolar epithelial cells. J Appl Physiol 2002; 93: 18521859.
287.
Matsuzawa Y, Fujimoto K, Kobayashi T, Namushi NR, Harada K, Kohno H,
Fukushima M, and Kusama S. Blunted hypoxic ventilatory drive in subjects
susceptible to high-altitude pulmonary edema. J Appl Physiol 1989; 66: 1152-1157.
288.
Matthay MA, Berthiaume Y, and Staub NC. Long-term clearance of liquid and
protein from the lungs of unanesthetized sheep. J Appl Physiol 1985; 59: 928-934.
289.
Matthay MA, Landolt CC, and Staub NC. Differential liquid and protein clearance
from the alveoli of anesthetized sheep. J Appl Physiol 1982; 53: 96-104.
290.
Matthay MA and Wiener-Kronish JP. Intact epithelial barrier function is critical for
the resolution of alveolar edema in humans. Am Rev Respir Dis 1990; 142: 1250-1257.
291.
Mazzone SB. An overview of the sensory receptors regulating cough. Cough 2005; 1:
2.
122
292.
Neurobiol 2006; 152: 363-371.
293.
Meehan RT, Uchakin PN, and Sams CF. High Altitude and Human Immune
Responsiveness. In: High Altitude: an exploration of human adaptation, edited by
Hornbein T and Schoene R. New York: Marcel Dekker, 2001, p. 537-539.
294.
Melon J. [Contribution to the study of the secretory activity of the nasal mucosa.
Characteristics of the permeability of the nasal epithelium. Chemical composition of
the nucus. The mechanism of its formation under normal conditions and during
paroxystic hypersecretion]. Acta Otorhinolaryngol Belg 1968; 22: 5-244.
295.
Michiels C, Minet E, Mottet D, and Raes M. Regulation of gene expression by
oxygen: NF-kappaB and HIF-1, two extremes. Free Radic Biol Med 2002; 33: 12311242.
296.
Miles DS, Cox MH, Bomze JP, and Gotshall RW. Acute recovery profile of lung
volumes and function after running 5 miles. J Sports Med Phys Fitness 1991; 31: 243248.
297.
Miles DS, Doerr CE, Schonfeld SA, Sinks DE, and Gotshall RW. Changes in
pulmonary diffusing capacity and closing volume after running a marathon. Respir
Physiol 1983; 52: 349-359.
298.
Miller MR, Dickinson SA, and Hitchings DJ. The accuracy of portable peak flow
meters. Thorax 1992; 47: 904-909.
299.
Miniati M, Pistolesi M, Milne EN, and Giuntini C. Detection of lung edema. Crit
Care Med 1987; 15: 1146-1155.
300.
Molliex S, Crestani B, Dureuil B, Bastin J, Rolland C, Aubier M, and Desmonts
JM. Effects of halothane on surfactant biosynthesis by rat alveolar type II cells in
primary culture. Anesthesiology 1994; 81: 668-676.
301.
Morice AH and Kastelik JA. Cough. 1: Chronic cough in adults. Thorax 2003; 58:
901-907.
302.
Morrell ED, Tsai BM, Crisostomo PR, Hammoud ZT, and Meldrum DR.
Experimental therapies for hypoxia-induced pulmonary hypertension during acute
lung injury. Shock 2006; 25: 214-226.
123
303.
Everest region of Nepal. Aviat Space Environ Med 1995; 66: 148-151.
304.
Nadel JA and Widdicombe JG. Effect of changes in blood gas tensions and carotid
sinus pressure on tracheal volume and total lung resistance to airflow. J Physiol 1962;
163: 13-33.
305.
Naeije R and Melot C. Acute pulmonary oedema on the Ruwenzori mountain range.
Br Heart J 1990; 64: 400-402.
306.
Negrini D, Passi A, de Luca G, and Miserocchi G. Pulmonary interstitial pressure
and proteoglycans during development of pulmonary edema. Am J Physiol 1996; 270:
H2000-2007.
307.
Negrini D, Passi A, De Luca G, and Miserochi G. Proteoglycan involvement during
development of lesional pulmonary edema. Am J Physiol 1998; 274: L203-211.
308.
Nicoloff DM, Ballin HM, and Visscher MB. Hypoxia and edema of the perfused
isolated canine lung. Proc Soc Exp Biol Med 1969; 131: 22-26.
309.
Nielsen VG, Baird MS, Geary BT, and Matalon S. Halothane does not decrease
amiloride-sensitive alveolar fluid clearance in rabbits. Anesth Analg 2000; 90: 14451449.
310.
Nieminen MS, Bohm M, Cowie MR, Drexler H, Filippatos GS, Jondeau G, Hasin
Y, Lopez-Sendon J, Mebazaa A, Metra M, Rhodes A, Swedberg K, Priori SG,
Garcia MA, Blanc JJ, Budaj A, Dean V, Deckers J, Burgos EF, Lekakis J,
Lindahl B, Mazzotta G, Morais J, Oto A, Smiseth OA, Dickstein K, Albuquerque
A, Conthe P, Crespo-Leiro M, Ferrari R, Follath F, Gavazzi A, Janssens U,
Komajda M, Moreno R, Singer M, Singh S, Tendera M, and Thygesen K.
Executive summary of the guidelines on the diagnosis and treatment of acute heart
failure: the Task Force on Acute Heart Failure of the European Society of Cardiology.
Eur Heart J 2005; 26: 384-416.
311.
Noble TJ, Harris ND, Morice AH, Milnes P, and Brown BH. Diuretic induced
change in lung water assessed by electrical impedance tomography. Physiol Meas
2000; 21: 155-163.
312.
Noble TJ, Morice AH, Channer KS, Milnes P, Harris ND, and Brown BH.
Monitoring patients with left ventricular failure by electrical impedance tomography.
Eur J Heart Fail 1999; 1: 379-384.
124
313.
Noble WH, Kay JC, and Obdrzalek J. Lung mechanics in hypervolemic pulmonary
edema. J Appl Physiol 1975; 38: 681-687.
314.
Noda M, Suzuki S, Tsubochi H, Sugita M, Maeda S, Kobayashi S, Kubo H, and
Kondo T. Single dexamethasone injection increases alveolar fluid clearance in adult
rats. Crit Care Med 2003; 31: 1183-1189.
315.
Norlin A, Baines DL, and Folkesson HG. Role of endogenous cortisol in basal liquid
clearance from distal air spaces in adult guinea-pigs. J Physiol 1999; 519 Pt 1: 261272.
316.
Norlin A, Finley N, Abedinpour P, and Folkesson HG. Alveolar liquid clearance in
the anesthetized ventilated guinea pig. Am J Physiol 1998; 274: L235-243.
317.
Norlin A, Lu LN, Guggino SE, Matthay MA, and Folkesson HG. Contribution of
amiloride-insensitive pathways to alveolar fluid clearance in adult rats. J Appl Physiol
2001; 90: 1489-1496.
318.
Noviski N, Bar-Yishay E, Gur I, and Godfrey S. Exercise intensity determines and
climatic conditions modify the severity of exercise-induced asthma. Am Rev Respir
Dis 1987; 136: 592-594.
319.
O'Brodovich H. Fetal lung liquid secretion: insights using the tools of inhibitors and
genetic knock-out experiments. Am J Respir Cell Mol Biol 2001; 25: 8-10.
320.
O'Brodovich H, Canessa C, Ueda J, Rafii B, Rossier BC, and Edelson J.
Expression of the epithelial Na+ channel in the developing rat lung. Am J Physiol
1993; 265: C491-496.
321.
O'Brodovich H, Hannam V, and Rafii B. Sodium channel but neither Na(+)-H+ nor
Na-glucose symport inhibitors slow neonatal lung water clearance. Am J Respir Cell
Mol Biol 1991; 5: 377-384.
322.
O'Connell F. Central pathways for cough in man--unanswered questions. Pulm
Pharmacol Ther 2002; 15: 295-301.
323.
O'Grady SM, Jiang X, and Ingbar DH. Cl-channel activation is necessary for
stimulation of Na transport in adult alveolar epithelial cells. Am J Physiol Lung Cell
Mol Physiol 2000; 278: L239-244.
324.
O'Grady SM and Lee SY. Chloride and potassium channel function in alveolar
epithelial cells. Am J Physiol Lung Cell Mol Physiol 2003; 284: L689-700.
125
325.
Oelz O, Maggiorini M, Ritter M, Waber U, Jenni R, Vock P, and Bartsch P.
Nifedipine for high altitude pulmonary oedema. Lancet 1989; 2: 1241-1244.
326.
Ogawa S, Gerlach H, Esposito C, Pasagian-Macaulay A, Brett J, and Stern D.
Hypoxia modulates the barrier and coagulant function of cultured bovine endothelium.
Increased monolayer permeability and induction of procoagulant properties. J Clin
Invest 1990; 85: 1090-1098.
327.
Olivera W, Ridge K, Wood LD, and Sznajder JI. ANF decreases active sodium
transport and increases alveolar epithelial permeability in rats. J Appl Physiol 1993;
75: 1581-1586.
328.
Olivera WG, Ciccolella DE, Barquin N, Ridge KM, Rutschman DH, Yeates DB,
and Sznajder JI. Aldosterone regulates Na,K-ATPase and increases lung edema
clearance in rats. Am J Respir Crit Care Med 2000; 161: 567-573.
329.
Olver RE, Ramsden CA, Strang LB, and Walters DV. The role of amilorideblockable sodium transport in adrenaline-induced lung liquid reabsorption in the fetal
lamb. J Physiol 1986; 376: 321-340.
330.
Otulakowski G, Flueckiger-Staub S, Ellis L, Ramlall K, Staub O, Smith D, Durie
P, and O'Brodovich H. Relation between alpha, beta, and gamma human amiloridesensitive epithelial Na+ channel mRNA levels and nasal epithelial potential difference
in healthy men. Am J Respir Crit Care Med 1998; 158: 1213-1220.
331.
Paintal AS. Mechanism of stimulation of type J pulmonary receptors. J Physiol 1969;
203: 511-532.
332.
Pantaleo T, Bongianni F, and Mutolo D. Central nervous mechanisms of cough.
Pulm Pharmacol Ther 2002; 15: 227-233.
333.
Parsons JP and Mastronarde JG. Exercise-induced bronchoconstriction in athletes.
Chest 2005; 128: 3966-3974.
334.
Passi A, Negrini D, Albertini R, De Luca G, and Miserocchi G. Involvement of
lung interstitial proteoglycans in development of hydraulic- and elastase-induced
edema. Am J Physiol 1998; 275: L631-635.
335.
Pedersen OF, Miller MR, Sigsgaard T, Tidley M, and Harding RM. Portable peak
flow meters: physical characteristics, influence of temperature, altitude, and humidity.
Eur Respir J 1994; 7: 991-997.
126
336.
Pilewski JM and Frizzell RA. Role of CFTR in airway disease. Physiol Rev 1999;
79: S215-255.
337.
Pitkanen O, Tanswell AK, Downey G, and O'Brodovich H. Increased PO2 alters
the bioelectric properties of fetal distal lung epithelium. Am J Physiol 1996; 270:
L1060-1066.
338.
Pitkanen OM and O'Brodovich HM. Significance of ion transport during lung
development and in respiratory disease of the newborn. Ann Med 1998; 30: 134-142.
339.
Pitkanen OM, Smith D, O'Brodovich H, and Otulakowski G. Expression of alpha-,
beta-, and gamma-hENaC mRNA in the human nasal, bronchial, and distal lung
epithelium. Am J Respir Crit Care Med 2001; 163: 273-276.
340.
Planes C, Blot-Chabaud M, Matthay MA, Couette S, Uchida T, and Clerici C.
Hypoxia and beta 2-agonists regulate cell surface expression of the epithelial sodium
channel in native alveolar epithelial cells. J Biol Chem 2002; 277: 47318-47324.
341.
Planes C, Escoubet B, Blot-Chabaud M, Friedlander G, Farman N, and Clerici
C. Hypoxia downregulates expression and activity of epithelial sodium channels in rat
alveolar epithelial cells. Am J Respir Cell Mol Biol 1997; 17: 508-518.
342.
Planes C, Friedlander G, Loiseau A, Amiel C, and Clerici C. Inhibition of Na-KATPase activity after prolonged hypoxia in an alveolar epithelial cell line. Am J
Physiol 1996; 271: L70-78.
343.
Fraser RS, Miller MR, and Milledge JS. Hypoxia, hypocapnia and spirometry at
altitude [published erratum appears in Clin Sci (Colch) 1997 Dec;93(6):611]. Clin Sci
(Colch) 1997; 92: 593-598.
344.
Pollard AJ, Mason NP, Barry PW, Pollard RC, Collier DJ, Fraser RS, Miller
MR, and Milledge JS. Effect of altitude on spirometric parameters and the
performance of peak flow meters. Thorax 1996; 51: 175-178.
345.
(previously referred to as postnasal drip syndrome): ACCP evidence-based clinical
practice guidelines. Chest 2006; 129: 63S-71S.
346.
guidelines. Chest 2006; 129: 72S-74S.
127
347.
Pratter MR. Overview of common causes of chronic cough: ACCP evidence-based
clinical practice guidelines. Chest 2006; 129: 59S-62S.
348.
Pugh L, Gill M, Lahiri S, Milledge J, Ward M, and West J. Muscular exercise at
great altitude. J Appl Physiol 1964; 19: 431-440.
349.
Rabold M. High altitude bronchitis on Cerro Aconcagua (abstract). Wilderness
Medical Society Newsletter 1987.
350.
Rahn H and Hammond D. Vital capacity at reduced barometric pressure. J Appl
Physiol 1952; 4: 715 - 725.
351.
Ramminger SJ, Baines DL, Olver RE, and Wilson SM. The effects of PO2 upon
transepithelial ion transport in fetal rat distal lung epithelial cells. J Physiol 2000; 524
Pt 2: 539-547.
352.
Raymond L and Severinghaus J. Static pulmonary compliance of man during
altitude hypoxia. J Appl Physiol 1971; 31: 785-787.
353.
Reeves JT, Halpin J, Cohn JE, and Daoud F. Increased alveolar-arterial oxygen
difference during simulated high-altitude exposure. J Appl Physiol 1969; 27: 658-661.
354.
Reynolds AM and McEvoy RD. Tachykinins mediate hypocapnia-induced
bronchoconstriction in guinea pigs. J Appl Physiol 1989; 67: 2454-2460.
355.
Rezaiguia-Delclaux S, Jayr C, Luo DF, Saidi NE, Meignan M, and Duvaldestin P.
Halothane and isoflurane decrease alveolar epithelial fluid clearance in rats.
Anesthesiology 1998; 88: 751-760.
356.
Richalet JP. The Endocrine System. In: High Altitude: an exploration of human
adaptation, edited by Hornbein T and Schoene R. New York: Marcel Dekker, 2001, p.
601- 644.
357.
Richalet JP, Robach P, Jarrot S, Schneider JC, Mason NP, Cauchy E, Herry JP,
Bienvenu A, Gardette B, and Gortan C. Operation Everest III (COMEX '97).
Effects of prolonged and progressive hypoxia on humans during a simulated ascent to
8,848 M in a hypobaric chamber. Adv Exp Med Biol 1999; 474: 297-317.
358.
Ridge KM, Dada L, Lecuona E, Bertorello AM, Katz AI, Mochly-Rosen D, and
Sznajder JI. Dopamine-induced exocytosis of Na,K-ATPase is dependent on
activation of protein kinase C-epsilon and -delta. Mol Biol Cell 2002; 13: 1381-1389.
128
359.
Ridge KM, Olivera WG, Saldias F, Azzam Z, Horowitz S, Rutschman DH,
Dumasius V, Factor P, and Sznajder JI. Alveolar type 1 cells express the alpha2
Na,K-ATPase, which contributes to lung liquid clearance. Circ Res 2003; 92: 453460.
360.
Roach R, Bartsch P, Hackett P, and Oelz O. The Lake Louise acute mountain
sickness scoring system. In: Hypoxia and Mountain Medicine, edited by Sutton J,
Houston C and Coates G. Burlington: Queen City Printers, 1993, p. 327-330.
361.
Rocker GM. Bedside measurement of pulmonary capillary permeability in patients
with acute lung injury. What have we learned? Intensive Care Med 1996; 22: 619-621.
362.
Rosenstein BJ and Cutting GR. The diagnosis of cystic fibrosis: a consensus
statement. Cystic Fibrosis Foundation Consensus Panel. J Pediatr 1998; 132: 589-595.
363.
Roux J, Kawakatsu H, Gartland B, Pespeni M, Sheppard D, Matthay MA,
Canessa CM, and Pittet JF. Interleukin-1beta decreases expression of the epithelial
sodium channel alpha-subunit in alveolar epithelial cells via a p38 MAPK-dependent
signaling pathway. J Biol Chem 2005; 280: 18579-18589.
364.
Roy SB, Balasubramanian V, Khan MR, Kaushik VS, Manchanda SC, and Guha
SK. Transthoracic electrical impedance in cases of high-altitude hypoxia. Br Med J
1974; 3: 771-775.
365.
Roy SB, Guleria JS, Khanna PK, Manchanda SC, Pande JN, and Subba PS.
Haemodynamic studies in high altitude pulmonary oedema. Br Heart J 1969; 31: 5258.
366.
Rutschman DH, Olivera W, and Sznajder JI. Active transport and passive liquid
movement in isolated perfused rat lungs. J Appl Physiol 1993; 75: 1574-1580.
367.
Ryan US. Metabolic activity of pulmonary endothelium: modulations of structure and
function. Annu Rev Physiol 1986; 48: 263-277.
368.
Said SI. Metabolic functions of the pulmonary circulation. Circ Res 1982; 50: 325333.
369.
Sakka SG, Ruhl CC, Pfeiffer UJ, Beale R, McLuckie A, Reinhart K, and MeierHellmann A. Assessment of cardiac preload and extravascular lung water by single
transpulmonary thermodilution. Intensive Care Med 2000; 26: 180-187.
129
370.
Sakuma T, Folkesson HG, Suzuki S, Okaniwa G, Fujimura S, and Matthay MA.
Beta-adrenergic agonist stimulated alveolar fluid clearance in ex vivo human and rat
lungs. Am J Respir Crit Care Med 1997; 155: 506-512.
371.
Sakuma T, Okaniwa G, Nakada T, Nishimura T, Fujimura S, and Matthay MA.
Alveolar fluid clearance in the resected human lung [see comments]. Am J Respir Crit
Care Med 1994; 150: 305-310.
372.
Sakuma T, Zhao Y, Sugita M, Sagawa M, Toga H, Ishibashi T, Nishio M, and
Matthay MA. Malnutrition impairs alveolar fluid clearance in rat lungs. Am J Physiol
Lung Cell Mol Physiol 2004; 286: L1268-1274.
373.
Saldias FJ, Lecuona E, Comellas AP, Ridge KM, and Sznajder JI. Dopamine
restores lung ability to clear edema in rats exposed to hyperoxia. Am J Respir Crit
Care Med 1999; 159: 626-633.
374.
Sampson JB, Cymerman A, Burse RL, Maher JT, and Rock PB. Procedures for
the measurement of acute mountain sickness. Aviat Space Environ Med 1983; 54:
1063-1073.
375.
Sant'Ambrogio G and Widdicombe J. Reflexes from airway rapidly adapting
receptors. Respir Physiol 2001; 125: 33-45.
376.
Sartori C, Allemann Y, Duplain H, Lepori M, Egli M, Lipp E, Hutter D, Turini
P, Hugli O, Cook S, Nicod P, and Scherrer U. Salmeterol for the prevention of highaltitude pulmonary edema. N Engl J Med 2002; 346: 1631-1636.
377.
Sartori C, Duplain H, Lepori M, Egli M, Maggiorini M, Nicod P, and Scherrer U.
High altitude impairs nasal transepithelial sodium transport in HAPE-prone subjects.
Eur Respir J 2004; 23: 916-920.
378.
Sartori C, Vollenweider L, Loffler BM, Delabays A, Nicod P, Bartsch P, and
Scherrer U. Exaggerated endothelin release in high-altitude pulmonary edema.
Circulation 1999; 99: 2665-2668.
379.
Saunders NA, Betts MF, Pengelly LD, and Rebuck AS. Changes in lung mechanics
induced by acute isocapnic hypoxia. J Appl Physiol 1977; 42: 413-419.
380.
Scadding G and Scadding GK. Update on the use of nitric oxide as a noninvasive
measure of airways inflammation. Rhinology 2009; 47: 115-120.
130
381.
Scherrer U, Vollenweider L, Delabays A, Savcic M, Eichenberger U, Kleger GR,
Fikrle A, Ballmer PE, Nicod P, and Bartsch P. Inhaled nitric oxide for high-altitude
pulmonary edema. N Engl J Med 1996; 334: 624-629.
382.
Schneeberger EE and Lynch RD. Structure, function, and regulation of cellular tight
junctions. Am J Physiol 1992; 262: L647-661.
383.
Schneeberger EE and Lynch RD. The tight junction: a multifunctional complex. Am
J Physiol Cell Physiol 2004; 286: C1213-1228.
384.
Schoene RB, Hackett PH, Henderson WR, Sage EH, Chow M, Roach RC, Mills
WJ, Jr., and Martin TR. High-altitude pulmonary edema. Characteristics of lung
lavage fluid. Jama 1986; 256: 63-69.
385.
Schoene RB, Swenson ER, Pizzo CJ, Hackett PH, Roach RC, Mills WJ, Jr.,
Henderson WR, Jr., and Martin TR. The lung at high altitude: bronchoalveolar
lavage in acute mountain sickness and pulmonary edema. J Appl Physiol 1988; 64:
2605-2613.
386.
Schoner W. Endogenous cardiac glycosides, a new class of steroid hormones. Eur J
Biochem 2002; 269: 2440-2448.
387.
Schuster DP and Calandrino FS. Single versus double indicator dilution
measurements of extravascular lung water. Crit Care Med 1991; 19: 84-88.
388.
Schwiebert EM, Potter ED, Hwang TH, Woo JS, Ding C, Qiu W, Guggino WB,
Levine MA, and Guggino SE. cGMP stimulates sodium and chloride currents in rat
tracheal airway epithelia. Am J Physiol 1997; 272: C911-922.
389.
Scillia P, Delcroix M, Lejeune P, Melot C, Struyven J, Naeije R, and Gevenois
PA. Hydrostatic pulmonary edema: evaluation with thin-section CT in dogs.
Radiology 1999; 211: 161-168.
390.
Scillia P, Kafi SA, Melot C, Keyzer C, Naeije R, and Gevenois PA. Oleic acidinduced lung injury: thin-section CT evaluation in dogs. Radiology 2001; 219: 724731.
391.
Scoggin CH, Hyers TM, Reeves JT, and Grover RF. High-altitude pulmonary
edema in the children and young adults of Leadville, Colorado. N Engl J Med 1977;
297: 1269-1272.
131
392.
Seals D and Jones P. Autonomic nervous system. In: High Altitude: an exploration of
human adaptation, edited by Hornbein T and Schoene R. New York: Marcel Dekker,
2001, p. 426-433.
393.
Senn O, Clarenbach CF, Fischler M, Thalmann R, Brunner-La Rocca H, Egger
P, Maggiorini M, and Bloch KE. Do changes in lung function predict high-altitude
pulmonary edema at an early stage? Med Sci Sports Exerc 2006; 38: 1565-1570.
394.
Serikov VB, Grady M, and Matthay MA. Effect of temperature on alveolar liquid
and protein clearance in an in situ perfused goat lung. J Appl Physiol 1993; 75: 940947.
395.
Sharma S and Brown B. Spirometry and respiratory muscle function during ascent to
higher altitudes. Lung 2007; 185: 113-121.
396.
Sheppard DN and Welsh MJ. Structure and function of the CFTR chloride channel.
Physiol Rev 1999; 79: S23-45.
397.
Shields JL, Hannon JP, Harris CW, and Platner WS. Effects of altitude
acclimatization on pulmonary function in women. J Appl Physiol 1968; 25: 606-609.
398.
Singh I, Khanna PK, Srivastava MC, Lal M, Roy SB, and Subramanyam CS.
Acute mountain sickness. N Engl J Med 1969; 280: 175-184.
399.
Skou JC. The identification of the sodium pump. Biosci Rep 2004; 24: 436-451.
400.
Smedira N, Gates L, Hastings R, Jayr C, Sakuma T, Pittet JF, and Matthay MA.
Alveolar and lung liquid clearance in anesthetized rabbits. J Appl Physiol 1991; 70:
1827-1835.
401.
402.
Song Y, Fukuda N, Bai C, Ma T, Matthay MA, and Verkman AS. Role of
aquaporins in alveolar fluid clearance in neonatal and adult lung, and in oedema
formation following acute lung injury: studies in transgenic aquaporin null mice. J
Physiol 2000; 525 Pt 3: 771-779.
403.
Staub NC. Pulmonary edema. Physiol Rev 1974; 54: 678-811.
404.
Staub NC. Pulmonary edema due to increased microvascular permeability to fluid and
protein. Circ Res 1978; 43: 143-151.
405.
Staub NC, Nagano H, and Pearce ML. Pulmonary edema in dogs, especially the
sequence of fluid accumulation in lungs. J Appl Physiol 1967; 22: 227-240.
132
406.
Steele P. Medicine on Mount Everest 1971. Lancet 1971; 2: 32-39.
407.
Steinacker JM, Tobias P, Menold E, Reissnecker S, Hohenhaus E, Liu Y,
Lehmann M, Bartsch P, and Swenson ER. Lung diffusing capacity and exercise in
subjects with previous high altitude pulmonary oedema. Eur Respir J 1998; 11: 643650.
408.
Stenmark KR, Davie NJ, Reeves JT, and Frid MG. Hypoxia, leukocytes, and the
pulmonary circulation. J Appl Physiol 2005; 98: 715-721.
409.
Stokes JB and Sigmund RD. Regulation of rENaC mRNA by dietary NaCl and
steroids: organ, tissue, and steroid heterogeneity. Am J Physiol 1998; 274: C16991707.
410.
Strauss RH, McFadden ER, Jr., Ingram RH, Jr., Deal EC, Jr., and Jaeger JJ.
Influence of heat and humidity on the airway obstruction induced by exercise in
asthma. J Clin Invest 1978; 61: 433-440.
411.
Strauss RH, McFadden ER, Jr., Ingram RH, Jr., and Jaeger JJ. Enhancement of
exercise-induced asthma by cold air. N Engl J Med 1977; 297: 743-747.
412.
Sutton JR, Bryan AC, Gray GW, Horton ES, Rebuck AS, Woodley W, Rennie
ID, and Houston CS. Pulmonary gas exchange in acute mountain sickness. Aviat
Space Environ Med 1976; 47: 1032-1037.
413.
Sutton JR, Gray GW, McFadden MD, Bryan AC, Horton ES, and Houston CS.
Nitrogen washout studies in acute mountain sickness. Aviat Space Environ Med 1977;
48: 108-110.
414.
Sutton JR, Reeves JT, Wagner PD, Groves BM, Cymerman A, Malconian MK,
Rock PB, Young PM, Walter SD, and Houston CS. Operation Everest II: oxygen
transport during exercise at extreme simulated altitude. J Appl Physiol 1988; 64: 13091321.
415.
Suzuki S, Tsubochi H, Suzuki T, Darnel AD, Krozowski ZS, Sasano H, and
Kondo T. Modulation of transalveolar fluid absorption by endogenous aldosterone in
adult rats. Exp Lung Res 2001; 27: 143-155.
416.
Svartengren M, Anderson M, Philipson K, and Camner P. Human lung deposition
of particles suspended in air or in helium/oxygen mixture. Exp Lung Res 1989; 15:
575-585.
133
417.
Svensson S, Olin AC, and Hellgren J. Increased net water loss by oral compared to
nasal expiration in healthy subjects. Rhinology 2006; 44: 74-77.
418.
Swenson ER. Renal function and fluid homeostasis. In: High Altitude: an exploration
of human adaptation, edited by Hornbein T and Schoene R. New York: Marcel
Dekker, 2001, p. 537-539.
419.
Swenson ER, Maggiorini M, Mongovin S, Gibbs JS, Greve I, Mairbaurl H, and
Bartsch P. Pathogenesis of high-altitude pulmonary edema: inflammation is not an
etiologic factor. Jama 2002; 287: 2228-2235.
420.
Sznajder JI, Ridge KM, Yeates DB, Ilekis J, and Olivera W. Epidermal growth
factor increases lung liquid clearance in rat lungs. J Appl Physiol 1998; 85: 10041010.
421.
422.
Tenney S, Rahn H, Stroud R, and Mithoefer J. Adaptation to high altitude: changes
in lung volumes during the first seven days at Mt. Evans, Colorado. J Appl Physiol
1953; 5: 607-613.
423.
Tharaux PL, Dussaule JC, Couette S, and Clerici C. Evidence for functional ANP
receptors in cultured alveolar type II cells. Am J Physiol 1998; 274: L244-251.
424.
Therien AG and Blostein R. Mechanisms of sodium pump regulation. Am J Physiol
Cell Physiol 2000; 279: C541-566.
425.
Thome UH, Davis IC, Nguyen SV, Shelton BJ, and Matalon S. Modulation of
sodium transport in fetal alveolar epithelial cells by oxygen and corticosterone. Am J
Physiol Lung Cell Mol Physiol 2003; 284: L376-385.
426.
response to carbon dioxide on ascent to high altitude. Respir Med 2009; 103: 11821188.
427.
Tomlinson LA, Carpenter TC, Baker EH, Bridges JB, and Weil JV. Hypoxia
reduces airway epithelial sodium transport in rats. Am J Physiol 1999; 277: L881-886.
428.
Trapnell BC, Chu CS, Paakko PK, Banks TC, Yoshimura K, Ferrans VJ,
Chernick MS, and Crystal RG. Expression of the cystic fibrosis transmembrane
134
conductance regulator gene in the respiratory tract of normal individuals and
individuals with cystic fibrosis. Proc Natl Acad Sci U S A 1991; 88: 6565-6569.
429.
Tsukimoto K, Mathieu-Costello O, Prediletto R, Elliott AR, and West JB.
Ultrastructural appearances of pulmonary capillaries at high transmural pressures. J
Appl Physiol 1991; 71: 573-582.
430.
Turner J, Mead J, and Wohl M. Elasticity of human lungs in relation to age. J Appl
Physiol 1968; 25: 664-671.
431.
Ulvedal F, Morgan TE, Cutler RG, and Welch BE. Ventilatory capacity during
prolonged exposure to simulated altitude without hypoxia. J Appl Physiol 1963; 18:
904-908.
432.
van Os CH, Kamsteeg EJ, Marr N, and Deen PM. Phsyiological relevance of
aquaporins: luxury or necessity? [In Process Citation]. Pflugers Arch 2000; 440: 513520.
433.
Varene P, Timbal J, and Jacquemin C. Effect of different ambient pressures on
airway resistance. J Appl Physiol 1967; 22: 699-706.
434.
Vejlstrup NG, Boyd CA, and Dorrington KL. Effect of lung inflation on active and
passive liquid clearance from in vivo rabbit lung. Am J Physiol 1994; 267: L482-487.
435.
Verkman AS, Matthay MA, and Song Y. Aquaporin water channels and lung
physiology. Am J Physiol Lung Cell Mol Physiol 2000; 278: L867-879.
436.
Visscher M. The Discussant. Pulmonary edema at high altitude. Med Thorac 1962;
19: 326-327.
437.
Vivona ML, Matthay M, Chabaud MB, Friedlander G, and Clerici C. Hypoxia
reduces alveolar epithelial sodium and fluid transport in rats: reversal by betaadrenergic agonist treatment. Am J Respir Cell Mol Biol 2001; 25: 554-561.
438.
Vock P, Brutsche MH, Nanzer A, and Bartsch P. Variable radiomorphologic data
of high altitude pulmonary edema. Features from 60 patients. Chest 1991; 100: 13061311.
439.
Vock P, Fretz C, Franciolli M, and Bartsch P. High-altitude pulmonary edema:
findings at high-altitude chest radiography and physical examination. Radiology 1989;
170: 661-666.
135
440.
Voilley N, Lingueglia E, Champigny G, Mattei MG, Waldmann R, Lazdunski M,
and Barbry P. The lung amiloride-sensitive Na+ channel: biophysical properties,
pharmacology, ontogenesis, and molecular cloning. Proc Natl Acad Sci U S A 1994;
91: 247-251.
441.
Wagner PD. Gas exchange. In: High Altitude: an exploration of human adaptation,
edited by Hornbein T and Schoene R. New York: Marcel Dekker, 2001, p. 199-234.
442.
Wagner PD, Gale GE, Moon RE, Torre-Bueno JR, Stolp BW, and Saltzman HA.
Pulmonary gas exchange in humans exercising at sea level and simulated altitude. J
Appl Physiol 1986; 61: 260-270.
443.
Wagner PD, Sutton JR, Reeves JT, Cymerman A, Groves BM, and Malconian
MK. Operation Everest II: pulmonary gas exchange during a simulated ascent of Mt.
Everest. J Appl Physiol 1987; 63: 2348-2359.
444.
Wagner WW, Jr., Latham LP, and Capen RL. Capillary recruitment during airway
hypoxia: role of pulmonary artery pressure. J Appl Physiol 1979; 47: 383-387.
445.
Wang Y, Folkesson HG, Jayr C, Ware LB, and Matthay MA. Alveolar epithelial
fluid transport can be simultaneously upregulated by both KGF and beta-agonist
therapy. J Appl Physiol 1999; 87: 1852-1860.
446.
Ward M, Milledge JS, and West JB. Ventilatory response to hypoxia and carbon
dioxide. In: High altitude medicine and physiology (3rd ed.). London: Arnold, 2000, p.
50-64.
447.
Ware LB and Matthay MA. The acute respiratory distress syndrome. N Engl J Med
2000; 342: 1334-1349.
448.
Ware LB and Matthay MA. Alveolar fluid clearance is impaired in the majority of
patients with acute lung injury and the acute respiratory distress syndrome. Am J
449.
Weiskopf RB and Severinghaus JW. Lack of effect of high altitude on hemoglobin
oxygen affinity. J Appl Physiol 1972; 33: 276-277.
450.
Weiss J, Haefeli WE, Gasse C, Hoffmann MM, Weyman J, Gibbs S, Mansmann
U, and Bartsch P. Lack of evidence for association of high altitude pulmonary edema
and polymorphisms of the NO pathway. High Alt Med Biol 2003; 4: 355-366.
136
451.
Welsh CH, Wagner PD, Reeves JT, Lynch D, Cink TM, Armstrong J, Malconian
MK, Rock PB, and Houston CS. Operation Everest. II: Spirometric and radiographic
changes in acclimatized humans at simulated high altitudes. Am Rev Respir Dis 1993;
147: 1239-1244.
452.
Welsh DA, Guery BP, Deboisblanc BP, Dobard EP, Creusy C, Mercante D,
Nelson S, Summer WR, and Mason CM. Keratinocyte growth factor attenuates
hydrostatic pulmonary edema in an isolated, perfused rat lung model. Am J Physiol
Heart Circ Physiol 2001; 280: H1311-1317.
453.
Welsh DA, Summer WR, Dobard EP, Nelson S, and Mason CM. Keratinocyte
growth factor prevents ventilator-induced lung injury in an ex vivo rat model. Am J
454.
West J. Obstructive diseases: chronic obstructive pulmonary disease. In: Pulmonary
Pathophysiology - The Essentials (6th ed.). Baltimore: Williams and Wilkins, 2003, p.
51-69.
455.
West J. Vascular diseases: pulmonary edema. In: Pulmonary Pathophysiology - The
Essentials (6th ed.). Baltimore: Williams and Wilkins, 2003, p. 101-111.
456.
West J. Ventilation/blood flow and gas exchange. Oxford: Blackwell Scientific
Publications, 1990.
457.
West JB. Diffusing capacity of the lung for carbon monoxide at high altitude. J Appl
Physiol 1962; 17: 421-426.
458.
West JB. Thoughts on the pulmonary blood-gas barrier. Am J Physiol Lung Cell Mol
Physiol 2003; 285: L501-513.
459.
West JB, Colice GL, Lee YJ, Namba Y, Kurdak SS, Fu Z, Ou LC, and MathieuCostello O. Pathogenesis of high-altitude pulmonary oedema: direct evidence of stress
failure of pulmonary capillaries [see comments]. Eur Respir J 1995; 8: 523-529.
460.
West JB and Mathieu-Costello O. Stress failure of pulmonary capillaries: role in
lung and heart disease [see comments]. Lancet 1992; 340: 762-767.
461.
Whayne TF, Jr. and Severinghaus JW. Experimental hypoxic pulmonary edema in
the rat. J Appl Physiol 1968; 25: 729-732.
137
462.
Widdcombe JH. How does cAMP increase active Na absorption across alveolar
epithelium? [editorial; comment]. Am J Physiol Lung Cell Mol Physiol 2000; 278:
L231-232.
463.
Widdicombe JG. Afferent receptors in the airways and cough. Respir Physiol 1998;
114: 5-15.
464.
Widdicombe JG. Nasal airflow resistance at simulated altitude. Eur Respir J 2002;
19: 4-5.
465.
Widdicombe JG. Neurophysiology of the cough reflex. Eur Respir J 1995; 8: 11931202.
466.
Wigglesworth JS, Desai R, and Hislop AA. Fetal lung growth in congenital
laryngeal atresia. Pediatr Pathol 1987; 7: 515-525.
467.
Wodopia R, Ko HS, Billian J, Wiesner R, Bartsch P, and Mairbaurl H. Hypoxia
decreases proteins involved in epithelial electrolyte transport in A549 cells and rat
lung. Am J Physiol Lung Cell Mol Physiol 2000; 279: L1110-1119.
468.
Yan SF, Ogawa S, Stern DM, and Pinsky DJ. Hypoxia-induced modulation of
endothelial cell properties: regulation of barrier function and expression of interleukin6. Kidney Int 1997; 51: 419-425.
469.
Zavorsky GS. Evidence of pulmonary oedema triggered by exercise in healthy
humans and detected with various imaging techniques. Acta Physiol (Oxf) 2007; 189:
305-317.
470.
Zhao Y, Packer CS, and Rhoades RA. Pulmonary vein contracts in response to
hypoxia. Am J Physiol 1993; 265: L87-92.
CHAPTER 2
COUGH FREQUENCY AND COUGH RECEPTOR SENSITIVITY TO CITRIC ACID
CHALLENGE DURING A SIMULATED ASCENT TO EXTREME ALTITUDE.
Mason, N. P., Barry, P. W., Despiau, G., Gardette, B., Richalet, J. P.
Cough frequency and cough receptor sensitivity to citric acid challenge
during a simulated ascent to extreme altitude.
Eur Respir J, 13: 508-513, 1999
Copyright #ERS Journals Ltd 1999
European Respiratory Journal
ISSN 0903-1936
Eur Respir J 1999; 13: 508±513
Printed in UK ± all rights reserved
Cough frequency and cough receptor sensitivity to citric acid
challenge during a simulated ascent to extreme altitude
N.P. Mason*, P.W. Barry**, G. Despiau+, B. Gardette{, J-P. Richalet#
Cough frequency and cough receptor sensitivity to citric acid challenge during a simulated
ascent to extreme altitude. N.P. Mason, P.W. Barry, G. Despiau, B. Gardette, J-P. Richalet.
#ERS Journals Ltd 1999.
ABSTRACT: The aim of this study was to determine the frequency of cough and the
citric acid cough threshold during hypobaric hypoxia under controlled environmental
conditions.
Subjects were studied during Operation Everest 3. Eight subjects ascended to a
simulated altitude of 8,848 m over 31 days in a hypobaric chamber. Frequency of
nocturnal cough was measured using voice-activated tape recorders, and cough
threshold by inhalation of increasing concentrations of citric acid aerosol. Spirometry
was performed before and after each test. Subjects recorded symptoms of acute
mountain sickness and arterial oxygen saturation daily. Air temperature and
humidity were controlled during the operation.
Cough frequency increased with increasing altitude, from a median of 0 coughs
(range 0±4) at sea level to 15 coughs (range 3±32) at a simulated altitude of 8,000 m.
Cough threshold was unchanged on arrival at 5,000 m compared to sea level (geometric mean difference (GMD) 1.0, 95% confidence intervals (CI) 0.5±2.1, p=0.5), but fell
on arrival at 8,000 m compared to sea level (GMD 3.3, 95% CI 1.1±10.3, p=0.043). There
was no relationship between cough threshold and symptoms of acute mountain sickness,
oxygen saturation or forced expiratory volume in one second. Temperature and
humidity in the chamber were controlled between 18±248C and 30±60%, respectively.
These results confirm an increase in cough frequency and cough receptor sensitivity
associated with hypobaric hypoxia, and refute the hypothesis that high altitude cough
is due to the inhalation of cold, dry air. The small sample size makes further
conclusions difficult, and the cause of altitude-related cough remains unclear.
Eur Respir J 1999; 13: 508±513.
Numerous anecdotal reports exist of paroxysmal cough
in climbers and travellers to high altitude [1±3] which may
be severe enough to cause rib fractures [2, 3]. The cause of
this cough is not known, but has been attributed to the
inspiration of cold, dry air, acute mountain sickness (AMS),
high altitude pulmonary oedema (HAPO), bronchoconstriction or respiratory tract infection [4, 5].
In the first systematic study of cough at high altitude [6],
an increase in cough frequency and cough receptor sensitivity was reported in a group of subjects ascending to
Mount Everest Base Camp in Nepal at an altitude of 5,300
m. However, because of the nature of the study, subjects at
Base Camp were unavoidably exposed to cold, dry air,
which has been shown to cause cough [7]. The aim of this
study was therefore to measure cough frequency and
cough receptor sensitivity in a group of subjects making a
simulated ascent of Mount Everest (8,848 m) in a hypobaric chamber in which the temperature and humidity were
controlled within normal sea level limits, and also to study
the effects of extreme hypobaric hypoxia on cough.
Methods
Subjects
Eight subjects and one reserve were initially selected
from healthy volunteers after physical and psychological
*Service D'AnestheÂsie-ReÂanimation, HoÃpital Tenon, Paris, France. **Dept of Child
Health, University of Leicester, Leicester,
UK. +UniversiteÂ Paul Sabatier, FaculteÂ de
MeÂdecine, Toulouse, France. {COMEX Industries,Marseillex,France. #ARPE,Laboratoire de Physiologie, UFR de Medicine,
Bobigny, France.
Correspondence: N.P. Mason
Service D'AnestheÂsie-ReÂanimation
HoÃpital Tenon
75020 Paris
France
Fax: 33 156017007
Keywords: Citric acid
cough
high altitude
hypobaric chamber
Received: June 25 1998
Accepted after revision November 7 1998
Supported by grants from the ReÂgion
Provence-Alps-CoÃte d'Azur and the MinisteÁre de la Jeune et des Sports (France)
assessment. All subjects had previous mountaineering
exposure to high altitude. Subject demographics are shown
in table 1. None of the subjects had a history of atopy,
asthma, exercise-induced bronchoconstriction or cough.
Two subjects were occasional smokers, smoking <20 cigarettes.week-1. None were taking regular medication.
Ascent profile
To minimize the time spent confined in the hypobaric
chamber, after baseline tests at sea level, the subjects and
reserve ascended by helicopter to the Vallot Observatory
on Mont Blanc at 4,350 m for a 6-day period of acclimatization, spending the first night en route at the Cosmiques Hut (3,613 m). During the period at the Vallot one
subject developed evidence of HAPO and was therefore
withdrawn from the study being replaced by the reserve.
These eight subjects then returned to Marseille where they
spent a night before re-entering the chamber which was
immediately depressurized to an altitude of 4,500 m. Over
the next 31 days the subjects ascended to the barometric
equivalent of the summit of Mount Everest (altitude 8,848
m, barometric pressure 33.6 kPa (253 mmHg)). The ascent
profile is illustrated in figure 1. A period of recovery at
5,000 m was included prior to exposure to altitudes of
8,000 m and above, as would normally be adopted during
a real mountaineering ascent to such altitudes.
509
COUGH FREQUENCY AND RECEPTOR SENSITIVITY DURING HYPOBARIC HYPOXIA
Table 1. ± Subject demographics and previous altitude
exposure
Citric acid cough challenge
Sub- Age Height Weight Smoker Maximum
ject
yrs
m
kg
cigarettes. previous
No.
day-1 altitude m
Solutions of increasing concentrations of citric acid were
inhaled via an ultrasonic nebulizer (Sonix 2000; Clement
Clarke International, Harlow, UK) during a slow vital
capacity inspiration over 5 s as previously described [6].
Citric acid was chosen as it is less likely to cause bronchoconstriction than other tussive agents such as capsaicin and is well tolerated. Beginning with a 0.3125 g.L-1
solution of citric acid in 0.9% saline, each solution was
inhaled three times until the maximum concentration of
160 g.L-1 or the cough threshold was reached. The cough
threshold was defined as the lowest concentration which
provoked a cough providing that a cough was also provoked at the next concentration. Cough challenge was
performed at sea level, 5,000 m, recovery at 5,000 m and at
8,000 m.
The pattern of deposition of an aerosol within the airways changes with different inspiratory flow rates [9], and
variations in the inspiratory flow rate alter the citric acid
cough threshold (CACT) [10]. To maintain a reproducible
stimulus to the airways a citric acid aerosol was administered using a single-breath technique from residual volume (RV) to total lung capacity (TLC) over 5 s [11]. This
technique has been reported to give a reproducible result
for the CACT [12].
Before and after each cough challenge forced expiratory
volume in one second (FEV1), forced vital capacity (FVC)
and peak expiratory flow (PEF) were recorded using a
Micromedical Microloop turbine spirometer (Micromedical Ltd., Rochester, UK). The accuracy of this device is
unaffected by low barometric pressures. Each day arterial
oxygen saturation (Sa,O2) was recorded using a Hewlett
Packard pulse oximeter (Hewlett Packard, Boulogne,
France). The standardized Lake Louise Scoring System
[13] was recorded to assess the severity of AMS. This
assigned a score from 0 (no symptoms) to 3 (severe and
incapacitating) to the following symptoms: headache; gastrointestinal symptoms (anorexia, nausea and vomiting);
weakness or fatigue; vertigo or dizziness; and the quality of
sleep from the preceding night.
The study was approved by the ethics committee of the
University of Marseille. All subjects gave their written
1
23
1.80
65
2
4807
2
37
1.83
75.5
No
8760
3
25
1.90
81
No
>6000
4
25
1.89
82
No
>6000
5
26
1.76
72.5
No
>6000
6
7
25
25
1.77
1.72
70
66
4
No
4100
5200
8
26
1.76
78
No
4100
Previous
altitude
illness
No
Mild
(insomnia)
Mild
(headaches)
Mild
(nausea)
Mild
(nausea)
Mild
(headaches)
No
Mild
(headaches)
Nocturnal cough frequency
Nocturnal cough frequency (NCF) was measured using
portable voice-activated tape recorders (Panasonic RQL317, Panasonic, Bracknell, UK). Microphones were placed adjacent to the subjects head each night. The threshold
for voice activation was adjusted to an appropriate sensitivity to be activated by coughing but not by extraneous
noise. Subjects recorded the start and finishing times of
recording on the tape. All recordings were stopped at 07:30
h if not otherwise indicated on the tape. Cough recording
was performed at sea level, 5,000 m, 7,000 m, recovery at
5,000 m and nights at 7,000 m after having spent the day at
8,000 m. Tapes were replayed the next day by one of two
observers and the total number of coughs noted, Coughs
were easily distinguished from other noise. The number of
coughs on the first technically satisfactory recording was
taken as the cough frequency for each subject at that
altitude. All tapes recorded at 7,000 m were assessed
independently by both observers and good agreement in
the recorded frequency of cough was observed between
them (k=0.84) [8].
HA5
9,000
HA4
Simulated altitude m
8,000
HA3
7,000
HA2
6,000
HA1
PA
5,000
Re
4,000
3,000
2,000
1,000
0
SL
RSL
-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
Days in the chamber
Fig. 1. ± Operation Everest-3 ascent profile. SL: sea level, undertaken 10±16 March; PA: pre-acclimatization at Vallot, from 25±31 March; HA1 to HA5:
high altitude, starting 1 April; Re: recovery before final assault; RSL: return to sea level on 2 May. D: night.
N.P. MASON ET AL.
informed consent. The hypobaric chamber was located at
COMEX Industries (Marseille, France). Temperature and
relative humidity in the chamber varied during the day and
with changes in the chamber pressure, but were maintained
between 18 and 248C and 30±60%, respectively. Two
technicians monitored the environmental conditions at all
times. The chamber was equipped with supplementary
oxygen in all areas and three backup oxygenation systems.
Any subject could be removed from the chamber within
minutes.
Completeness of the data
Subject 1 developed a severe migraine-like headache
which was associated with focal neurological signs on day
26 at an altitude of 8,000 m and the data at this altitude are
therefore not available. NCF data are not available from
subject 8 at 7,000 m, from subjects 6 and 7 at 8,000 m,
owing to technical problems with the tape recorders.
Subjects 2 and 8 did not collect NCF on return to sea level
after 31 days in the chamber. Thus, NCF recordings were
made on eight subjects at sea level, 5,000 m, and return to
5,000 m, on seven subjects at 7,000 m; on five subjects at
8,000 m; and on six subjects on return to sea level. CACT
was not available for subject 8 on return to 5,000 m, and
CACT was therefore measured on eight subjects at sea
level and 5,000 m, and on seven subjects at return to 5,000
m and 8,000 m.
Statistical analysis
NCF was normalized by logarithmic transformation, and
the effect of altitude on NCF was determined by analysis of
variance. Fisher's pairwise comparisons were used to give
the 95% confidence intervals (CI) for the difference
between the mean NCFs at different altitudes. The values
are expressed as the geometric mean differences (GMD) and
represent the ratio between the geometric means.
The distribution of cough threshold values was positively skewed, and was normalized by logarithmic transformation of the data. Cough thresholds for the subjects at
each altitude are described by the mean of the logarithm of
the cough threshold (the geometric mean). Differences
between groups studied at different locations or times are
given as the GMD and 95% CI. Cough thresholds for each
individual were compared using paired t-tests, without
correction for multiple comparisons.
The relationship between change in cough threshold
from sea level and AMS score, Sa,O2 and lung function
(prior to the CACT) were compared using a general linear
model [8]. The model allows multiple factors to be analysed to explain the observed change in cough threshold at
altitude. Terms for subject number, altitude, AMS score,
oxygen saturation and lung function were included in the
model. Terms were fitted to the model by backwards stepwise regression. Data was analysed using Minitab Release
10 (Clecom Ltd., Birmingham, UK). Statistical significance was assumed at p<0.05.
Table 2. ± Nocturnal cough frequency
Subject
No.
1
2
3
4
5
6
7
8
Median
Altitude m
Sea 5,000 7,000 Return to 8,000 Return to
level
5,000
sea level
2
0
2
0
3
0
0
0
0
0
2
9
10
2
13
8
1
5
12
18
10
32
18
11
8
±
12
7
4
3
3
2
2
2
0
2.5
±
3
12
32
9
±
±
27
12
0
±
0
0
0
0
1
±
0
Results
Nocturnal cough frequency
Analysis of variance showed that NCF (table 2) increased from sea level (median number of coughs 0, range
0±3) as the subjects ascended (p<0.0005). At 5,000 m (median number of coughs 5, range 0±13) the NCF was between 1.7 and 10 times (95% CI) the sea level value, and
at 7,000 m (median number of coughs 12, range 8±32) the
NCF was between 6 and 39 times the sea level value. On
return to 5,000 m NCF fell in comparison with 7,000 m
(median number of coughs at 5,000 m 2.5, range 0±7).
Ascent to 8,000 m, with nights spent at 7,000 m, resulted
in a further rise in NCF in comparison with the return to
5,000 m (median NCF at 8,000 m 12 coughs, range 3±
32), but was not significantly different from NCF at 7,000
m (95% CI 0.3±2.5 times the 7,000 m NCF). NCF fell
with the return to sea level (median 0, range 0±1).
Citric acid cough challenge
The CACT (fig. 2) was unchanged at both 5,000 m
(GMD) 1.0, 95% CI 0.5±2.1, p=0.5) and the return to 5,000
m in comparison with sea level (GMD 0.8, 95% CI 0.3±2.1,
p=0.35) and there was no difference between the first stay
1.5
Log citric acid cough threshold
510
1.0
0.5
0
-0.5
-1.0
-1.5
-2.0
Sea level
5,000 m
Return to
5,000 m
Altitude m
8,000 m
Fig. 2. ± Log (citric acid cough threshold) against altitude. The figure
shows the log of the individual citric acid cough thresholds at each
altitude, including sea level for the eight subjects. The geometric means
are represented by the solid horizontal bars.
at 5,000 m and return to 5,000 m (GMD 0.8, 95% CI 0.5±
1.5, p=0.261). The cough threshold was, however,
reduced at 8,000 m in comparison with both sea level
(GMD 0.3, 95% CI 0.1±0.95, p=0.043) and arrival at 5,000
m (GMD 0.35, 95% CI 0.15±0.8, p=0.047). CACT, and
prechallenge spirometric values, AMS score and oxygen
saturation are given in table 3.
AMS score increased with altitude, and at 8,000 m all
subjects had scores of $3. Sa,O2 fell to a mean of 86% at
5,000 m and 68% at 8,000 m. At 8,000 m PEF increased
by a mean of 27%, FVC fell by 6.8% and FEV1 was
unchanged. These changes in spirometry are in keeping
with previous reports [14]. Citric acid challenge did not
produce a significant fall in FEV1 in comparison with
prechallenge values. There was no statistically significant
relationship between CACT and the AMS score or Sa,O2.
Discussion
The authors have previously [6] demonstrated a significant increase in the frequency of nocturnal cough and a
decrease in the CACT in a group of 42 subjects ascending
Table 3. ± Citric acid cough threshold, and prechallenge
spirometric values, acute mountain sickness (AMS) score
and arterial oxygen saturation (Sa,O2)
Altitude
m
0
Subject Cough FVC FEV1 PEF AMS Sa,O2
No. threshold
L
L
L.s-1 score %
log [g.L-1]
1
2
3
4
5
7
8
9
0.0
1.2
0.0
0.3
0.6
-0.6
0.0
0.0
5.1
5.0
6.0
6.4
6.0
5.0
4.8
4.1
4.2
3.8
4.9
5.0
5.0
4.4
4.2
3.1
10.4
8.2
8.9
12.0
8.7
13.0
9.0
8.8
0
0
0
0
0
0
0
±
99
99
98
98
99
98
98
98
5,000
1
2
3
4
5
7
8
9
0.6
0.3
0.0
0.6
-0.3
-0.6
-0.3
0.0
5.4
4.9
5.9
6.5
5.9
4.8
5.0
3.6
4.6
4.0
5.4
5.1
5.1
4.4
4.4
2.9
12.2
11.2
10.3
14.5
10.7
14.0
11.5
9.3
0
3
0
0
1
1
0
0
84
86
87
84
85
84
82
86
Return
to 5,000
1
2
3
4
5
7
8
9
0.3
0.0
0.0
0.6
-0.6
-0.9
0.3
±
5.4
4.7
5.8
5.8
5.9
±
4.8
±
4.7
3.9
5.3
5.1
5.2
±
4.4
±
11.6
10.3
10.3
11.0
10.9
±
12.6
±
0
1
0
2
1
0
0
0
88
86
86
88
92
88
87
88
8,000
1
2
3
4
5
7
8
9
±
-0.6
-0.3
-0.3
-1.5
-0.6
0.0
0.0
±
4.4
5.7
5.6
5.6
4.9
4.3
4.1
±
3.6
5.4
4.7
5.0
4.4
4.1
3.3
±
11.5
12.1
14.7
11.9
15.2
12.4
8.9
±
1
6
6
7
7
6
10
±
80
70
67
69
57
66
66
FVC: forced vital capacity; FEV1: forced expiratory volume in
one second; PEF: peak expiratory flow.
511
to Everest Base Camp at 5,300 m in the Nepalese
Himalaya. The changes in NCF were most obvious in a
subgroup of three climbers ascending to 7,000 m or above.
The present study has confirmed those findings and
extended them in a larger group to the extreme altitudes
above 7,000 m, with climatic conditions which were controlled throughout the duration of the study. This study,
therefore, refutes the hypothesis that high altitude cough is
due to changes in inspired air temperature or humidity.
Despite the present findings, drying of the respiratory
mucosa, which is known to precipitate cough [15, 16], may
still have occurred, not through the inhalation of cold, dry
air, but because the increase in ventilation at altitude in
response to hypoxia requires larger volumes of air to be
warmed and humidified than at sea level. Unfortunately,
the only studies measuring respiratory water loss at altitude
have been confined to exercise [17±19] while the measurements made in this study were at rest. In addition,
ANDERSEN et al. [20] have demonstrated that nasal breathing at sea level was able to satisfactorily condition air
inspired at 238C and with a relative humidity of 9% even at
high minute volumes (up to 107 L.min-1). It is not known
how altitude effects the ability of the upper respiratory tract
to condition inspired air.
Cough may be the sole symptom of asthma. Although
bronchoconstriction has been demonstrated after hyperpnoea with cold air [21], it is dissociated from the cough
reflex, being abolished by pretreatment with b2-agonists,
which had no effect on cough [22]. Likewise bronchodilator therapy does not alter the CACT in nonasthmatic
subjects [23]. In the present study, the change in cough
threshold was not associated with changes in FEV1 or PEF
and these findings do not support a role for bronchoconstriction in cough at high altitude.
Most subjects ascending to high altitude almost certainly
develop a degree of subclinical interstitial pulmonary oedema [24, 25]. While it is generally considered that pulmonary oedema causes cough, and one of the recognized
symptoms of HAPO is cough [26, 27], there are, to the
authors' knowledge, no controlled studies demonstrating a
clear relationship between pulmonary oedema and cough.
In addition, the major known anatomical sites which stimulate cough are located above the segmental airways
(division 4) [28]. However, there is growing evidence that
inflammatory processes [29, 30] and alterations in vascular
permeability occur throughout the body upon exposure to
high altitude [31, 32], often without any symptoms. If
subclinical pulmonary oedema is a result of an overall
increase in vascular permeability, changes around rapidly
acting pulmonary receptors might produce a change in the
cough threshold. The fall in FVC with altitude supports the
suggestion that mild pulmonary oedema occurred in these
subjects.
There was no relationship between AMS score or hypoxia (as indicated by oxygen saturation) and change in
CACT. This does not exclude a relationship between
cough and AMS, as it may simply mean that the numbers
were too small to detect changes, particularly at the low
AMS scores seen in the study.
One of the changes seen during acclimatization to high
altitude is an increase in the hypercapnic ventilatory
response (HCVR) [33]. BANNER [34] has demonstrated a
relationship between HCVR and cough threshold to hypotonic saline, with those subjects with the greatest HCVR
512
N.P. MASON ET AL.
having the lowest cough threshold, suggesting that there
may a role for the "cough centre" in the changes seen in
cough threshold at high altitude. This is supported from
observational data from high altitude [35].
The increase in cough seen in this study may be owing
to changes at any point in the cough reflex arc. At sea level
the commonest precipitants of cough are upper respiratory
tract infections, environmental irritants, and in particular
cigarette smoke [36]. The subjects in the study demonstrated no evidence of respiratory tract infection. Although
three subjects were occasional smokers before entry into
the chamber, smoking during the duration of the study was
not permitted. Air quality at high altitudes in the mountains
is generally of a considerably higher quality than at sea
level and is relatively free of allergens. It is unlikely
therefore that inhaled irritants are responsible for high
altitude cough.
The logistics of a hypobaric chamber study limit the
sample size, and restricted the ability to answer the questions posed in this study. Using data from a previous study
[6], it was estimated that nine subjects would be needed to
demonstrate a significant fall in the cough threshold with a
study power of 80% [37]. It was only possible to accommodate eight subjects in the chamber, and a number of
data points were missed owing to either subject illness or
logistical problems. This means that a change in the cough
threshold at lower altitudes may have been missed, and
limits the ability to explore relationships between cough
and lung function.
The other difference from the authors' previous study is
that the change in CACT only becomes significant at 8,000
m, whereas previously a change was demonstrable after
acclimatization to 5,000 m. Airway deposition of inhaled
particles changes with air density [38], and it is possible
that the increase in the cough threshold seen at 8,000 m is
due to changes in the amount of citric acid delivered to the
cough receptors. In contrast, NCF varies strikingly with
altitude, increasing on ascent to 7,000 m, decreasing on
descent to 5,000 m, and then increasing again on return to
7,000 m and above. This suggests the possibility of a direct
relationship between cough and altitude, independent of
any tests using a tussive stimulus.
In summary, an increase in cough frequency and cough
receptor sensitivity was demonstrated on simulated ascent
to 8,000 m, despite controlled inspired air temperature and
humidity. This study refutes the hypothesis that high altitude cough is simply due to the prolonged inhalation of
cold, dry air. The small sample size makes further conclusions difficult, and the cause of altitude-related cough
remains unclear.
Acknowledgements: The authors gratefully
acknowledge the participation of the "altinauts",
K. Bodin, E. Cauchy, G. Despiau, J-C. Finance,
M. Gayet, V. Marchand, G. Sabin, P. Serpollet
and A. Heretier, without whose dedication and
cooperation the study would not hve been possible, the staff of COMEX, and of Clement
Clarke International for the loan of the Sonix
2000 nebulizer.
References
1.
Somervell TH. After Everest. London, Hodder and
Stoughton, 1936.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
Tasker J. Everest the Cruel Way. London, Eyre Methuen,
1981.
Steele P. Medicine on Mount Everest 1971. Lancet 1971;
ii: 32±39.
Murdoch DR. Symptoms of infection and altitude illness
among hikers in the Mount Everest region of Nepal. Aviat
Space Environ Med 1995; 66: 148±151.
Ward MP, Milledge JS, West JB. High Altitude Medicine
and Physiology, 2nd edn. London, Chapman and Hall,
1995; p. 390.
Barry PW, Mason NP, Riordan M, O©Callaghan C. Cough
frequency and cough receptor sensitivity are increased in
man at altitude. Clin Sci 1997; 93: 181±186.
Banner AS, Chausow A, Green J. The tussive effect of
hyperpnea with cold air. Am Rev Respir Dis 1985; 131:
362±367.
Altman DG. Practical Statistics for Medical Research.
London, Chapman and Hall, 1991.
Ryan G, Dolovich MB, Obminski G, Cockroft DW,
Juniper E, Hargreaves FE. Standardisation of inhalation
provocation tests: influence of nebuliser output, particle
size and method of inhalation. J Allergy Clin Immunol
1981; 67: 156±161.
Barros MJ, Zammattio SJ, Rees PJ. Importance of inspiratory flow rate in the cough response to citric acid inhalation in normal subjects. Clin Sci 1990; 78: 521±525.
Pounsford JC, Saunders KB. Diurnal variation and adaption of the cough response to citric acid in normal subjects. Thorax 1985; 40: 657±661.
Empey DW, Laitinen LA, Young GA, Bye CA, Hughes
DTD. Comparison of the antitussive effect of codeine
phosphate 20 mg, dextromethorphan 30 mg and noscapine 30 mg using citric acid induced cough in normal
subjects. Eur J Clin Pharmacol 1979; 16: 393±397.
Hackett P. Lake Louise Consensus statement on the
definition of altitude illness. In: Sutton JR, Coates G,
Houston CS, eds. Hypoxia and Mountain Medicine.
Burlington, CS Houston, 1992; pp. 327±330.
Pollard AJ, Mason NP, Barry PW, et al. Effect of altitude
on spirometric parameters and the performance of peak
flow meters. Thorax 1996; 51: 175±178.
Irwin RS, Rosen MJ. Cough: a comprehensive review.
Arch Intern Med 1977; 137: 1186±1191.
Banner AS, Green J, O©Conner M. Relation of respiratory
water loss to coughing after exercise. N Engl J Med 1984;
311: 833±836.
Pugh LGCE, Gill MB, Lahiri S, et al. Muscular exercise
at great altitude. J Appl Physiol 1964; 19: 431±440.
West JB, Boyer SJ, Graber DJ, et al. Maximal exercise at
extreme altitudes on Mount Everest. J Appl Physiol 1983;
55: 688±698.
Ward MP, Milledge JS, West JB. High Altitude Medicine
and Physiology, 2nd edn. London, Chapman & Hall,
1995; p. 478.
Andersen I, Lundqvist GR, Jensen PL, Proctor DF. Human response to 78-hour exposure to dry air. Arch
Environ Health 1974; 29: 319±324.
Strauss RH, McFadden ER, Ingram RH, Jaegar JJ.
Enhancement of exercise-induced asthma by cold air. N
Engl J Med 1977; 297: 743±747.
Pounsford JC, Birch MJ, Saunders KB. Effect of bronchodilators on the cough response to citric acid in normal
and asthmatic subjects. Thorax 1985; 40: 662±667.
Belcher N, Rees PJ. Effects of pholcodine and salbutamol
on citric acid induced cough in normal subjects. Thorax
1986; 41: 74±75.
Gray GW, McFadden MD, Houston CS, Bryan AC.
25.
26.
27.
28.
29.
30.
31.
Changes in the single-breath nitrogen washout curve on
exposure to 17,600 ft. J Appl Physiol 1975; 39: 652±656.
Kronenburg RS, Safar P, Lee J, et al. Pulmonary artery
pressure and alveolar gas exchange in man during acclimatisation to 12,470 ft. J Clin Invest 1971; 150: 827±
837.
Richalet J-P. High altitude pulmonary oedema: still a
place for controversy? Thorax 1995; 50: 923±929.
Peacock AJ. High altitude pulmonary oedema: who gets it
and why? Eur Respir J 1995; 8: 1819±1821.
Widdecombe JG. Neurophysiology of the cough reflex.
Eur Respir J 1995; 8: 1193±1202.
Maggiorini M, Bartsch P, Oelz O. Association between
raised body temperature and acute mountain sickness:
cross sectional study. Br Med J 1997; 315: 403±404.
Schoene RB, Swenson ER, Pizzo CJ, et al. The lung at
high altitude: bronchoalveolar lavage in acute mountain
sickness and pulmonary oedema. J Appl Physiol 1988;
64: 2605±2613.
Hansen JM, Olsen NV, Feldt-Rasmussen B, et al. Albuminuria and overall capillary permeability of albumin in
acute hypoxia. J Appl Physiol 1994; 76: 1922±1927.
32.
33.
34.
35.
36.
37.
38.
513
Richalet J-P, Hornych A, Rathat C, Aumont J, Larmignat
P, Remy P. Plasma prostaglandins, leukotrienes and thromboxane in acute high altitude hypoxia. Respir Physiol
1991; 85: 205±215.
Kellog RH. The role of CO2 in altitude acclimatization.
In: Cunningham DJC, Lloyd BB, eds. The Regulation of
Human Respiration. Oxford, Blackwell Science, 1963;
pp. 379±394.
Banner AS. Relationship between cough due to hypotonic
aerosol and the ventilatory response to CO2 in normal
subjects. Am Rev Respir Dis 1988; 137: 647±650.
Barry PW, Mason NP, Nickol A, et al. Cough receptor
sensitivity and dynamic ventilatory response to carbon
dioxide in man acclimatised to high altitude (Abstract). J
Physiol 1996; 497P: 29±30.
Irwin SI, Rosen MJ, Braman SS. Cough: a comprehensive
review. Arch Intern Med 1997; 137: 1186±1191.
Campbell MJ, Machin D. Medical Statistics: A Common
Sense Approach. Chichester, John Wiley & Sons, 1994.
Lippman M, Esch J. Effect of lung airway branching
pattern and gas composition on particle deposition. Exp
Lung Res 1988; 14: 311±320.
CHAPTER 3
SERIAL CHANGES IN SPIROMETRY DURING AN ASCENT TO 5300 m
IN THE NEPALESE HIMALAYA.
Mason, N. P., Barry, P. W., Pollard, A. J., Collier, D. J., Taub, N. A., Miller, M. R. Milledge, J. S.
High Alt Med Biol, 1: 185-195, 2000.
HIGH ALTITUDE MEDICINE & BIOLOGY
Volume 1, Number 3, 2000
Mary Ann Liebert, Inc.
Serial Changes in Spirometry During an Ascent to
5300 m in the Nepalese Himalayas
NICHOLAS P. MASON,1 PETER W. BARRY, 2 ANDREW J. POLLARD,3
DAVID J. COLLIER,4 NICHOLAS A. TAUB,5 MARTIN R. MILLER,6
and JAMES S. MILLEDGE 7
ABSTRACT
Mason, Nicholas P., Peter W. Barry, Andrew J. Pollard, David J. Collier, Nicholas A. Taub,
Martin R. Miller, and James S. Milledge. Serial changes in spirometry during an ascent to
5300 m in the Nepalese Himalayas. High Alt Med Biol 1:185–195, 2000.— The aims of the present study were to determine the changes in forced vital capacity (FVC), forced expiratory volume in 1 sec (FEV 1 ) and peak expiratory flow (PEF), during an ascent to 5300 m in the Nepalese
Himalayas, and to correlate the changes with arterial oxygen saturation measured by pulse
oximetry (SpO 2 ) and symptoms of acute mountain sickness (AMS). Forty-six subjects were studied twice daily during an ascent from 2800 m (mean barometric pressure 550.6 mmHg) to 5300
m (mean barometric pressure 404.3 mmHg) during a period of between 10 and 16 days. Measurements of FVC, FEV 1 , PEF, SpO 2 , and AMS were recorded. AMS was assessed using a standardized scoring system. FVC fell with altitude, by a mean of 4% from sea level values [95%
confidence intervals (CI) 0.9% to 7.4%] at 2800 m, and 8.6% (95% CI 5.8 to 11.4%) at 5300 m.
FEV 1 did not change with increasing altitude. PEF increased with altitude by a mean of 8.9%
(95% CI 2.7 to 15.1%) at 2800 m, and 16% (95% CI 9 to 23%) at 5300 m. These changes were not
significantly related to SpO 2 or AMS scores. These results confirm a progressive fall in FVC and
increase in PEF with increasing hypobaric hypoxia while FEV 1 remains unchanged. The increase
in PEF is less than would be predicted from the change in gas density. The fall in FVC may be
due to reduced inspiratory force producing a reduction in total lung capacity; subclinical pulmonary edema; an increase in pulmonary blood volume, or changes in airway closure. The absence of a correlation between the spirometric changes and SpO 2 or AMS may simply reflect
that these measurem ents of pulmonary function are not sufficiently sensitive indicators of altitude-related disease. Further studies are required to clarify the effects of hypobaric hypoxia on
lung volumes and flows in an attempt to obtain a unifying explanation for these changes.
Key Words: ventilatory function; altitude; respiratory tract; acute mountain sickness; pulmonary
oedema
1
Laboratoire de Physiologie et de Physiopathologie, Faculté de Médecine, Université Libre de Bruxelles, Bruxelles,
Belgium.
2
Department of Child Health, University of Leicester, Leicester, England.
3
Department of Paediatrics, Imperial College School of Medicine, St. Mary’s Hospital, London, England.
4
Department of Clinical Pharmacology, St. Bartholomew’s Hospital, London, England.
5
Department of Epidemiology and Public Health, University of Leicester, Leicester, England.
6
Department of Medicine, Selly Oak Hospital, Birmingham, England.
7
Institute of Medical Research, Northwick Park Hospital, Harrow , England.
185
186
MASON ET AL.
exposure to hypobaric
hypoxia leads to changes in spirometry,
including a fall in forced vital capacity (FVC)
(Rahn and Hammond, 1952; Tenney et al. 1953;
Ulvedal et al., 1963; Welsh et al., 1993; Pollard
et al., 1996; Cogo et al., 1997). The etiology behind these changes remains uncertain. Interpretation of previous studies is difficult due to
their small numbers of subjects. In addition,
there is little clear information available on the
changes in resting lung function at moderate
altitudes below 3500 m (Dillard et al., 1998).
We have previously reported the changes
compared with sea level in PEF, FVC, and
forced expiratory volume in one second (FEV 1 )
at Mount Everest Base Camp, at 5300 m in the
Nepalese Himalayas, in 51 members of the 1994
British Mount Everest Medical Expedition (Pollard et al., 1996). In that study, designed to compare the performance of a variable orifice peak
flow meter with that of a turbine spirometer,
mean FVC fell by 5%; PEF rose by 25.5% and
FEV 1 remained unchanged.
During the trek into Everest Base Camp, over
a period of between 9 and 14 days, members of
the same Expedition made twice daily measurements of spirometry, arterial oxygen saturation by pulse oximetry (SpO 2 ), and acute
mountain sickness (AMS) scores at altitudes
from 2800 m [mean barometric pressure (BP)
550.6 mmHg] to 5300 m (mean BP 404.3 mmHg).
The size and continuity of this data is unique
and, in addition, presents for the first time a
large volume of field data from lower altitudes.
mmHg). All groups flew to Lukla from Kathmandu (altitude 1250 m, mean BP 654.8
mmHg). Each group followed a similar, but not
identical, ascent profile including rest days, designed to minimize AMS and maximize acclimatizatio n. Variations between groups’ rates
of ascent were primarily due to differences in
fitness and speed of acclim atization.
During the walk-in to Base Camp, twice
daily measurements were taken of PEF, FEV 1 ,
and FVC using a Micro Medical Microplus turbine spirometer (Micro Medical Ltd, Rochester,
Kent, UK). This device is an accurate handheld
spirometer (Miller et al., 1992), which is unaffected by changes in barometric pressure (Pedersen et al., 1994) and remains accurate after exposure to subzero temperatures (M. Miller
personal observation). All volumes are given at
body temperature, ambient pressure and fully
saturated with water vapor (B.T.P.S.). SpO 2
was recorded using a Nellcor N20P pulse
oximeter (Nellcor Puritan Bennet Ltd, Warwick, UK). The Lake Louise Consensus scoring
system was used to rank the severity of symptoms of AMS (Roach et al., 1993). Recordings
were made on the afternoon of arrival at any
new altitude after at least 1 hours’ rest, and in
the mornings before departure. Spirometry
was performed according to the guidelines of
the British Thoracic Society (British Thoracic
Society, 1994) with the best of three attempts
being recorded for each subject. Barometric
pressures were recorded using a Vertech wrist
altimeter-b arometer (Avocet, Union City, CA).
Complete data sets were collected from 46 subjects, and these data are analyzed further here.
METHODS
STATISTICAL METHODS
Fifty-five members of the 1994 British Mount
Everest Medical Expedition (age range 19 to 55
years) trekked to Everest Base Camp at 5300 m
(mean BP 404.3 mmHg) in the Nepalese Himalaya from Lukla (altitude 2800 m, mean BP
550.6 mmHg) in 6 groups of up to 10 members
(Fig. 1). Prior to departure from the United
Kingdom, and having given written informed
consent, baseline sea-level measurements were
made during two weekends in London and
Stirling (barometric pressure 759.2 to 761.2
The spirometric measurem ents are represented throughout as percentage change from
the baseline (sea-level) values. Measurements
made on arrival at each new altitude were analyzed first. The relationship between the
change in spirometric measurement and altitude was plotted, and 95% confidence intervals
(CIs) given to illustrate the precision of the
results. The 95% CI was calculated using the
1.96SD
formula: mean 6 } .
Ï wn
INTRODUCTION
A
CU TE A N D CH RO N IC
SPIROMETRY AT ALTITUDE
FIG. 1.
187
Altitude profile for walk-in to Everest Base Camp (distances are in kilometers from starting point at Lukla).
Multilevel modelling was then used to examine the involvement of potential aetiological
factors (AMS score, oxygen saturation, and day
of the trek) with spirometric values. Multilevel
models are an extension of linear regression
models in that they not only express the spirometric measurem ent in terms of a linear combination of the possible aetiological factors, but
also allow for biases that may arise from the relationship between multiple measurements
made on each individual in the study (Goldstein, 1995). Modelling was used to analyze the
relationship between spirometric measurements on arrival at a particular altitude, and on
leaving that altitude. AMS score was analyzed
as a dichotomous variable (score # 3 or $ 4).
Both linear and quadratic relationships were
examined, and the backwards selection procedure was used to simplify the models, rejecting those factors not significant at the p , 0.01
level. Statistical significance in all other cases
was assumed at p , 0.05.
PEF has been shown to rise with ascent to altitude, and it has been suggested that this is
due to changes in barometric pressure. If so,
the predicted increase in PEF will be inversely
proportional to the square root of the gas density ratio, and this can be calculated directly
from the change in barometric pressure (Dawson and Elliot, 1977):
theoretical increase in PEF 5
Ï
w (Sw Lw Bw Pw /Aw Bw Pw )
where, SLBP is the sea-level barometric pressure and ABP is the barometric pressure at altitude. Thus, the predicted increase at Everest
base camp (mean measured barometric pressure 404.3 mmHg, mean sea-level barometric
pressure 760.6 mmHg) is:
Ï
w (7w 6w 0w .6w /w 4w 0w 4w .3w ) 5 1.372, or 37.2%.
Observed changes in PEF were compared with
the predicted value at each altitude.
RESULTS
The relationship between the changes in
spirometric measurem ents and altitude are
given in Figs. 2–4. Overall FVC falls with altitude, by a mean of 4% (95% (CI 0.9 to 7.4%) at
2800 m (mean BP 550.6 mmHg), and 8.6% (95%
CI 5.8 to 11.4% ) at 5300 m (mean BP 404.3
mmHg). FEV 1 does not change with increasing
altitude. PEF increases with altitude by a mean
188
MASON ET AL.
FIG. 2.
Change in FVC with altitude showing mean values and 95% CI.
of 8.9% (95% CI 2.7 to 15.1%) at 2800 m (mean
BP 550.6 mmHg), and 16% (95% CI 9 to 23%)
at 5300 m (mean BP 404.3 mmHg).
Multilevel modelling suggests that the percentage change in FVC from sea level was inversely related to altitude (coefficient of the
mean slope of the change 21.68% per 1000 m,
SE 0.45% , p , 0.001), but not independently to
the day of the trek, AMS score or oxygen saturation (all p . 0.05). Percentage change in
FEV 1 tended to fall with altitude, by a mean coefficient of 0.26% per 1000-m altitude gain (SE
0.43% ), but this change was not statistically significant. The day of the trek, AMS score or oxygen saturation were not related to FEV 1 in the
FIG. 3.
model, independently of the relationship with
altitude. PEF increased significantly with altitude (coefficient 3.61, SE 0.80, p , 0.001). None
of the other terms in the model were independently significant.
Within these overall changes, there was considerable variability in each individual’s change
in lung function. For FVC and FEV 1, this variability was greater than would have been anticipated by chance. Some of this variability was
explained by the individuals’ age and height.
During the stay at each altitude, FVC and
FEV 1 tended to increase [by a mean of 0.51%
(SE 0.41) and 0.22% (SE 0.49) of the sea-level
value, respectively], but this was not statisti-
Change in FEV1 with altitude showing mean values and 95% CI.
FIG. 4.
189
Change in PEF with altitude showing mean values and 95% CIs.
cally significant. PEF tended to fall by a mean
of 1.23% (SE 0.64% ). It should be remembered
that this comparison is between measurements
made at different times of the day and also after different levels of activity.
Comparison of the measured changes in PEF
with predicted changes based on the change in
barometric pressure showed that the mean ratio of the measured changes to predicted was
0.55 (95% CI 0.45 to 0.65).
DISCUSSION
Previous studies of spirometry at high altitude have been limited by small numbers of
subjects and often by a lack of careful control
data. The data presented here are unique in size
and serial nature, with recordings made at sea
level and then at 7 increasing altitudes during
a period of between 9 and 14 days.
FVC
We have demonstrated a fall in FVC with increasing altitude (mean of 4% at 2800 m, mean
BP 550.6 mmHg, and 8.6% at 5300 m, mean BP
404.3 mmHg, in comparison with sea level).
The 8.6% reduction at 5300 m is greater than
the 5.2% fall that we have previously reported
(Pollard et al., 1996). In the previous study mea-
surements on a group of 51 subjects, which included the subjects of the present study, were
taken on average 2 to 3 days after arrival at base
camp, whereas those reported here are the first
measurements taken on arrival. The marked
drop in FVC between Gorak Shep (5180 m,
mean BP 411.8 mmHg) and Base Camp (5300
m, mean BP 404.3 mmHg) remains unexplained. There was no identifiable difference in
the subjects’ routine nor in environmental factors between these two places. The fall in FVC
at altitude has been reported by a number of
authors (Rahn and Hammond, 1952; Tenney et
al., 1953; Ulvedal et al., 1963; Welsh et al., 1993;
Cogo et al., 1997) and various explanations
have been suggested including respiratory
muscle weakness; an increase in pulmonary
blood volume; an increase in residual volume
(RV) consequent upon abdominal gas distension and early airways closure secondary to
subclinical pulmonary edema.
It is unlikely that the fall in FVC is due to
respiratory muscle fatigue. Although there is
some evidence that diaphragm atic fatigue contributes to exercise limitation at high altitude
(Kayser et al., 1993) such changes are only seen
during heavy prolonged whole body exercise,
rather than during a vital capacity maneuver.
Reduced inspiratory muscle power, however,
could be responsible for the fall in FVC via a
reduction in total lung capacity (TLC).
Rahn and Hammond (1952) conducted a
190
complicated series of experiments in which between 4 and 18 subjects were exposed to a variety of environments. Their results demonstrated that FVC fell more with acute hypobaric
hypoxia than with the equivalent acute normobaric hypoxia. Acute hypobaria with oxygen supplementation only produced a fall in
FVC at the extreme altitudes of 9144 m and
12,192 m. Chronic hypobaric hypoxia on
Mount Evans (4267 m) produced a fall in FVC
on the third day of exposure, which gradually
returned to sea-level values before increasing
to values in excess of sea-level values during
the 7-day stay. During acute normobaric hypoxia, they demonstrated a reduction in maximal expiratory pressure, although there was no
statistically significant difference in FVC between normoxia and normobaric hypoxia except at inspired oxygen tensions equivalent to
5486 m. No measurem ent was made of maximal inspiratory pressure. The administration of
supplementary oxygen during acute hypobaric
hypoxia quickly restored FVC to its sea level
values, a fact that argues in favor of a reduction in respiratory muscle power.
We have previously reported no increase in
FVC after supplementary oxygen at 5300 m in
subjects exposed to at least 10 days of increasing altitude (Pollard et al., 1997). However, the
duration of altitude exposure was much longer
than in the study of Rahn and Hammond and,
in addition, the oxygen flow rate was limited
to 1 L/min during 5 min. Although sufficient
to increase arterial oxygen saturation to a mean
of 94%, this may have been an insufficient increase, or of an insufficient duration, to produce a change in FVC.
Rahn and Hammond (1952) postulated that
the mechanism by which hypobaria per se
could bring about a fall in FVC would be via
increased abdominal distension producing a
cephalad displacem ent of the diaphragm .
However, they demonstrated that abdominal
distension of between 700 to 800 mL (a little
less than would be expected if acutely decompressed to an altitude equivalent to Everest
Base Camp) produced no significant change in
FVC. In addition Gilroy et al. (1985) and Kimball et al. (1986) demonstrated that abdominal
distension by a gastric balloon or by blood produced a cranial displacem ent of the diaphragm ,
MASON ET AL.
but that any effects upon functional residual
capacity (FRC) were largely negated by an outward movement of the rib cage.
In contrast, a role for hypobaria per se in the
reduction of FVC is supported by the work of
Ulvedal et al. (1963) in a study involving prolonged exposure to normoxic or supraoxic hypobaria. They demonstrated a fall in FVC with
increasing simulated altitude (3.1% at 5500 m;
2.9% at 8500 m, and 7.6% at 10,000 m). These
changes occurred rapidly upon exposure to reduced barometric pressure and did not improve with continued stay of up to 17 days at
the reduced barometric pressure. However, the
change is less than half the reduction that we
measured at an equivalent altitude. It would
thus appear unlikely that a direct effect of hypobaria is responsible for the fall in FVC seen
in our study.
Klocke and Rahn (1959) calculated that the
theoretical fall in lung volume due to oxygen
absorption from the lungs during 4 to 7 sec of
breath-holding would account for 1.6% in a
7.6% fall of FVC at 10,000 m. This clearly cannot explain all of the fall in FVC that we measured at a much lower altitude, although it
could contribute if the cause for the fall in FVC
is multifactorial.
An increase in pulmonary blood volume
could be responsible for the changes seen in
FVC. Increased capillary recruitment attributed
to an increase in pulmonary artery pressure has
been described in the upper lung zones of anesthetized dogs following exposure to acute hypoxia (Wagner et al., 1979). In contrast however, Doyle et al. (1952) found no increase in
pulmonary blood volume in human subjects
during acute hypoxia. Coates et al. (1979), in
studying acute exposure to a barometric equivalent of 4268 m in a hypobaric chamber, found
no evidence of an increase in pulmonary blood
volume up to 19 h after decompression. It is
important to note that all these studies looked
at the effects of acute hypoxia and while the
subjects in our study were being repeatedly exposed to ever reduced barometric pressures
during their walk-in, their overall state was one
of subacute or chronic hypoxia.
During Operation Everest II, Welsh et al.
(1993) calculated the theoretical change in
blood volume occurring upon descent from
4267 m to sea level. They found that it correlated well with the immediate increase in FVC
observed upon return to sea level, although this
does not prove that it is the cause of the change
in FVC.
There is evidence that even small clinically
undetectable amounts of pulmonary edema
can effect lung function (Hales and Kazemi,
1974). The fall in FVC, which occurs in longdistance runners, is thought to occur as a result
of early airways closure secondary to pulmonary edema, and is thus accompanied by an
increase in RV (Maron et al., 1979). Unfortunately, the data available on changes in other
lung volumes at altitude are limited (Tenney et
al., 1953; Frisancho, 1975; Gaultier and Crapo,
1997). It would appear that total lung capacity
and RV may initially increase on exposure to
altitude and then gradually return toward
baseline values with continuing time at altitude. This could be consistent with pulmonary
edema resolving with time at altitude.
A subclinical form of high altitude pulmonary edema (HAPE) could explain the
change in FVC. There is some evidence that
most subjects exposed to high altitude develop
a degree of subclinical pulmonary edema (Kronenberg et al., 1971). Coates et al. (1979) concluded that this was the cause of gas trapping
and a maldistribution of ventilation in their
acute hypobaric chamber study. Gray et al.
(1975) felt that the presence of subclinical pulmonary edema was responsible for changes observed in the phase III slope of the nitrogen
washout curve in a study that took place over
up to 7 days at 5364 m on Mount Logan, while
Grissom et al. (1992) found an increase in the
alveolar-arterial oxygen (A-aO 2 ) gradient in 6
control subjects during the 24 h after arrival at
4200 m on Mount McKinley. The authors suggested that this rise, which was not seen in
three out of six subjects taking acetazolam ide
in whom measurement of the A-aO 2 gradient
was possible, was due to increased extravascular lung water. Cogo et al. (1997) found a fall
in the maximal expiratory flow at 25% of FVC
on the second day at 3500 m and at higher altitudes. Clinically, basal crackles were observed by Houston in 22% of climbers on reaching the summit of Mt. Rainier (4392 m) (Gray
et al., 1975).
191
During Operation Everest II, Welsh et al.
(1993) obtained chest X rays 2 h after descent
from a simulated altitude of 8848 m, which
showed the presence of interstitial edema in
subjects who did not exhibit frank signs of pulmonary edema. The fact that the fall in FVC at
5300 m (mean BP 404.3 mmHg) reported in this
present study on arrival at base camp is greater
than the observed change 2 to 3 days later on
a group that included the subjects of the present study (Pollard et al., 1996) would also be
compatible with interstitial edema resolving
with time after arrival at base camp. Other authors have also reported this trend towards
normal with time (Ulvedal et al., 1963).
There are several possible mechanisms to explain an increase in extravascular lung water
without the occurrence of frank HAPE. AMS is
characterized by peripheral edema and it is
possible that it could also produce a subclinical interstitial pulmonary edema. Exercise at
altitude can cause asymptomatic interstitial
edema (Anholm et al., 1999), although in the
study of Anholm et al. the level of exercise at
2800 m, equivalent to the lowest altitude in our
study, was considerably greater than that of
our subjects. Exercise increases pulmonary
artery pressure (PAP) and if PAP rose to a sufficiently high level it could result in a hydrostatic pulmonary edema. It is therefore conceivable that a lower level of exercise at a
higher altitude (with a resultant stronger hypoxic pulmonary vasoconstriction) a subclinical pulmonary edema could be produced. To
minimize any effect of exercise, our subjects
rested for a minimum of 1 h before the afternoon measurem ents.
Assuming a hydrostatic mechanism, it would
not necessarily be expected that the morning
measurements of FVC should be lower after a
night of rest as the supine position produces a
marked increase in PAP (Gibbs, 1999). Recently,
the importance of the amiloride-sensitive alveolar epithelial sodium channel in the clearance
of lung water has been recognized (Matalon and
O’Brodovich, 1999). Hypoxia down-regulates
the expression and activity of this channel
(Clerici and Matthay, 2000).
We believe that the explanation for the fall
in FVC is probably multifactorial, but that the
predominant mechanism is most likely to be
192
MASON ET AL.
subclinical interstitial pulmonary edema. This
would be supported by the fact that 30 min after the return to sea level from 8848 m during
Operation Everest II, a time period sufficient to
allow a change in pulmonary blood volume to
occur (Doyle et al., 1952), FVC had normalized
only partially, and of the remaining difference,
only 50% had resolved after a further 48 h, consistent with the resolution of pulmonary edema
(Welsh et al., 1993). Proving this, however, presents a considerable challenge. Further, large
studies are required to define the effects of hypobaric hypoxia on lung volumes, maximum
inspiratory pressure (MIP) and maximal expiratory pressure (MEP), along with an accurate
measure of lung water that can be used easily
in the mountain environment. To date no such
system exists. Clinical examination is insensitive (Marantz et al., 1988), while chest radiography may also miss small increases in extra
vascular lung water (Miniati et al., 1987; Vock
et al., 1989). Magnetic resonance imaging (MRI)
is highly sensitive, but of limited practical use
at high altitude (Cutillo et al., 1984). It is possible to measure the permeability of the alveolar-capillary membrane, but this does not give
information on changes in lung water (Jones et
al., 1983). The double indicator technique can
determine small changes in extravascular lung
water, but would be difficult to use accurately
in large numbers of subjects at altitude (Rocker,
1996). Transthoracic electrical impedance measurement has been tried (Roy et al., 1974; Hoon
et al., 1977), but the results have been disappointing. Recently, however, the technique has
been refined, with increased accuracy and spatial resolution in an easily portable form (Kunst
et al., 1999), and may prove to be useful in attempting to clarify the changes in extravascular lung water on ascent to high altitude.
FEV 1
FEV 1 did not increase with altitude, consistent with previous reports (Miller et al., 1992;
Welsh et al., 1993; Pollard et al., 1996, 1997).
PEF is measured in the first 10 to 90 msec of a
forced expiration and achieved with very little
displacement of air (Cotes, 1993) while FEV 1
represents the integral of overall flow during
the much longer period of the first second of
the maneuver. PEF, which is measured on the
first effort-dependent part of the FVC reflects
predominantly proximal large airway flow.
FEV 1 , however, is influenced by the effort-dependent and effort-independent parts of the
FVC. In a study of lung function in patients
within 2 weeks of acute myocardial infarction,
although the mean closing volume (CV) was
found to be 125% of the predicted value, suggesting a degree of subclinical edema, FEV 1
was unaltered (Hales, 1974). In addition, the
changes in small airway flow, particularly in
the presence of small airways disease and altered gas density, are extremely variable and
notoriously difficult to interpret (Macnee et al.,
1983). If the reduced FVC was due to a reduction in TLC, then this would be consistent with
the finding of FEV 1 being preserved, as is seen
in restrictive lung disorders.
PEF
PEF increases by 15.1% between sea level
(mean BP 760.6 mmHg) and 5300 m (mean BP
404.3 mmHg). The calculated increase is 3.1%
for each 1000 m of ascent. This is slightly less
than either we (Pollard et al., 1996) or other authors (Forster and Parker, 1983; Thomas et al.,
1990; Pedersen et al., 1994) have previously reported and it should be noted that, like FVC,
PEF inexplicably fell between Gorak Shep (altitude 5180 m, mean BP 411.8 mmHg) and Base
Camp (mean BP 404.3 mmHg). If the increase
in PEF is due to changes in gas density, assuming that the PEF is turbulent, or determined by a wave speed flow limiting mechanism, the change in PEF can be calculated by
the equation given in the Methods section. Possible causes for the discrepancy between predicted and observed PEF include increased
viscosity dependent frictional losses in the airways (reducing the intralum inal pressure, and
so the maximal flow, at the flow limiting segment or choke point); failure to reach wave
speed; reduced muscle power; reduced TLC; or
reduced elastic recoil.
Changes in gas viscosity at altitude will be
mainly due to changes in temperature. The
only direct effect of hypobaria will be an in-
crease in water vapor with decreasing barometric pressure. As water vapor has a low viscosity, this will lower the overall air viscosity
at altitude (e.g., BP 759.8 mmHg, 37°C, viscosity 5 182 micropoises; BP 439.6 mmHg, 37°C,
viscosity 5 178 micropoises). If the countercurrent heat transfer in the lungs fails to sufficiently warm expired air at altitude, then the
expired air temperature will fall and with it,
viscosity. Thus, altitude will tend to reduce viscosity and with it frictional loss. The observed
changes in PEF cannot therefore be accounted
for by changes in frictional losses.
Reduced muscle power would produce reduced lung inflation and a consequent reduction in TLC. This would explain the reduced
FVC and lower than predicted increase in PEF
(as PEF would be attained at a lower lung volume). If this was the case then maximum inspiratory and expiratory pressures (PIM A X and
PE M A X , respectively) would be reduced. However, as previously stated, the information on
the effect of altitude on TLC is contradictory
(Tenney et al., 1953; Frisancho, 1975; Gaultier
and Crapo, 1997). Although Rahn and Hammond (1952) demonstrated a reduction in
PE M A X during acute normobaric hypoxia, this
was not associated with changes in FVC. No
information is available on the changes in PIM AX at altitude. It is unlikely that muscle power
is so reduced that wave speed cannot be
reached (Kayser et al., 1993) nor that hypobaric
hypoxia should change lung elastic recoil.
The ratio of the observed to predicted increase in PEF for all our altitudes is of a similar order to that of Pedersen et al. in a hypobaric chamber decompressed to a barometric
pressure equivalent to 3000 m, and when
breathing a helium oxygen mixture equivalent
in density to an altitude of around 7500 m (Pedersen et al., 1994), suggesting that similar factors were operating to limit PEF in all three situations.
CONCLUSION
These results confirm a progressive fall in
FVC and increase in PEF with increasing hypobaric hypoxia, while FEV 1 remains unchanged. The increase in PEF is less than would
193
be predicted from the change in gas density.
We believe that the fall in FVC is multifactorial, although the predominant mechanism is
most likely to be a subclinical pulmonary
edema. Other factors may include reduced inspiratory force producing a reduction in TLC;
an increase in pulmonary blood volume, or
changes in airway closure. A sensitive measure
of lung water that can be easily used in the
mountain environment, and further studies to
clarify the effects of hypobaric hypoxia on TLC,
RV, MIP, and MEP are required in an attempt
to obtain a unifying explanation for these
changes.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the participation of the members of the 1994 British
Mount Everest Medical Expedition without
whose enthusiasm and dedication this study
would not have been possible; Micro Medical,
UK for supplying the spirometers and for technical advice and Nellcor Puritan Bennet for the
loan of the oximeters.
REFERENCES
Anholm J.D., Milne E.N.C., Stark P., Bourne J.C., and
Friedman P. (1999). Radiographic evidence of interstitial pulmonary edema after exercise at altitude. J. Appl.
Physiol. 86:503– 509.
British Thoracic Society and the Association of Respiratory Technicians and Physiologists (1994). Guidelines
for the measurement of respiratory function: Topical review. Respir. Med. 88:165– 194.
Clerici C., and Matthay M.A. (2000). Hypoxia regulates
gene expression of alveolar epithelial transport proteins. J. Appl. Physiol. 88:1890– 1896.
Coates G., Gray G., Mansell A., Nahmias C., Powles A.,
Sutton J., and Webber C. (1979). Changes in lung volume, lung density, and distribution of ventilation during hypobaric decompression. J. Appl. Physiol. 46:752–
755.
Cogo A., Legnani D., and Allegra L. (1997). Respiratory
function at different altitudes. Respiration 64:416–421.
Cotes J.E. (1993). Lung function: Assessment and application in medicine, 5th ed. Blackwell Scientific Publications, Oxford; pp. 115–123.
Cutillo A.G., Morris A.H., Ailon D.C., Case T.A., Durney
C.H., Ganesan K., Watanabe F., and Akhtari M. (1984).
Assessment of lung water distribution by nuclear magnetic resonance. Am. Rev. Resp. Dis. 137:1371– 1375.
194
Dawson S.V., and Elliot E.A. (1977). Wave-speed limitation on expiratory flow—A unifying concept. J. Appl.
Physiol. 43:498– 515.
Dillard T.A., Rajagopal K.R., Slivka W.A., Berg B.W.,
Mehm W.J., and Lawless N.P. (1998). Lung function
during moderate hypobaric hypoxia in normal subjects
and patients with chronic obstructive pulmonary disease. Aviat. Space Environ. Med. 69:979– 985.
Doyle J.T., Wilson J.S., and Warren J.V. (1952). The pulmonary vascular responses to short term hypoxia in human subjects. Circulation 5:263– 271.
Forster P., and Parker R.W. (1983). Peak expiratory flow
rate at high altitude. Lancet ii:100.
Frisancho A.R. (1975). Functional adaptions to high altitude hypoxia. Science 187:313– 319.
Gaultier C., and Crapo R. (1977). Effects of nutrition,
growth hormone disturbances, training, altitude and
sleep on lung volumes. Eur. Respir. J. 10:2913– 2919.
Gibbs J.S. (1999). Pulmonary hemodynamics: implications
for high altitude pulmonary edema (HAPE). A review .
Adv. Exp. Med. Biol. 474:81–91.
Gilroy R.J., Lavietes M.H., Loring S.H., Mangura B.T., and
Mead J. (1985). Respiratory mechanical effects of abdominal distension. J. Appl. Physiol. 58:1997– 2003.
Goldstein H. (1995). Multilevel statistical models. Edward
Arnold, London UK.
Gray G.W ., McFadden M.D., Houston C.S., and Bryan
A.C. (1975). Changes in the single-breath nitrogen
washout curve on exposure to 17 600 ft. J. Appl. Physiol. 39:652– 656.
Grissom C.K., Roach R.C., Sarnquist F.H., and Hackett
P.H. (1992). Acetazolam ide in the treatment of acute
mountain sickness: clinical efficacy and effect on gas
exchange. Ann. Intern. Med. 116:461– 465.
Hales C.A., and Kazemi H. (1974). Small-airways function in myocardial infarction. New Engl. J. Med.
290:761– 765.
Hoon R.S., Balasubramanian V., Tiwari S.C., Mathew
O.P., Behl A., Sharma S.C., and Chadha K.S. (1977).
Changes in transthoracic electrical impedance at high
altitude. Br. Heart J. 39:61– 66.
Jones J.G., Royston D., and Minty B.D. (1983). Changes in
the alveolar–capillary barrier function in animals and
humans. Am. Rev. Resp. Dis. 127:S51.
Kayser B., Narici M., and Cibella F. (1993). Fatigue and
muscle performance at high altitude. In: Hypoxia and
Molecular Medicine. J.R. Sutton, G. Coates, and C.S.
Houston, eds. Queen City Printers, Burlington; pp.
222–234.
Kimball W.R., Kelly K.B., and Mead, J. (1986). Thoracoabdominal blood volume change and its effect on lung and
chest wall volumes. J. Appl. Physiol. 61:953–959.
Klocke P.J., and Rahn H. (1959). Breath holding after
breathing of oxygen. J. Appl. Physiol. 14:689.
Kronenberg R.S., Safar P., Lee J., Wright F., Noble W.,
Wahrenbrock E., Hickey R., Nemoto E., and Severinghaus J.W . (1971). Pulmonary artery pressure and alveolar gas exchange in man during acclimatisation to
12 470 ft. J. Clin. Invest. 50:827– 837.
Kunst P.W., Vonk Noordegraaf A.V., Raaijmakers E.,
MASON ET AL.
Bakker J., Groeneveld A.B., Postmus P.E., and de Vries
P.M. (1999). Electrical impedance tomography in the assessment of extravascu lar lung water in noncardiogenic
acute respiratory failure. Chest 116:1695– 1702.
Macnee W., Power J., Innes A., Douglas N.J., and Sudlow
M.F. (1983). The dependence of maximal flow in man
on the airway gas physical properties. Clin. Sci.
65:273–279.
Marantz P.O.R., Tobin J.N., Wassertheil-Smoller S., Steingart R.M., Wexler J.P., Budner N., Lense L., and Wachspress J. (1988). The relationship between left ventricular systolic function and congestive heart failure
diagnosed by clinical criteria. Circulation 77:607–612.
Maron M.B., Hamilton L.H., and Maksud M.G. (1979). Alterations in pulmonary function consequent to competitive marathon running. Med. Sci. Sports 11:244–249.
Matalon S., and O’Brodovich H. (1999). Sodium channels
in alveolar epithelial cells: Molecular characteriza tion,
biophysical properties, and physiological significance.
Annu. Rev. Physiol. 61:627– 661.
Miller M.R., Dickinson S.A., and Hitchings D.J. (1992). The
accuracy of portable peak flow meters. Thorax
47:904–909.
Miniati M., Pistolesi M., Milne E.N.C., and Giutini C.
(1987). Detection of lung oedema. Crit. Care Med.
15:1146– 1155.
Pedersen O.F., Miller M.R., Sigsgaard T., Tidley M., and
Harding R.M. (1994). Portable peak flow meters: Physical characteristics, influence of temperature, altitude,
and humidity. Eur. Respir. J. 7:991–997.
Pollard A.J., Barry P.W., Mason N.P., Collier D.J., Pollard
R.C., Pollard P.F., Martin I., Fraser R.S., Miller M.R.,
and Milledge J.S. (1997). Hypoxia, hypocapnia and
spirometry at altitude. Clin. Sci. 92:593– 598.
Pollard A.J., Mason N.P., Barry P.W., Pollard R.C., Collier D.J., Fraser R.S., Miller M.R., and Milledge J.S.
(1996). Effect of altitude on spirometric param eters and
the performance of peak flow meters. Thorax 51:175–
178.
Rahn H., and Hammond D. (1952). Vital capacity at reduced barometric pressure. J. Appl. Physiol. 4:715–725.
Roach R.C., Bärtsch P., Hackett P.H., and Oelz O. (1993)
The Lake Louise acute mountain sickness scoring system. In: Hypoxia and Mountain Medicine. J.R. Sutton,
C.S. Houston, and G. Coates, eds. Queen City Printers,
Burlington; pp. 327–330.
Rocker G.M. (1996). Bedside measurement of pulmonary
capillary permeability in patients with acute lung injury. What have we learned? Intensive Care Med.
22:619–621.
Roy S.B., Balasubramanian V., Khan M.R., Kaushik V.S.,
Manchanda S.C., and Guha S.K. (1974). Transthoracic
electrical impedance in cases of high-altitude hypoxia.
Br. Med. J. 3:771–775.
Tenney S.M., Rahn H., Stroud R.C., and Mithoefer J.C.
(1953). Adaptation to high altitude: changes in lung volumes during the first seven days at Mt. Evans, Colorado. J Appl. Physiol. 5:607– 613.
Thomas P.S., Harding R.M., and Milledge J.S. (1990). Peak
expiratory flow at altitude. Thorax 45:620– 622.
Ulvedal F., Morgan T.E., Cutler J.R., and Welch B.E.
(1963). Ventilatory capacity during prolonged exposure
to simulated altitude without hypoxia. J Appl. Physiol.
2:904– 908.
Vock P., Fretz C., Franciolli M., and Bartsch P. (1989).
High-altitude pulmonary edema: Findings at high-altitude chest radiography and physical examination. Radiology 170:661– 666.
Wagner W.W., Latham L.P., and Capen R.L. (1979). Capillary recruitment during airway hypoxia: Role of pulmonary artery pressure. J. Appl. Physiol. 47:383–337.
Welsh C.H., Wagner P.D., Reeves J.T., Lynch D., Cink
T.M., Armstrong J., Malconian M.K., Rock P.B., and
Houston C.S. (1993). Operation Everest II: Spirometric
and radiographic changes in acclimatised humans at
simulated high altitudes. Am. Rev. Respir. Dis.
147:1239– 1244.
195
Address reprint requests to:
Dr. NP Mason
Université Libre de Bruxelles
Laboratoire de Physiologie et de Physiopathologie
Faculté de Médecine—Campus Erasme, Bat E2.4
Route de Lennik 808— CP604
B-1070 Bruxelles
Belgium
E-mail: [email protected]
Received November 25, 1999; accepted in
final form June 12, 2000
CHAPTER 4
SERIAL CHANGES IN NASAL POTENTIAL DIFFERENCE AND LUNG ELECTRICAL
IMPEDANCE TOMOGRAPHY AT HIGH ALTITUDE.
Mason, N. P., Petersen, M., Melot, C., Imanow, B., Matveykine, O., Gautier, M. T., Sarybaev, A.,
Aldashev, A., Mirrakhimov, M. M., Brown, B. H., Leathard, A. D., Naeije, R.
Serial changes in nasal potential difference and lung electrical impedance tomography at high altitude.
J Appl Physiol, 94: 2043-2050, 2003.
.
J Appl Physiol 94: 2043–2050, 2003.
First published December 6, 2002; 10.1152/japplphysiol.00777.2002.
Serial changes in nasal potential difference and lung
electrical impedance tomography at high altitude
Nicholas P. Mason,1 Merete Petersen,2 Christian Mélot,3 Bakyt Imanow,4
Olga Matveykine,4 Marie-Therese Gautier,1 Akpay Sarybaev,4 Almaz Aldashev,4
Mirsaid M. Mirrakhimov,4 Brian H. Brown,5 Andrew D. Leathard,5 and Robert Naeije1
1
Department of Physiology, Free University of Brussels, B1070 Brussels, Belgium; 2Faculty of Health Sciences,
University of Copenhagen, DK-2000 Copenhagen N, Denmark; 3Department of Intensive Care Medicine, Erasme
Hôspital, B1070 Brussels, Belgium; 4National Center for Cardiology and Internal Medicine, Bishkek, Kyrgyzstan;
5
Department of Medical Physics, Royal Hallamshire Hospital, Sheffield S10 2JF, United Kingdom
Submitted 26 August 2002; accepted in final form 20 November 2002
Mason, Nicholas P., Merete Petersen, Christian
Mélot, Bakyt Imanow, Olga Matveykine, Marie-Therese Gautier, Akpay Sarybaev, Almaz Aldashev, Mirsaid
M. Mirrakhimov, Brian H. Brown, Andrew D. Leathard, and Robert Naeije. Serial changes in nasal potential
difference and lung electrical impedance tomography at high
altitude. J Appl Physiol 94: 2043–2050, 2003. First published
December 6, 2002; 10.1152/japplphysiol.00777.2002.—Recent work suggests that treatment with inhaled ␤2-agonists
reduces the incidence of high-altitude pulmonary edema in
susceptible subjects by increasing respiratory epithelial sodium transport. We estimated respiratory epithelial ion
transport by transepithelial nasal potential difference (NPD)
measurements in 20 normal male subjects before, during,
and after a stay at 3,800 m. NPD hyperpolarized on ascent to
3,800 m (P ⬍ 0.05), but the change in potential difference
with superperfusion of amiloride or isoprenaline was unaffected. Vital capacity (VC) fell on ascent to 3,800 m (P ⬍
0.05), as did the normalized change in electrical impedance
(NCI) measured over the right lung parenchyma (P ⬍ 0.05)
suggestive of an increase in extravascular lung water. EchoDoppler-estimated pulmonary artery pressure increases
were insufficient to cause clinical pulmonary edema. There
was a positive correlation between VC and NCI (R2 ⫽ 0.633)
and between NPD and both VC and NCI (R2 ⫽ 0.267 and
0.418). These changes suggest that altered respiratory epithelial ion transport might play a role in the development of
subclinical pulmonary edema at high altitude in normal
subjects.
pulmonary edema; hypobaric hypoxia
ALTHOUGH ONLY A MINORITY OF those who go to high
altitude develop the potentially fatal condition of highaltitude pulmonary edema (HAPE), there is increasing
evidence that the majority of people ascending to altitude may develop subclinical pulmonary edema (9).
Forced vital capacity (FVC) falls on ascent to high
altitude and is thought to be primarily due to subclinical pulmonary edema (32). The control of pulmonary
extravascular lung water (EVLW) has traditionally
Address for reprint requests and other correspondence: N. Mason,
Université Libre de Bruxelles, Laboratoire de Physiologie et de
Physiopathologie, Faculté de Médecine-Campus Erasme, Bat. E2,
niveau 4, Route de Lennik 808 - CP604, B-1070 Bruxelles, Belgium
(E-mail: [email protected]).
http://www.jap.org
been attributed to the interplay of Starling forces, with
the pulmonary capillary pressure attributed the major
regulatory role. It is now realized that sodium transport across the respiratory epithelium plays an important role in the removal of alveolar water (34) and
that an intact epithelial barrier is necessary for the
resolution of alveolar edema (35). Hypoxia reduces
epithelial sodium transport in cultured rat alveolar
epithelial monolayers (29) and decreases the expression of alveolar epithelial ion transport proteins (8). In
addition, treatment of HAPE-susceptible individuals
with inhaled ␤2-agonists, which are known to increase
transepithelial sodium transport, decreases pulmonary edema without any effect on pulmonary hemodynamics (44).
Measurement of alveolar ion transport in vivo in
human subjects is not feasible; however, measurement
of the potential difference (PD) generated by ion transport in the nasal mucosa can be used as a marker of
transport in the distal respiratory epithelium (22, 23).
By perfusing the nasal mucosa with substances that
alter the conductance of specific ion channels, the relative contribution of these channels to the PD may be
estimated (21). Amiloride inhibits the amiloride-sensitive epithelial sodium channel (ENaC), whereas a lowconcentration chloride solution containing isoprenaline
can be used to stimulate chloride secretion, predominantly via the cystic fibrosis transmembrane regulator.
Assessing changes in lung water at high altitude has
presented problems because of the difficulty in detecting early changes in EVLW. The Sheffield electrical
impedance tomography (EIT) system is a portable device that measures the change in electrical resistivity
of lung tissue occurring in the presence of EVLW to
give a cross-sectional image of the resistive changes in
a tissue plane (5).
We therefore measured nasal potential difference
(NPD), FVC, EIT changes, and pulmonary circulation
pressures by Doppler echocardiography in a group of
The costs of publication of this article were defrayed in part by the
payment of page charges. The article must therefore be hereby
marked ‘‘advertisement’’ in accordance with 18 U.S.C. Section 1734
solely to indicate this fact.
8750-7587/03 $5.00 Copyright © 2003 the American Physiological Society
2043
2044
ION TRANSPORT AND ALTITUDE EDEMA
normal male volunteers before, during, and after a
2-wk stay at 3,800 m in the Tien Shan mountains
of Kyrgyzstan.
METHODS
Subjects.
Twenty male Kyrgyz lowlander volunteers (age range 18–
35 yr) with no previous exposure to high altitude underwent
baseline tests (BL) in Bishkek (altitude 700 m) before being
transported by road in 7 h to Kumtor gold mine at an altitude
of 3,800 m. High-altitude measurements were repeated on
days 1, 2, 5, and 10 for all subjects, on day 14 for half of the
subjects, and day 15 for the remaining half (HA01, HA02,
HA05, HA10, and HA14/15, respectively). Subjects returned
to Bishkek by road, and return to baseline (RBL) measurements were made within 24 h of their departure from high
altitude. All subjects gave written, informed consent, and the
project was approved by the ethics committee of the Faculty
of Medicine of the Free University of Brussels.
Transepithelial NPD. Transepithelial NPD was measured
using a 5-French umbilical catheter (Tyco Healthcare, Tullamore, Ireland) as the exploring bridge. The reference
bridge was a 20-gauge catheter inserted into a forearm vein
and perfused with 0.9% NaCl solution. The bridges were
connected by Driref KCl electrodes (World Precision Instruments, Sarasota, Florida) to a high-impedance voltmeter
(M-4640A, Metex, Seoul, Korea) The output from the voltmeter was recorded onto a personal computer using software
supplied with the meter. The nasal catheter was perfused
consecutively at a rate of ⬃0.5 ml/min with solutions of 1) 154
mM sodium chloride, 2) 154 mM sodium chloride with 10⫺4
M amiloride hydrochloride to assess the amiloride-inhibitable sodium current, and 3) 134 mM sodium gluconate and 5
mM sodium chloride containing 10⫺4 M amiloride hydrochloride and 10⫺5 M isoprenaline hydrochloride to stimulate
chloride secretion. Each solution was perfused until the voltage had reached a stable plateau lasting for at least 6 s.
Measurements were always made at the same distance into
the same nostril. Technical problems have been reported
because of the nasal drying and crusting that occurs at high
altitude (30). To avoid this, subjects bathed the test nostril
with 10 ml of 0.9% sodium chloride solution morning and
evening throughout the study. The liquid junction potential
was calculated for the solutions used, and the measured
results were corrected accordingly (2). In this article, a potential that becomes more negative is said to hyperpolarize,
whereas a potential that becomes less negative is said to
depolarize. Because of a technical problem with the electrodes, no recordings were obtained at RBL.
EIT. EIT measurements were performed by use of the
Mark 1 DAS-01P Sheffield Applied Potential Tomograph
(Department of Medical Physics, Royal Hallamshire Hospital, Sheffield, UK). For each measurement, 16 silver-silver
chloride ECG electrodes (Kendall, Neustadt/Donau, Germany) were placed horizontally, equidistant around the thorax at the height of the xiphisternum. Pairs of electrodes
were stimulated with an alternating current at a frequency of
51 kHz, and recordings were made by the remaining electrodes. This process was then repeated consecutively with
the measurement module making a set of transfer impedance
measurements at 25 frames/s. With the subject seated, measurements made at total lung capacity (TLC) were compared
with a reference measurement made at residual volume
(RV). From these paired measurements, cross-sectional images of the changes in impedance from RV to TLC were
J Appl Physiol • VOL
generated (5, 6). The change in impedance between RV and
TLC was calculated with an analysis program written using
MATLAB software (The MathWorks, Natick, MA), which
permits a zone of interest to be delineated. The zone chosen
was over the parenchyma of the right lung to avoid interference from changes in cardiac volume with respiration, and
excluding the chest wall. Results represent the fractional
increase in impedance on inspiration from RV to TLC with
the presence of EVLW suggested by a smaller fractional
increase. The fractional impedance changes were normalized
to correct for changes in lung volume by dividing by FVC and
are expressed as the fractional change per liter of FVC.
Measurements were repeated three times, and results given
are the mean of each group of three measurements. After the
first set of recordings at BL, the positions of the electrodes
were marked with an indelible marker pen; these marks
were renewed daily so that subsequent recordings were made
with the electrodes in an identical position.
FVC. FVC was measured by use of a Micro Medical MicroPlus Spirometer (Micro Medical, Rochester, Kent, UK). Measurements were made in accordance with the guidelines of
the British Thoracic Society (4). The best of three blows were
recorded. All volumes are given at body temperature, ambient pressure, and fully saturated with water vapor (BTPS).
Echocardiography. Echocardiography and Doppler estimation of pressures were performed with a Philips SSD-800
echocardiograph (Philips Medical Systems, Bothell, WA).
Mean pulmonary arterial pressure (Ppa) was estimated from
pulmonary artery acceleration time, and systolic Ppa was
estimated from the peak velocity of the tricuspid regurgitant
jet, if present, by using the modified Bernoulli formula (36).
Left ventricular diastolic function was assessed by comparing the initial peak transmitral velocity of early ventricular
filling (E) and the late transmitral velocity of atrial contraction (A) and deriving the E/A ratio (38).
Clinical data. Pulse, blood pressure, temperature, and
oxygen saturation (Nellcor N20P pulse oximeter. NellcorPuritan Bennet, Warwick, UK) were recorded on the morning
of each measurement day. Acute mountain sickness (AMS)
was assessed by use of the Lake Louise consensus scoring
system (41).
Statistical analysis. Data are presented in text and tables
as means ⫾ SE. Normality was assessed by using the Shapiro-Wilks test. Serial measures were tested for statistical
significance by using one-way ANOVA for repeated measures
with significant post hoc differences being further analyzed
by use of a Student-Newman-Keuls test for parametric data
and a Dunnett’s test for nonparametric data. Statistical
significance was assumed at P ⬍ 0.05.
Because of the repeated nature of the measures, correlation analysis was performed by using the Poon test for the
analysis of linear and mildly nonlinear relationships using
pooled subject data from all altitudes (40). Analyses were
performed by use of SigmaStat 2.0 software (Jandell, San
Rafael, CA) and a program written with Microsoft Excel for
the Poon test.
RESULTS
NPD. Data were obtained on 18 of the 20 subjects.
One subject could not tolerate the procedure because of
repeated gagging, and in one subject a stable trace was
never obtained. NPD hyperpolarized on ascent to
HA01 and remained hyperpolarized at HA02 (both P ⬍
0.05 vs. BL) before returning to BL values on HA05.
The PD during infusion with amiloride hyperpolarized
on ascent to HA but did not reach statistical signifi-
94 • MAY 2003 •
www.jap.org
cance. The PD during perfusion with the low-chloride
solution containing isoprenaline hyperpolarized on ascent to HA01 (P ⬍ 0.05 vs. BL). Neither the amilorideinhibitable proportion nor the stimulated chloride proportion of the PD changed significantly on ascent to or
during stay at altitude compared with baseline (Fig. 1).
Electrical impedance tomography. Satisfactory data
were obtained on 18 of 20 subjects with data on two
subjects being rejected because of consistently poorquality recordings. The standardized change in impedance between RV and TLC fell on ascent to HA01 (P ⬍
0.05) and then returned to BL values on subsequent
days (Fig. 2).
FVC. Data were obtained on all subjects. FVC fell on
ascent to HA01 compared with BL (P ⬍ 0.05) and
remained significantly reduced compared with BL
throughout the stay at altitude and at RBL (P ⬍ 0.05,
Table 1).
Ppa. Mean Ppa was measurable in 17 subjects at BL
and in all subjects at HA and RBL. Systolic Ppa was
measurable in 15 subjects at BL and 16 and 18 subjects
at HA and RBL, respectively. Both mean and systolic
Ppa increased on ascent with maximum values at
HA01 and HA02 (P ⬍ 0.01 vs. BL) and then fell at
HA05 although remaining significantly higher than BL
throughout the rest of the stay at altitude (P ⬍ 0.05 vs.
BL). Mean Ppa returned to BL values at RBL, although
systolic Ppa remained elevated. The E/A ratio fell on
ascent to HA and remained reduced throughout the
stay at HA and at RBL (P ⬍ 0.05 vs. BL). These results
are summarized in Table 1.
Oxygen saturation and AMS score. Data were obtained on all subjects. Oxygen saturation fell on ascent
to HA01, remained significantly reduced throughout
the stay at altitude compared with BL (all altitudes
2045
Fig. 2. Normalized fractional change in impedance during an inspiration from residual lung volume (RV) to total lung capacity (TLC).
Results are normalized by dividing the fractional change in impedance from RV to TLC by the forced vital capacity (FVC) in liters and
are thus expressed as the fractional change per liter of FVC. The
increase in the volume of air in the lungs on inspiration will cause
impedance to increase. The presence of extravascular lung water will
reduce this increase in impedance. On ascent to HA01, the increase
in impedance was reduced compared with BL and other altitudes.
RBL, return to baseline. *P ⬍ 0.05.
P ⬍ 0.05), and then returned to BL values at RBL.
Despite a statistically significant increase, AMS score
values never exceeded the accepted value of 3 for clinically relevant AMS. These results are summarized
in Table 1.
Relationship between EIT and FVC changes and
changes in NPD and Ppa. There was a positive correlation for FVC and the standardized change in EIT
(R2 ⫽ 0.633, P ⬍ 0.001; see Fig. 3) and between the
basal nasal potential and both FVC and the standardized change in EIT (R2 ⫽ 0.418 and 0.267 respectively,
both P ⬍ 0.001; see Fig. 4). There was no relationship
between mean Ppa and FVC or the standardized
change in EIT (R2 ⫽ 0.0046 and 0.0411, respectively)
or systolic Ppa and FVC or the standardized change in
EIT (R2 ⫽ 0.00006 and 0.0108, respectively).
DISCUSSION
Fig. 1. Changes in nasal potential differences (NPD) with time at
altitude. Values are means ⫾ SE. HA01, HA02, HA05, HA10, and
HA14/12 represent 1, 2, 5, 10, and 14–15 days at altitude, respectively. The 154 mM NaCl potential fell at HA01 and HA02 compared
with baseline (BL) before returning to BL values. Changes in the
amiloride potential did not reach statistical significance. Isoprenaline
potential fell at HA01 compared with BL. Neither the amiloride inhibitable proportion of the NPD (represented by the difference between the
154 mM NaCl line and the 154 mM NaCl ⫹ 10⫺4 M amiloride line) nor
the difference between the amiloride and low-chloride-isoprenaline
NPDs changed with ascent to altitude. *P ⬍ 0.05
This work demonstrates a hyperpolarization in the
NPD in normal subjects on ascent to high altitude, a
simultaneous fall in FVC, and changes in EIT, which
would be consistent with an increase in EVLW. In
addition, these findings occurred in the absence of
signs of AMS and at the relatively low altitude of
3,800 m.
A fall in FVC on ascent to high altitude has been
consistently reported by a number of authors, and our
data are in keeping with this (32). The most likely
mechanism behind this fall is the presence of subclinical pulmonary edema, although other suggested etiologies include an increase in pulmonary blood volume or
reduced respiratory muscle strength. To date there is
no evidence to support a sufficient reduction in resting
muscle strength at altitude to produce a reduction
in FVC.
Despite this circumstantial evidence, the major
problem in proving the presence of subclinical pulmo-
94 • MAY 2003 •
www.jap.org
2046
Table 1. Changes in cardiorespiratory parameters and AMS
FVC, liters
Mean Ppa, Torr
Systolic Ppa, Torr
E/A ratio
Heart rate, beats/min
Systolic BP, mmHg
Diastolic BP, mmHg
O2 saturation, %
AMS score
BL
HA01
HA02
HA05
HA10
HA14/15
RBL
5.19 ⫾ 0.11
11.6 ⫾ 0.8
19.7 ⫾ 1.1
2.4 ⫾ 0.1
65 ⫾ 2.6
129.7 ⫾ 3.6
70.4 ⫾ 2
97 ⫾ 0.3
0
4.94 ⫾ 0.08*
20.2 ⫾ 0.9*
34.2 ⫾ 1.7*
1.89 ⫾ 0.12*
83 ⫾ 2.4*
139.6 ⫾ 2.8*
84.4 ⫾ 1.6*
89 ⫾ 0.5*
2.3 ⫾ 0.4*
4.88 ⫾ 0.10*
19.6 ⫾ 0.9*
35.4 ⫾ 1.7*
1.69 ⫾ 0.09*
89 ⫾ 2.3*
132.6 ⫾ 2
80.4 ⫾ 1.5*
89 ⫾ 0.4*
1.7 ⫾ 0.5*
4.87 ⫾ 0.09*
15 ⫾ 0.7*
32.4 ⫾ 1.4*
1.8 ⫾ 0.09*
87 ⫾ 1.8*
126.6 ⫾ 1.5
77.5 ⫾ 1.7*
89 ⫾ 0.5*
1.3 ⫾ 0.4
4.83 ⫾ 0.09*
15.6 ⫾ 0.6*
29.4 ⫾ 1.2*
1.78 ⫾ 0.05*
89 ⫾ 2*
132 ⫾ 1.6
75.2 ⫾ 1.5*
91 ⫾ 0.4*
0.4 ⫾ 0.2
4.93 ⫾ 0.10*
15.9 ⫾ 0.58*
27.9 ⫾ 1.0*
1.75 ⫾ 0.06*
91 ⫾ 1.3*
127.9 ⫾ 1.8
77.1 ⫾ 1.5*
91 ⫾ 0.3*
0.4 ⫾ 0.2
4.97 ⫾ 0.13*
11.7 ⫾ 0.5
26.9 ⫾ 1.0*
1.94 ⫾ 0.01*
84 ⫾ 1.6*
122.6 ⫾ 1.7*
70 ⫾ 1.7
97 ⫾ 0.2
0
Values are means ⫾ SE. BL, baseline; HA01, HA02, HA05, HA10, and HA14/15, 1, 2, 5, 10, and 14–15 days at high altitude, respectively;
RBL, return to baseline; FVC, forced vital capacity; Ppa, pulmonary arterial pressure; E/A ratio, ratio of early transmitral ventricular filling
to late transmitral velocity of atrial contraction; BP, blood pressure; AMS, acute mountain sickness. * P ⬍ 0.05 vs. BL.
nary edema has been the absence of an accurate measure of EVLW that can be used in the mountain environment. Clinical examination and chest radiography
are insensitive to small increases in EVLW (31, 47).
Magnetic resonance imaging is highly sensitive but of
limited practical use at high altitude (10). The thermal
dye double-indicator dilution technique remains one of
the more accurate methods in intact humans and can
determine small changes in EVLW, but it is highly
invasive and would be difficult to perform in large
numbers of subjects at altitude (42). Transthoracic
electrical impedance measurement has been used at
altitude (14, 43), but the results have been disappointing and changes in lung volume with ventilation and
the lack of spatial resolution further complicate the
interpretation of results (11).
The Sheffield Applied Potential Tomograph system
was developed to overcome these problems; it provides
a portable device that generates a two-dimensional
image of the impedance changes occurring during ventilation in a transverse cut through the thorax and
allows a region of interest to be delineated for analysis.
The parenchyma of the right lower lobe was chosen to
be analyzed because in the upright position it has the
maximum volume change with ventilation without cardiac interference. There is good correlation between
EVLW measured by EIT and the double-indicator dilution technique in ventilated patients with acute respiratory failure (25) and in an oleic acid animal model
of the acute respiratory distress syndrome (7).
Possible factors that would confound the EIT results
are changes in pulmonary blood volume and lung vol-
Fig. 3. Relationship between FVC and the normalized change in
electrical impedance tomography (EIT) between RV and TLC for
pooled data from all altitudes. A statistically significant relationship
is present between the change in EIT and FVC. The correlation
coefficient and statistical significance are indicated.
Fig. 4. Relationship between NPD and the fractional change in EIT
(A) and between NPD and FVC (B) for pooled data from all altitudes.
A statistically significant relationship is present between NPD and
both EIT changes and FVC. The correlation coefficient and statistical
significance are indicated.
94 • MAY 2003 •
www.jap.org
umes. According to the model of Nopp et al. (37), an
increase in pulmonary blood volume from 500 to 600 ml
(i.e., by 20%) will cause a drop in resistivity of 7% at 50
kHz, whereas a 20% change in extracellular fluid will
cause a 40% change in resistivity at 50 kHz. To take
account of changes in lung volume, our results were
normalized with reference to the FVC at that altitude.
However, the raw, nonnormalized results showed a
mean reduction in the change in impedance of 22%. For
this to be caused by an increase in pulmonary blood
volume would require an increase in blood volume of
over 300 ml or 60%, and we therefore conclude that it
is highly improbable that a change in blood volume
explains the change in EIT seen in this study.
Evidence of the effects of acute exposure to hypoxia
on RV and TLC suggests that both increase on initial
exposure to altitude but return to baseline after around
a month at altitude (12). An increase in RV or a fall in
TLC could produce a fall in FVC, but as both these
volumes increase they would tend to cancel each other
out, making it unlikely that changes in lung volume
are a sufficient explanation for the observed impedance
changes. In addition, the rapidity with which the impedance changes return to normal compared with the
much longer time course for the normalization of lung
volumes argues against the impedance changes being
due to changes in lung volumes. Finally, an increase in
lung volume at high altitude might be expected to
decrease, rather than increase, the amount of estimated EVLW by augmenting the air content, which
would increase resistivity of the lung tissue.
Using EIT, we have demonstrated a marked fall in
the normalized change in impedance between RV and
TLC on ascent to 3,800 m, which would be consistent
with an increase in EVLW. This fall in impedance
shows a strong relationship with FVC, with the smallest changes in impedance being associated with the
lowest FVCs, suggesting that an increase in EVLW
may be responsible for the fall in FVC seen at high
altitude. That FVC did not return to baseline levels
during the high-altitude sojourn, whereas EIT did,
does not exclude the possibility that the fall in FVC is
due to increased EVLW but may simply reflect differing sensitivities to the presence of EVLW by EIT
and FVC.
Two possible causes for a possible increase in EVLW
were addressed by this study. The control of EVLW has
conventionally been attributed to the balance between
Starling forces causing extravasation of water from the
pulmonary capillaries and the ability of the pulmonary
lymphatics to clear this water (46). The most important
variable that influences the development of clinical
pulmonary edema is the pulmonary capillary pressure
(Ppc). In an invasive hemodynamic study at 4,559 m in
the Swiss Alps, Maggiorini et al. (28) found a cutoff Ppc
of 19 Torr for the development of HAPE in a group of
HAPE-susceptible subjects. All subjects who developed
HAPE had a Ppc ⬎19 Torr, which corresponded to a
mean Ppa ⬎ 35 Torr. No HAPE-susceptible subject
who developed HAPE had a mean Ppa ⬍ 35 Torr.
2047
We chose to use Doppler echocardiography to assess
Ppa because it is noninvasive and has been shown to
correlate well with the results obtained with right
heart catheterization at high altitude (1). None of our
subjects’ mean Ppa approached the cutoff point of 35
mmHg observed by Maggiorini (maximum mean Ppa
20.2 ⫾ 0.9 on HA1 of our study) excluding HAPE as a
cause of any changes in EVLW in this study. However,
if subclinical pulmonary edema is of hydrostatic origin,
it is likely to occur at much lower Ppas than those seen
in clinical HAPE. Hargreaves et al. (13) demonstrated
increased activity in rabbit airway rapidly adapting
receptors suggestive of a physiological effect from interstitial edema after an increase in left atrial pressure
of as little as 5 mmHg. In keeping with the findings of
Boussuges et al. (3), we also found alterations in transmitral flow on echo-Doppler with a decrease in the E/A
ratio on ascent to high altitude, suggestive of a reduction in left ventricular diastolic compliance. A fall in
diastolic compliance would increase the risk of developing pulmonary edema on exercise (45), and one may
argue that some of the increase in EVLW seen in this
study could be due to repeatedly elevated Ppc during
physical activity. Nevertheless, the failure of the Poon
analysis to demonstrate any relationship between either mean or systolic Ppa and either the changes in
EIT or FVC, while demonstrating a strong relationship
between the changes in EIT or FVC and NPD, argues
against the subclinical pulmonary edema seen in this
study being of predominantly hydrostatic origin. This
is also consistent with the work of Sartori et al. (44), in
which inhaled ␤2-agonists, known to increase transepithelial sodium transport, decreased pulmonary edema
in HAPE-susceptible individuals without any effect on
pulmonary hemodynamics.
During the last decade, it has become apparent that
an intact alveolar epithelium is necessary for the resolution of pulmonary edema (35) and that uptake of
water across the alveolar-capillary barrier is an active
process dependent on transepithelial sodium transport. The sodium current is generated by the basolateral Na-K-ATPase pump, and sodium enters the epithelium through the apical ENaC, the rate-limiting
step of this process (34). In patients with acute lung
injury and the acute respiratory distress syndrome,
reduced alveolar water clearance is associated with
increased morbidity and mortality (48), whereas homozygote knockout mice born lacking the ␣-subunit of
ENaC died within 40 h of birth from pulmonary edema
(15). A number of factors may influence the sodium
current, and thus water uptake across the alveolar
epithelium, including ␤-adrenergic agonists, glucocorticoids, thyroid hormones, insulin, and certain growth
factors (34). Studies of cultured respiratory epithelium
suggest that hypoxia inhibits both the activity and
expression of both ENaC and the Na-K-ATPase
pump (29).
It is not possible to measure alveolar epithelial sodium transport in vivo in the human subject, but measurement of the PD generated by ion transport across
the respiratory mucosa under the inferior turbinate of
94 • MAY 2003 •
www.jap.org
2048
the nose can be used as a surrogate measure for ion
transport in the distal respiratory epithelium tract and
is a simple and well-tolerated measure to perform
(21–23). Measurement of this NPD correlates well with
the pathological changes seen in the airways of patients with cystic fibrosis (24). In a study of HAPEsusceptible individuals, Sartori et al. (44) found that
their NPD at sea level was 32% less negative than in
non-HAPE-susceptible subjects and that, in addition,
superperfusion with amiloride produced a significantly
smaller depolarization in NPD in the HAPE-susceptible compared with the HAPE-resistant subjects. This
work was extended by Mairbäurl et al. (30), who confirmed a less negative NPD in HAPE-susceptible individuals, compared with nonsusceptible controls at low
altitude, and stimulated chloride transport with a chloride-free solution containing amiloride and isoprenaline. Although there was no difference between susceptible or resistant subjects at low altitude, ascent to
4,559 m produced a large hyperpolarization in NPD,
which was most marked in the nonsusceptible group,
in whom it was due to a twofold increase in stimulated
chloride secretion whereas sodium absorption was inhibited.
Our results differ from those of Mairbäurl et al. (30)
in a number of ways. Although our baseline potentials
were only slightly lower, ascent to 3,800 m did not
produce such a large hyperpolarization in NPD, nor did
we see any reduction in the amiloride-dependent sodium transport or a significant increase in isoprenaline-stimulated chloride transport at altitude. Our
study took place at an altitude 750 m lower than that
of Mairbäurl et al. and our subjects had almost no
AMS. A prevalence of AMS of 53% has been reported at
the Margharita hut used by Mairbäurl et al. for the
high-altitude part of their study (27). Either the lower
altitude with the resultant higher partial pressure of
oxygen or the lower incidence of AMS might be responsible for the difference between our results. In addition, the study of Mairbäurl et al. was on white Caucasians, whereas the subjects in the present study
were Altai-subtype Mongolians. Ireson et al. (17) have
demonstrated a significant difference in NPD between
blacks and whites, but little is known about other
racial differences in NPD or respiratory epithelial ion
transport. It is possible that differences between our
results and those of Mairbäurl may in part be explained by racial differences.
The hyperpolarization in NPD on ascent to high
altitude could be due either to changes in ion currents
or to a change in transepithelial resistance. There is
little information available on the changes in transepithelial resistance in hypoxia, and that which is available comes from work in cell monolayers. Mairbaurl et
al. (29) demonstrated both small increases and decreases in the transepithelial resistance of rat type II
pneumocytes exposed to different levels of hypoxia for
between 4 and 24 h. However, in all cases the transepithelial PD fell, equivalent to a depolarization in
vivo. From cellular work we can find no evidence to
support an increase in transepithelial resistance as a
cause for our findings.
Changes in ion currents could be due to either increased cation absorption or increased anion secretion.
The exact nature of cation channels in the alveolar
epithelium has been debated because of the differing
biophysical properties of channels encountered in cultured cells. However, Jain and colleagues (18) demonstrated that the culture conditions under which type II
pneumocytes are grown influence their biophysical
properties. When ATII cells were grown on glass plates
submerged in media, the predominant channel was a
21-pS nonselective cation channel. If they were grown
on a permeable support and in the presence of steroids
and an air interface, the predominant channel was a
low-conductance (6.6 ⫾ 3.4 pS), highly Na⫹-selective
channel, inhibited by submicromolar concentrations of
amiloride, and similar in biophysical properties to
ENaC. It now seems that the variety of different cation
channels seen in alveolar epithelium in part stems
from the assembly of different combinations of the ␣-,
␤-, and ␥-subunits of ENaC with the highly selective 4to 6-pS channel containing all three of the subunits
(33). The absence of any change in the proportion of the
NPD inhibited by amiloride on ascent to altitude excludes this being due to increased sodium absorption
via an amiloride-sensitive ENaC and suggests that it
could be all or partly explained by an increase in
sodium absorption via amiloride-insensitive nonspecific cation channels.
If the hyperpolarization is not due to increased cation absorption then it must be due to increased anion
secretion. Both chloride and bicarbonate channels have
been reported in rat fetal distal lung epithelium (26),
although only chloride channels have to date been
found in adult rat epithelium (20). Anion secretion
would be associated with the secretion of water into the
luminal space. This is the normal state in the fetus
(39). Chloride transport takes place by a number of
apical channels, the major ones being the cystic fibrosis
transmembrane regulator (CFTR), the outwardly rectifying chloride channel, and the calcium-activated
chloride channel (19). Sympathomimetic agents such
as isoprenaline may activate CFTR (16). Such activation in the presence of a favorable chemical gradient
will stimulate apical chloride secretion, estimating the
transepithelial chloride transport capacity of CFTR.
The fact that stimulation of CFTR with isoprenaline
and a low-chloride solution showed no statistically
significant increase at high altitude in the present
study only allows us to conclude that CFTR Cl⫺ transport was already maximally stimulated by the lowchloride isoprenaline solution at sea level and was not
further stimulated by the increased levels of sympathetic activity that occur at altitude. This does not
exclude an alteration in chloride transport via a channel other than CFTR or exclude bicarbonate secretion.
Although emphasizing that the presence of a correlation does not prove causality, the relationship demonstrated between NPD and both FVC and the change
in EIT (Fig. 4) would argue in favor of anion secretion
94 • MAY 2003 •
www.jap.org
over sodium reabsorption as the cause for the hyperpolarization in NPD. The most negative PDs were
associated with the lowest FVCs and the lowest
changes in EIT between RV and TLC. Both a fall in
FVC and a reduced increase in impedance on inspiration would be consistent with an increase in EVLW, a
situation more compatible with increased anion secretion than sodium reabsorption.
In conclusion, this work demonstrates a hyperpolarization of the NPD in asymptomatic individuals on
ascent to 3,800 m and simultaneous changes in EIT
and FVC that may be consistent with an increase in
EVLW. The changes in NPD suggest either an increase
in sodium absorption via an amiloride-resistant cation
pathway, either bicarbonate secretion or chloride secretion via a non-CFTR channel, or a combination of
both mechanisms. Predominance of anion secretion
over sodium reabsorption, if present in the alveoli,
would be associated with the secretion of water into the
lumen as occurs in the fetal lung (44). Further work is
required to confirm the precise nature of these
changes.
We express gratitude to the members of the Kyrgyz Presidential
Guard who participated in this study; the staff of Kumtor Gold Mine,
Kyrgyzstan, and in particular Dr. Francois du Toit, without whose
help the project would not have been realized; Dr. Renaud Beauwens
of the Department of Physiology of the Free University of Brussels
for advice in the preparation of the manuscript; and Dr. Heimo
Mairbäurl of the Department of Sports Physiology, University of
Heidelberg for teaching the nasal potential technique.
This work was supported by Grant no. 3.4567.00 of the Fonds de
la Recherche Scientifique Médical of Belgium.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
REFERENCES
1. Allemann Y, Sartori C, Lepori M, Pierre S, Melot C, Naeije
R, Scherrer U, and Maggiorini M. Echocardiographic and
invasive measurements of pulmonary artery pressure correlate
closely at high altitude. Am J Physiol Heart Circ Physiol 279:
H2013–H2016, 2000.
2. Barry PH and Lynch JW. Liquid junction potentials and small
cell effects in patch clamp analysis. J Membr Biol 121: 101–117,
1991.
3. Boussuges A, Molenat F, Burnet H, Cauchy E, Gardette B,
Sainty JM, Jammes Y, and Richalet JP. Operation Everest
III (Comex ’97): modifications of cardiac function secondary to
altitude-induced hypoxia. An echocardiographic and Doppler
study. Am J Respir Crit Care Med 161: 264–270, 2000.
4. British Thoracic Society and the Association of Respiratory Technicians and Physiologists. Guidelines for the measurement of respiratory function. Respir Med 88: 165–194, 1994.
5. Brown BH, Barber DC, Wang W, Lu L, Leathard AD, Smallwood RH, Hampshire AR, Mackay R, and Hatzigalanis K.
Multi-frequency imaging and modelling of respiratory related
electrical impedance changes. Physiol Meas 15, Suppl 2a: 1–12,
1994.
6. Brown BH, Flewelling R, Griffiths H, Harris ND, Leathard
AD, Lu L, Morice AH, Neufeld GR, Nopp P, and Wang W.
EITS changes following oleic acid induced lung water. Physiol
Meas 17: 117–130, 1996.
7. Campbell JH, Harris ND, Zhang F, Morice AH, and Brown
BH. Detection of changes in intrathoracic fluid in man using
electrical impedance tomography. Clin Sci (Lond) 87: 97–101,
1994.
8. Clerici C and Matthay MA. Hypoxia regulates gene expression
of alveolar epithelial transport proteins. J Appl Physiol 88:
1890–1896, 2000.
9. Cremona G, Asnaghi R, Baderna P, Brunetto A, Brutsaert
T, Cavallaro C, Clark TM, Cogo A, Donis R, Lanfranchi P,
21.
22.
23.
24.
25.
26.
27.
28.
29.
2049
Luks A, Novello N, Panzetta S, Perini L, Putnam M, Spagnolatti L, Wagner H, and Wagner PD. Pulmonary extravascular fluid accumulation in recreational climbers: a prospective
study. Lancet 359: 303–309, 2002.
Cutillo AG, Morris AH, Ailon DC, Case TA, Durney CH,
Ganesan K, Watanabe F, and Akhtari M. Assessment of lung
water distribution by nuclear magnetic resonance. Am Rev Respir Dis 137: 1371–1375, 1984.
Fein A, Grossman RF, Jones JG, Goodman PC, and Murray JF. Evaluation of transthoracic electrical impedance in the
diagnosis of pulmonary edema. Circulation 60: 1156–1160, 1979.
Gaultier C and Crapo R. Effects of nutrition, growth hormone
disturbances, training, altitude and sleep on lung volumes. Eur
Respir J 10: 2913–2919, 1997.
Hargreaves M, Ravi K, and Kappagoda CT. Responses of
slowly and rapidly adapting receptors in the airways of rabbits to
changes in the Starling forces. J Physiol 432: 81–97, 1991.
Hoon RS, Balasubramanian V, Tiwari SC, Mathew OP,
Behl A, Sharma SC, and Chadha KS. Changes in transthoracic electrical impedance at high altitude. Br Heart J 39: 61–66,
1977.
Hummler E, Barker P, Gatzy J, Beermann F, Verdumo C,
Schmidt A, Boucher R, and Rossier BC. Early death due to
defective neonatal lung liquid clearance in alpha-ENaC-deficient
mice. Nat Genet 12: 325–328, 1996.
Hwang TC and Sheppard DN. Molecular pharmacology of the
CFTR Cl⫺ channel. Trends Pharmacol Sci 20: 448–453, 1999.
Ireson NJ, Tait JS, MacGregor GA, and Baker EH. Comparison of nasal pH values in black and white individuals with
normal and high blood pressure. Clin Sci (Colch) 100: 327–333,
2001.
Jain L, Chen XJ, Ramosevac S, Brown LA, and Eaton DC.
Expression of highly selective sodium channels in alveolar type
II cells is determined by culture conditions. Am J Physiol Lung
Cell Mol Physiol 280: L646–L658, 2001.
Jentsch TJ, Stein V, Weinreich F, and Zdebik AA. Molecular structure and physiological function of chloride channels.
Physiol Rev 82: 503–568, 2002.
Jiang X, Ingbar DH, and O’Grady SM. Adrenergic regulation
of ion transport across adult alveolar epithelial cells: effects on
Cl⫺ channel activation and transport function in cultures with
an apical air interface. J Membr Biol 181: 195–204, 2001.
Kersting U, Schwab A, and Hebestreit A. Measurement of
human nasal potential difference to teach the theory of transepithelial fluid transport. Adv Physiol Educ 275: S72–S77, 1998.
Knowles MR, Buntin WH, Bromberg PA, Gatzy JT, and
Boucher RC. Measurements of transepithelial electric potential differences in the trachea and bronchi of human subjects in
vivo. Am Rev Respir Dis 12: 108–112, 1982.
Knowles MR, Carson JL, Collier AM, Gatzy JT, and
Boucher RC. Measurements of nasal transepithelial electric
potential differences in normal human subjects in vivo. Am Rev
Respir Dis 124: 484–490, 1981.
Knowles M, Gatzy J, and Boucher R. Increased bioelectric
potential difference across respiratory epithelia in cystic fibrosis.
N Engl J Med 305: 1489–1495, 1981.
Kunst PW, Vonk Noordegraaf AV, Raaijmakers E, Bakker
J, Groeneveld AB, Postmus PE, and de Vries PM. Electrical
impedance tomography in the assessment of extravascular lung
water in noncardiogenic acute respiratory failure. Chest 116:
1695–1702, 1999.
Lazrak A, Thome U, Myles C, Ware J, Chen L, Venglarik
CJ, and Matalon S. cAMP regulation of Cl⫺ and HCO⫺
3 secretion across rat fetal distal lung epithelial cells. Am J Physiol
Lung Cell Mol Physiol 282: L650–L658, 2002.
Maggiorini M, Buhler B, Walter M, and Oelz O. Prevalence
of acute mountain sickness in the Swiss Alps. Br Med J 301:
853–855, 1990.
Maggiorini M, Melot C, Pierre S, Pfeiffer F, Greve I, Sartori C, Lepori M, Hauser M, Scherrer U, and Naeije R.
High-altitude pulmonary edema is initially caused by an increase in capillary pressure. Circulation 103: 2078–2083, 2001.
Mairbaurl H, Mayer K, Kim KJ, Borok Z, Bartsch P, and
Crandall ED. Hypoxia decreases active Na transport across
94 • MAY 2003 •
www.jap.org
2050
30.
31.
32.
33.
34.
35.
36.
37.
38.
primary rat alveolar epithelial cell monolayers. Am J Physiol
Lung Cell Mol Physiol 282: L659–L665, 2002.
Mairbäurl H, Weymann J, Möhrlein A, Swenson E, Maggiorini M, Gibbs S, and Bärtsch P. Decreased Na but increased Cl transport across the nasal epithelium in high altitude
hypoxia (Abstract). High Alt Med Biol 2: A81, 2001.
Marantz PR, Tobin JN, Wassertheil-Smoller S, Steingart
RM, Wexler JP, Budner N, Lense L, and Wachspress J. The
relationship between left ventricular systolic function and congestive heart failure diagnosed by clinical criteria. Circulation
77: 607–612, 1988.
Mason NP, Barry PW, Pollard AJ, Collier DJ, Taub NA,
Miller MR, and Milledge JS. Serial changes in spirometry
during an ascent to 5,300 m in the Nepalese Himalayas. High Alt
Med Biol 1: 185–195, 2000.
Matalon S, Lazrak A, Jain L, and Eaton DC. Biophysical
properties of sodium channels in lung alveolar epithelial cells.
J Appl Physiol 93: 1852–1859, 2002.
Matalon S and O’Brodovich H. Sodium channels in alveolar
epithelial cells: molecular characterization, biophysical properties, and physiological significance. Annu Rev Physiol 61: 627–
661, 1999.
Matthay MA and Wiener-Kronish JP. Intact epithelial barrier function is critical for the resolution of alveolar edema in
humans. Am Rev Respir Dis 142: 1250–1257, 1990.
Naeije R and Torbicki A. More on the noninvasive diagnosis of
pulmonary hypertension: Doppler echocardiography revisited.
Eur Respir J 8: 1445–1449, 1995.
Nopp P, Harris ND, Zhao TX, and Brown BH. Model for the
dielectric properties of human lung tissue against frequency and
air content. Med Biol Eng Comput 35: 695–702, 1997.
Oh JK, Appleton CP, Hatle LK, Nishimura RA, Seward JB,
and Tajik AJ. The noninvasive assessment of left ventricular
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
diastolic function with two-dimensional and Doppler echocardiography. J Am Soc Echocardiogr 10: 246–270, 1997.
Pitkanen OM and O’Brodovich HM. Significance of ion transport during lung development and in respiratory disease of the
newborn. Ann Med 30: 134–142, 1998.
Poon CS. Analysis of linear and mildly nonlinear relationships
using pooled subject data. J Appl Physiol 64: 854–859, 1988.
Roach RC, Bärtsch P, Hackett PH, and Oelz O. The Lake
Louise acute mountain sickness scoring system. In: Hypoxia and
Mountain Medicine, edited by Sutton JR, Houston CS, and
Coates G. Burlington, VT: Queen City, 1993, p. 327–330.
Rocker GM. Bedside measurement of pulmonary capillary permeability in patients with acute lung injury. What have we
learned? Intensive Care Med 22: 619–621, 1996.
Roy SB, Balasubramanian V, Khan MR, Kaushik VS, Manchanda SC, and Guha SK. Transthoracic electrical impedance
in cases of high-altitude hypoxia. Br Med J 3: 771–775, 1974.
Sartori C, Allemann Y, Duplain H, Lepori M, Egli M, Lipp
E, Hutter D, Turini P, Hugli O, Cook S, Nicod P, and
Scherrer U. Salmeterol for the prevention of high-altitude pulmonary edema. N Engl J Med 346: 1631–1636, 2002.
Schwammenthal E, Vered Z, Agranat O, Kaplinsky E,
Rabinowitz B, and Feinberg MS. Impact of atrioventricular
compliance on pulmonary artery pressure in mitral stenosis: an
exercise echocardiographic study. Circulation 102: 2378–2384,
2000.
Staub NC. Pulmonary edema. Physiol Rev 54: 678–811, 1974.
Vock P, Fretz C, Franciolli M, and Bartsch P. High-altitude
pulmonary edema: findings at high-altitude chest radiography
and physical examination. Radiology 170: 661–666, 1989.
Ware LB and Matthay MA. Alveolar fluid clearance is impaired in the majority of patients with acute lung injury and the
acute respiratory distress syndrome. Am J Respir Crit Care Med
163: 1376–1383, 2001.
94 • MAY 2003 •
www.jap.org
CHAPTER 5
CHANGES IN PLASMA BRADYKININ CONCENTRATION
AND CITRIC ACID COUGH THRESHOLD AT HIGH ALTITUDE.
Mason, N. P., Petersen, M., Melot, C., Kim, E. V., Aldashev, A., Sarybaev, A.,
Mirrakhimov, M. M., Naeije, R.
Wilderness and Environmental Medicine, 20, 353–358 (2009)
BRIEF REPORT
Changes in Plasma Bradykinin Concentration and Citric
Acid Cough Threshold at High Altitude
Nicholas P. Mason, MB, ChB; Merete Petersen, MB, BS; Christian Mélot, PhD; Elena V. Kim, MD;
Almaz Aldashev, PhD; Akpay Sarybaev, PhD; Mirsaid M. Mirrakhimov, PhD; Robert Naeije, PhD
From the Department of Anaesthesia and Intensive Care Medicine, Royal Gwent Hospital, Newport, UK (Mason); Department of Physiology, Free
University of Brussels, Belgium (Mason, Naeije); Department of Anaesthesia, Nordsjaellands Hospital, Denmark (Petersen); Department of
Intensive Care Medicine, Erasmus Hospital, Brussels, Belgium (Mélot); and National Centre for Cardiology and Internal Medicine, Bishkek,
Kyrgyzstan (Kim, Aldashev, Sarybaev, Mirrakhimov).
Objective.—Altitude-related cough is a troublesome condition of unknown etiology. Inhaled tussive
agents are used to quantify cough, and the citric acid cough threshold has been shown to fall on ascent to
altitude. Cough can occur in patients taking angiotensin-converting enzyme inhibitors due to stimulation
of airway sensory receptors by increased levels of bradykinin. We hypothesized that increased levels of
bradykinin could be responsible for the decrease in citric acid cough threshold on exposure to altitude
and a possible etiologic factor in altitude-related cough.
Methods.—Twenty healthy volunteers underwent baseline tests at 700 m before a 2-week stay at
3800 m. Angiotensin-converting enzyme activity and plasma bradykinin were measured at baseline and
altitude. Citric acid cough threshold and nocturnal cough frequency were measured at baseline and
throughout the 2 weeks at altitude.
Results.—Citric acid cough threshold fell from 3.7 g/dL at baseline to 2.1 g/dL on the second day at
3800 m (geometric mean difference 1.8, 95% CIs 1.0–5.0, P 5 .025) and remained reduced throughout
the stay at altitude. Nocturnal cough frequency was unchanged compared to baseline. Plasma bradykinin
fell from 0.43 ng/mL at baseline to 0.08 ng/mL at altitude (geometric mean difference 5.7, 95% CIs
2.1–15.5, P 5 .002), but angiotensin-converting enzyme activity was unchanged (mean difference 0.06,
95% CIs –2.7–2.8, P 5 .97). There was no correlation between plasma bradykinin and citric acid cough
threshold.
Conclusions.—Increased levels of bradykinin are unlikely to be a significant factor in the increased
sensitivity to citric acid seen in hypobaric hypoxia. Further studies are required to elucidate the etiology
of altitude-related cough.
Key words: Altitude-related cough, bradykinin, pulmonary edema, citric acid cough threshold
Introduction
Anecdotal reports have long existed of a troublesome
paroxysmal cough affecting visitors to high altitude. In
the first systematic study of altitude-related cough,
nocturnal cough frequency and sensitivity to the inhaled
tussive agent citric acid were increased in a group of
subjects ascending to 5300 m in the Nepalese Himalaya.1
Traditionally, altitude-related cough was attributed to the
inspiration of cold, dry air, but during Operation EverestCorresponding author: Nicholas P. Mason, MB, ChB, Department of
Anaesthesia and Intensive Care Medicine, Royal Gwent Hospital,
Cardiff Rd, Newport, NP20 2UB, UK (e-mail: [email protected].
uk).
COMEX, a hypobaric chamber study mimicking an
ascent of Mount Everest, nocturnal cough frequency
increased and citric acid cough threshold fell, despite
careful control of temperature and humidity in the
chamber, refuting cold, dry air as the sole cause of
cough at altitude.2 Other possible etiologies (eg, water
loss from the respiratory tract, high altitude pulmonary
edema, bronchoconstriction, respiratory tract infection,
postnasal drip) have been discussed in a recent review.3
The precise neuronal pathways that mediate the citric
acid cough challenge remain debated but involve
stimulation of airway sensory nerves, such as airway
rapidly adapting receptors.4 Cough is a well-recognized
side effect in a proportion of patients taking angiotensin-
Mason et al
354
converting enzyme (ACE) inhibitors and is thought to be
due to sensitization of airway rapidly adapting receptors
by increased levels of plasma bradykinin and substance
P.5 In human serum, the majority of bradykinin metabolism occurs via ACE. The early literature on the response
of serum ACE activity to hypoxia in humans is confusing
and contradictory.6 Nothing is known about what happens
to bradykinin at altitude beyond exposure to 1 hour of
normobaric hypoxia.7 However, if overall ACE activity
falls, the result would be increased levels of bradykinin,
which could sensitize airway sensory nerve endings.
We therefore hypothesized that the decrease in citric
acid cough threshold on exposure to hypobaric hypoxia
is due to increased bradykinin levels’ sensitizing airway
sensory receptors and that this is an etiologic factor in
altitude-related cough.
Methods
SUBJECTS
Twenty male Kyrgyz altitude-naı̈ve lowlander volunteers
(age range 18–35 years) underwent baseline (BL) tests in
Bishkek, Kyrgyzstan (altitude 700 m) before being
transported by road in 7 hours to an altitude of 3800 m
in the Tian Shan Mountains. Subjects remained at
3800 m for 2 weeks, as reported previously,8 during
which time they were essentially sedentary. All subjects
gave written informed consent, and the project was
approved by the ethics committee of the Faculty of
Medicine of the Free University of Brussels in Belgium.
nocturnal cough frequency. For logistical reasons, it was
not possible to obtain recordings on RBL.
BLOOD SAMPLING
Blood samples were taken in the morning at BL and
on HA02 at high altitude. Blood was taken from a
vein in the antecubital fossa after the subject had
rested, sitting, for 30 minutes. Blood was collected into
EDTA tubes and then centrifuged for 15 minutes at
1600g in tubes containing aprotinin (0.6 TIU/mL of
blood). The collected plasma was stored at 270uC
until analysis.
PLASMA BRADYKININ ASSAY
Plasma bradykinin concentration was analyzed using a
bradykinin enzyme immunoassay kit (Phoenix Pharmaceuticals, Belmont, CA). Absorbance was read on a BioRad microtiter plate reader at 450 nm.
ANGIOTENSIN-CONVERTING ENZYME
ACTIVITY ASSAY
Serum ACE activity was assayed by hydrolysis of the
specific substrate Z-Phe-His-Leu. The fluorogen ophtaldialdehyde was then added, and fluorescence
emission was measured at 500 nm. Angiotensin-converting enzyme activity was calculated and expressed as
mU/mL of serum.
CLINICAL DATA
CITRIC ACID COUGH CHALLENGE
Solutions of increasing concentrations of citric acid in
0.9% saline were inhaled via an ultrasonic nebuliser
(Sonix 2000, Clement Clarke International, UK) during a
slow vital capacity inspiration over 5 seconds.2 The
cough threshold was defined as the lowest concentration
that provoked a cough, providing that a cough was also
provoked at the next concentration. Cough challenges
were performed at BL in Bishkek and on the 2nd (HA02)
and 15th (HA15) days at 3800 m and on return to BL
(RBL) in Bishkek.
NOCTURNAL COUGH MONITORING
Nocturnal cough frequency was measured using portable
voice-activated tape recorders (Panasonic RQ-L317,
Osaka, Japan), as previously described.2 Recordings
were made on consecutive nights at BL and on HA01,
HA02, HA12, and HA13 nights at 3800 m. The mean of
the results from each pair of nights is reported as the
Heart rate and oxygen saturation (Nellcor N20P pulse
oximeter, Nellcor-Puritan Bennet Ltd, Warwick, UK)
were recorded on the morning of each measurement day
during a general clinical interview and examination.
Acute mountain sickness was assessed using the Lake
Louise consensus self-assessment scoring system.9
STATISTICAL ANALYSIS
Normality was assessed using the Kolmogorov-Smirnov
test. Angiotensin-converting enzyme levels and heart rate
were normally distributed. Bradykinin levels and citric
acid cough threshold were not normally distributed but
were successfully log transformed. The acute mountain
sickness score, oxygen saturation, and nocturnal cough
frequency were not normally distributed, and neither
could they be normalized by transformation. Angiotensin-converting enzyme and bradykinin levels were
compared using paired t tests, and citric acid cough
threshold and heart rates using repeat measure analysis of
Altitude-Related Cough and Bradykinin
355
Table. Serum angiotensin-converting enzyme (ACE) activity, plasma bradykinin, citric acid cough threshold, heart rate, oxygen
saturation, and acute mountain sickness (AMS) score at baseline (BL), on 2nd (HA02) and 15th (HA15) days at 3800 m, and on
return to baseline (RBL)*
BL
HA02
HA15
RBL
Serum ACE activity (mU/mL serum)
40.1
(33.7–46.5)
Plasma bradykinin (ng/mL plasma)
0.43
(0.23–0.81)
Citric acid cough threshold (g/dL)
3.7
(2.3–5.8)
Heart rate (bpm)
65
(59–70)
Oxygen saturation (%)
98
(94–99)
AMS score
0
(0–0)
40.1
(33.9–46.2)
P 5 .97{
0.08
(0.03–0.16)
P 5 .002
2.1
(1.4–3.0)
P 5.025
89
(84–93)
P , .001
90
(84–92)
P , .05
0
(0–4)
NS{
1.9
(1.3–2.9)
P 5 .009
91
(88–94)
P , .001
91
(89–94)
P , .05
0
(0–2)
NS
2.4
(1.5–3.9)
P 5 .15
84
(81–87)
P , .001
97
(96–99)
NS
0
(0–0)
NS
*Serum ACE activity and heart rate are given as the mean and 95% CIs, citric acid cough threshold and plasma bradykinin as the geometric mean
and 95% CIs, and oxygen saturation and AMS score as the median and range.
{P values refer to comparison with baseline measurements.
{NS indicates not significant.
variance with posthoc analysis performed using the
Bonferroni test. They are presented as means or
geometric means and 95% CIs. Nocturnal cough
frequency, acute mountain sickness scores, and oxygen
saturation were compared using repeat measure analysis
of variance on ranks with posthoc analysis performed
using the Student-Newman-Keuls test and are presented
as medians and ranges. Analyses were performed using
SigmaStat 2.0 software (Jandell Corporation, San Rafael,
CA).
Results
COMPLETENESS OF DATA
Due to a technical problem with the assay, bradykinin
analysis was only possible on samples from 14 of 20
subjects. Citric acid data were obtained from 18 of 20
subjects. Nocturnal cough frequency recordings were
obtained from 19 of 20 subjects at all altitudes. Two
subjects were excluded from analysis because their BL
cough frequencies in Bishkek were very high and
remained high throughout the study compared with any
other subject.
CITRIC ACID COUGH THRESHOLD
Compared with BL, citric acid cough threshold fell at
HA02 (geometric mean difference 1.8, 95% CIs 1.0–5.0,
P 5 .025) and HA15 (geometric mean difference 1.9,
95% CIs 1.1–3.4, P 5 .009). Although still reduced on
return to the BL altitude, the reduction was not
statistically significant (geometric mean difference 1.5,
95% CIs 0.9–2.7, P 5 .15). These results are summarized in the table.
NOCTURNAL COUGH FREQUENCY
There was no change in nocturnal cough frequency
between BL (median cough frequency 0.5, range 0–4.5)
and HA01/HA02 (median cough frequency 1.0, range 0–
5.5) and HA12/HA13 (median cough frequency 0, range
0–5.5).
PLASMA BRADYKININ LEVELS AND
ANGIOTENSIN-CONVERTING ENZYME ACTIVITY
Plasma bradykinin concentrations fell from BL to HA02
(geometric mean difference 5.7, 95% CIs 2.1–15.5, P 5
356
.002). There was no change in serum ACE activity (mean
difference 0.06, 95% CIs –2.7–2.8, P 5 .97). These
results are summarized in the table.
CLINICAL DATA
Compared with BL, heart rate increased at HA02 (mean
difference 24 bpm, 95% CIs 16–32, p,0.001) and HA15
(mean difference 26, 95% CIs 19–34, P , .001) and
remained elevated at RBL (mean difference 19, 95% CIs
12–27, P , .001). Oxygen saturation fell at HA02 and
HA15 compared with BL (both P , .05). The acute
mountain sickness self-assessment score did not significantly increase during the stay at altitude. These results
are summarized in the table.
CORRELATION ANALYSIS
There was no relationship between citric acid cough
threshold and bradykinin levels, heart rate, or oxygen
saturation or between the changes in citric acid cough
threshold and bradykinin levels.
Discussion
As in previous studies,1,2 we have demonstrated a fall in
citric acid cough threshold on ascent to altitude, but this
study extends these previous findings to show that the
increased sensitivity to citric acid occurs at the lower,
more frequented altitude of 3800 m. The citric acid
cough threshold remained reduced throughout the 2
weeks at 3800 m and returned to control levels on
descent from altitude. The fall in citric acid cough
threshold was not, however, accompanied by a change in
nocturnal cough frequency. This is consistent with the
clinical observation that subjects were not troubled by
cough during the study period. Plasma bradykinin fell by
approximately 81% on day 2 at 3800 m compared with
its control value, although serum ACE activity did not
change. There was no relationship between citric acid
cough threshold and plasma bradykinin levels, heart rate,
or oxygen saturation.
Bradykinin is produced by plasma kallikrein from both
high- and low-molecular-weight kininogen. It may also
be generated by aminopeptidase-mediated cleavage of
kallidin peptides, and other enzyme pathways may
operate in certain disease states. Kallikrein activity is
controlled principally by a number of inhibitors,
including C1-inhibitor, a-2 macroglobulin, and antithrombin III.10 There is no available evidence on the
effects of hypoxia on the kallikrein-kinin system, and
what evidence exists on the changes in the coagulation
system indicate that hypoxia per se has minimal effects.11
Mason et al
The kallikrein-kinin system is predominantly tissue
based, and tissue levels of bradykinin exceed those in
blood. However, changes in plasma levels of bradykinin
have been shown to parallel those in the tissues.12
Previous work on ACE activity in hypoxia is
contradictory. Some studies demonstrate no change in
ACE activity on exposure to hypoxia; some studies
demonstrate a fall in activity; others demonstrate a
biphasic response with an initial fall in activity followed
by a return to baseline values.6 These differences reflect
technological uncertainties, small subject numbers, and
the widely different conditions under which ACE activity
was measured, ranging from acute exposure to normobaric
hypoxia of only 10 minutes to a field study on Everest
lasting 4 weeks.6 It is not possible to measure lung ACE
activity in living human subjects, and we therefore chose
to use serum ACE activity as a surrogate marker of overall
lung activity. There are limitations to this approach,
although measurement of whole lung and serum ACE
activity in dogs during exposure to chronic hypoxia
yielded comparable results.13 The most feasible explanations for the dramatic fall in plasma bradykinin levels seen
in this study are that serum ACE activity did not parallel
an increase in local tissue ACE activity and that a local
tissue increase in ACE activity in, for example, the
pulmonary endothelium14 was responsible for the fall in
plasma bradykinin or that it was metabolized by a kininase
other than ACE. Despite being unable to explain the
mechanisms behind the fall in bradykinin seen in this
study, we can tentatively exclude an increase in plasma
bradykinin as the cause for the increase in sensitivity to
inhaled citric acid seen in this and previous papers.
Although there was a fall in citric acid cough threshold
on ascent to 3800 m, we were unable to demonstrate any
clinical increase in cough frequency, despite the magnitude of the change in citric acid threshold being
comparable to previous work at altitude.2 One possible
explanation for this finding would be an alteration in
nebulizer output due to reduced barometric pressure.
Barry15 assessed the output from the ultrasonic device
used in this study in a hypobaric chamber. It fell by only
23% at 4200 m and 13% at 5300 m compared with sea
level. In a further study, serum salbutamol levels were
measured in human subjects after ultrasonic-nebulized
administration of salbutamol.16 There was no difference in
serum salbutamol levels between sea level and a simulated
altitude of 5000 m. These findings argue against changes
in nebulizer output’s being responsible for the change in
citric acid cough threshold reported here.
The increased sensitivity to citric acid in the absence
of a clinical change in cough is an important finding,
because the study was performed at an altitude of
3800 m, which is more frequented than the higher
Altitude-Related Cough and Bradykinin
altitudes of previous studies.1,2 It suggests that the
pathophysiologic process that is responsible for altituderelated cough is operating even before clinical cough is
apparent. The inhalation of nebulized citric acid and the
threshold concentration that produces cough is a wellrecognized method of measuring the sensitivity of the
cough reflex and induces cough in a dose-dependent and
reproducible manner.17 Citric acid stimulates airway
sensory nerves, such as rapidly adapting receptors, which
constitute a heterogeneous group known to be activated
by a number of stimuli, including gaseous or aerosolized
irritants, inflammatory and immunologic mediators (eg,
bradykinin), edema, and atelectasis.4 It has recently been
suggested that the term altitude-related cough may cover
at least 2 conditions: a cough that can occur at lower
altitudes, which is related to exercise and possibly trauma
or infection of the respiratory tract, and a cough that is
only a clinical problem at higher altitudes and that may
be due to subclinical pulmonary edema’s stimulating
airway sensory nerves or changes in the central control of
cough.3 None of our subjects, who were predominantly
sedentary throughout the study, exhibited any clinical
signs of respiratory tract infection. In addition, as
previously described,8 the subjects performed nasal
lavage with 0.9% sodium chloride solution morning
and evening, and vasomotor rhinitis and postnasal drip
were not a problem. This excludes trauma or infection of
the respiratory tract as a cause for the alteration in citric
acid cough threshold seen in this study.
Respiratory control undergoes profound changes on
exposure to high altitude, and although the central
control of cough is poorly understood, a number of
factors that suppress cough also suppress ventilation.18,19
A relationship at sea level has been reported between the
hypercapnic ventilatory response and cough threshold to
hypotonic saline.20 It has been suggested that changes in
ventilatory control on ascent to altitude could influence
or parallel central changes in the control of cough.3
However, a recently published study could demonstrate
no correlation between hypercapnic ventilatory response
and the citric acid cough threshold on ascent to
5200 m.21
Finally, the increase in sensitivity to citric acid in the
absence of clinical cough could represent early subclinical pulmonary edema. Considerable evidence exists to
support the presence of subclinical pulmonary edema at
high altitude, and the 2 most likely etiologic mechanisms
are hypoxic pulmonary vasoconstriction, resulting in an
increase in pulmonary capillary pressure, and altered
respiratory epithelial ion transport.8
In conclusion, we have demonstrated a decrease in
citric acid cough threshold in a group of individuals
during a 2-week stay at the relatively moderate altitude of
357
3800 m. The change in citric acid cough threshold was
not due to altered plasma bradykinin levels, which fell on
ascent to altitude. More studies are required to elucidate
the mechanism of altitude-related cough.
Acknowledgments
We are grateful to Pascale Jespers and Marie-Therese
Gautier of Erasme Hospital, Brussels, Belgium, and Dr
Peter Barry, Judith Jackson, and Judy White of the
University of Leicester, UK, for their invaluable
assistance during preparation for the project; the
members of the Kyrgyz Presidential Guard who
participated in the study and the staff of Kumtor Gold
Mine, Kyrgyzstan; and in particular Dr Francois du Toit,
without whose help the project would not have been
realized. This work was supported by grant no.
3.4567.00 of the Fonds de la Recherche Scientifique
Médical of Belgium. Some of the data presented in this
paper were previously published in abstract form (Am J
Respir Crit Care Med. 2002;165:A830).
References
1. Barry PW, Mason NP, Riordan M, O’Callaghan C. Cough
frequency and cough-receptor sensitivity are increased in
man at altitude. Clin Sci (Colch). 1997;93:181–186.
2. Mason NP, Barry PW, Despiau G, Gardette B, Richalet JP.
Cough frequency and cough receptor sensitivity to citric
acid challenge during a simulated ascent to extreme
altitude. Eur Respir J. 1999;13:508–513.
3. Mason NP, Barry PW. Altitude-related cough. Pulm Pharmacol Ther. 2007;20:388–395.
4. Canning BJ. Anatomy and neurophysiology of the cough
reflex: ACCP evidence-based clinical practice guidelines.
Chest. 2006;129:33S–47S.
5. Israili ZH, Hall WD. Cough and angioneurotic edema
associated with angiotensin-converting enzyme inhibitor
therapy: a review of the literature and pathophysiology.
Ann Intern Med. 1992;117:234–242.
6. Swenson E. Renal function and fluid homeostasis. In:
Hornbein TF Sr, ed. High Altitude, An Exploration of
Human Adaptation. 1st ed. New York, NY: Marcel Dekker; 2001:537–539.
7. Ashack R, Farber MO, Weinberger MH, et al. Renal and
hormonal responses to acute hypoxia in normal individuals.
J Lab Clin Med. 1985;106:12–16.
8. Mason NP, Petersen M, Melot C, et al. Serial changes in
nasal potential difference and lung electrical impedance
tomography at high altitude. J Appl Physiol. 2003;94:
2043–2050.
9. Roach R, Bartsch P, Hackett P, Oelz O. The Lake Louise
acute mountain sickness scoring system. In: Sutton J,
Houston C, Coates G, eds. Hypoxia and Mountain Medicine.
Burlington, VT: Queen City Printers; 1993:327–330.
358
10. Campbell DJ. The kallikrein-kinin system in humans. Clin
Exp Pharmacol Physiol. 2001;28:1060–1065.
11. Grover R, Bärtsch P. Blood. In: Hornbein TF Sr, ed. High
Altitude, An Exploration of Human Adaptation. 1st ed.
New York, NY: Marcel Dekker; 2001:493–523.
12. Campbell DJ, Kladis A, Duncan AM. Bradykinin peptides
in kidney, blood, and other tissues of the rat. Hypertension.
1993;21:155–165.
13. Molteni A, Zakheim RM, Mullis KB, Mattioli L. The effect
of chronic alveolar hypoxia on lung and serum angiotensin
I converting enzyme activity. Proc Soc Exp Biol Med.
1974;147:263–265.
14. Morrell NW, Atochina EN, Morris KG, Danilov SM,
Stenmark KR. Angiotensin converting enzyme expression
is increased in small pulmonary arteries of rats with
hypoxia-induced pulmonary hypertension. J Clin Invest.
1995;96:1823–1833.
15. Barry P. The output of nebulisers at high altitude [abstract].
High Alt Med Biol. 2000;2:114.
Mason et al
16. Barry P, Hart N, Wilson C, et al. Lung deposition of
therapeutic aerosols at simulated altitude [abstract]. High
Alt Med Biol. 2000;2:114.
17. Morice AH, Fontana GA, Belvisi MG, et al. ERS
guidelines on the assessment of cough. Eur Respir J.
2007;29:1256–1276.
18. Douglas NJ. Control of breathing during sleep. Clin Sci
(Lond). 1984;67:465–471.
19. Gutstein H, Akil H. Opioid analgesics: centrally acting
anti-tussive agents. In: Brunton L, ed. Goodman and
Gilman’s The Pharmacological Basis of Therapeutics. 11th
ed. New York, NY: McGraw-Hill; 2006:578–579.
20. Banner AS. Relationship between cough due to hypotonic
aerosol and the ventilatory response to CO2 in normal
subjects. Am Rev Respir Dis. 1988;137:647–650.
21. Thompson ARR, Baille JK, Bates MGD, et al. The
citric acid cough threshold and the ventilatory response
to carbon dioxide on ascent to high altitude. Respir Med.
2009;103:1182–1188.
CHAPTER 6
THE CITRIC ACID COUGH THRESHOLD AND THE VENTILATORY RESPONSE
TO CARBON DIOXIDE ON ASCENT TO HIGH ALTITUDE.
Thompson, A. A., Baillie, J. K., Bates, M. G., Schnopp, M. F., Simpson, A., Partridge, R. W.,
Drummond, G. B., Mason, N. P.
Respir Med 103: 1182-1188, 2009.
.
.
Respiratory Medicine (2009) 103, 1182e1188
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/rmed
The citric acid cough threshold and the ventilatory
response to carbon dioxide on ascent to high
altitude*
A.A. Roger Thompson a,b,*, J. Kenneth Baillie b,c, Matthew G.D. Bates b,
Martin F. Schnopp b, Alistair Simpson b, Roland W. Partridge b,
Gordon B. Drummond c, Nicholas P. Mason d,e
a
Academic Unit of Respiratory Medicine, University of Sheffield, Royal Hallamshire Hospital, Glossop Road, Sheffield
S10 2JF, UK
b
APEX (Altitude Physiology Expeditions), c/o Dr F. Kristmundsdottir, College Office, College of Medicine and Veterinary
Medicine, The University of Edinburgh, The Chancellor’s Building, 49 Little France Crescent, Edinburgh EH16 4SB, UK
c
Department of Anaesthesia, Intensive Care and Pain Medicine, Royal Infirmary of Edinburgh, 52 Little France Crescent,
Edinburgh EH16 4SA, UK
d
Department of Anaesthesia and Intensive Care Medicine, Royal Gwent Hospital, Newport NP20 2UB, UK
e
Department of Physiology, Free University of Brussels, B1070 Brussels, Belgium
Received 12 April 2008; accepted 12 February 2009
Available online 19 March 2009
KEYWORDS
High altitude;
Cough;
Citric acid cough
threshold;
Ventilatory control;
Hypercapnic ventilatory
response
Summary
Ventilatory control undergoes profound changes on ascent to high altitude. We hypothesized
that the fall in citric acid cough threshold seen on ascent to altitude is mediated by changes
in the central control of cough and would parallel changes in central ventilatory control assessed by the hypercapnic ventilatory response (HCVR). Twenty-five healthy volunteers underwent measurements of HCVR and citric acid sensitivity at sea level and during a 9 day sojourn
at 5200 m. None of the subjects had any evidence of respiratory infection. Citric acid cough
threshold fell significantly on ascent to 5200 m. The slope, S, of the HCVR increased significantly on ascent to 5200 m and during the stay at altitude. There was no correlation between
citric acid sensitivity and HCVR. We conclude that the change in citric acid cough threshold
seen on exposure to hypobaric hypoxia is unlikely to be mediated by changes in the central
control of cough. Sensitivity to citric acid may be due to early subclinical pulmonary edema
stimulating airway sensory nerve endings.
ª 2009 Elsevier Ltd. All rights reserved.
*
Funding: This study was funded by a minor research award from Chest Heart and Stroke, Scotland.
* Corresponding author at: Academic Unit of Respiratory Medicine, University of Sheffield, Royal Hallamshire Hospital, Glossop Road,
Sheffield S10 2JF, UK. Tel.: þ44 (0)114 271 2630; fax: þ44 (0)114 226 8898.
E-mail address: [email protected] (A.A.R. Thompson).
0954-6111/$ - see front matter ª 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.rmed.2009.02.007
High-altitude cough
Introduction
Climbers and travelers to high altitude have long reported
a troublesome paroxysmal cough severe enough to cause rib
fractures.1e3 In the first systematic study of high-altitude
cough an increase in nocturnal cough frequency and cough
receptor sensitivity to citric acid were shown in a group of
subjects ascending to 5300 m in the Nepalese Himalaya,
with a massive increase in nocturnal cough frequency in the
3 climbers in whom recordings were possible at 7000 m.4
High-altitude cough was traditionally attributed to the
inspiration of cold, dry air. However, observations and
experiments in two long-duration hypobaric chamber
studies mimicking ascents of Mount Everest showed that
cough still occurs despite controlled temperature and
humidity.5,6 During the second of these chamber studies
(Operation Everest-COMEX), nocturnal cough frequency
increased and citric acid cough threshold fell despite careful
control of temperature and humidity in the chamber.6
Because the cause of high-altitude cough remains
unclear, methods to treat or prevent this important
symptom may be difficult to develop. Suggested causes
have been discussed fully in a recent review.7 Respiratory
control undergoes profound changes with exposure to high
altitude, and several lines of evidence suggest that changes
in respiratory control also affect the tendency to cough.8 A
number of factors which suppress cough also suppress
ventilation, such as sleep and centrally acting anti-tussive
agents.9,10 Banner demonstrated a lower hypercapnic
ventilatory response (HCVR) in a subgroup of healthy
volunteers at sea level who did not cough in response to
inhaled hypotonic saline and suggested that cough sensitivity and HCVR might be mediated by a common central
mechanism.11 Little is known about any direct relationship
between HCVR and cough generation, but in anesthetized
humans hypercapnia was demonstrated to inhibit tracheal
cough.12 Further possible evidence for a link between
ventilatory control and cough at altitude came from a study
at 5300 m in which a post-hoc analysis of data demonstrated a relationship between citric acid cough threshold
and the ventilatory response to early inspired pulses of
carbon dioxide.13
We hypothesized that high-altitude cough could be
caused by central alterations in respiratory control. We
predicted that if the link between HCVR and cough was
strong, there would be a correlation between susceptibility
to cough at altitude, as measured by the citric acid cough
challenge, and either sea level HCVR or the change in HCVR
between sea level and altitude.
Methods
Subjects
25 healthy subjects (15 male, 10 female, age range 18e54
years) underwent baseline testing at sea level in the United
Kingdom before flying to La Paz, Bolivia (altitude 3630 m).
Subjects remained in La Paz for 4 days to acclimatize
before ascending by road over 90 min to the Chacaltaya
Laboratory at 5200 m where they stayed for 9 days. All
subjects gave written informed consent and the project
1183
was approved by the Lothian Research Ethics Committee.
All subjects were non-smokers and had no history of
respiratory disease; none were taking regular medication
before or during the study.
Citric acid cough threshold
After a control solution of 0.9% saline, subjects inhaled
solutions of citric acid via an ultrasonic nebulizer (Sonix
2000, Clement Clarke International, Harlow, UK) during
a slow vital capacity inspiration over 5 s as previously
described.4 Ten solutions of citric acid doubling in
concentration from 0.31 to 160 g l1 were used. To avoid
a learning effect the concentrations were administered in
random order, although the stronger concentrations were
not given early in the test to avoid airway irritation.4 Each
solution was inhaled 3 times and the cough threshold was
defined as the lowest concentration which provoked
a cough providing that a cough was also provoked at the
next concentration. To avoid diurnal variations in cough
sensitivity to citric acid14,15 all tests were performed
between 14:00 and 18:00 h. Citric acid cough challenges
were performed at sea level (SL) prior to departure and on
days 1, 3, 5 and 9 at 5200 m (HA1, HA3, HA5 and HA9
respectively). The method differs from recently-published
European Respiratory Society guidelines16 as the guidelines
were published after this study was completed.
Before and after each citric acid test peak expiratory
flow was recorded using a Mini-Wright peak flow meter
(Airmed, Clement Clarke International, UK). Although the
measurements made with this variable-orifice flow meter
are profoundly altered by the fall in air density with
increasing altitude17 it was only being used to exclude
bronchospasm after inhalation of citric acid, and so being
light and portable lent itself ideally to use at high altitude.
Hypercapnic ventilatory response
HCVR was measured using the rebreathing method of
Read.18 Subjects refrained from alcohol and caffeine for
12 h prior to each measurement. After breathing room air
for 5 min breathing was switched to the bag-in-box apparatus using a 3-way valve. The 6 l bag contained 95% O2
and 5% CO2 at sea level, while at altitude the CO2
concentration was adjusted to around 7.5% to exceed
slightly the end tidal partial pressure of CO2 ðPE0CO Þ.
2
Rebreathing continued for 4 min or until the CO2 concentration had risen by 4%. The test was stopped if the
subject experienced discomfort.
At sea level ventilation was measured using flow data
from a F1000L pneumotachograph (GM Instruments, Kilwinning, UK) and at altitude with a turbine spirometer
(VmaxST, Sensormedics, Yorba Linda, CA, USA). End tidal
CO2 was measured using a sidestream, infra-red analyser
(Datex Normocap CD200 or Datex Oxicap, Datex-Ohmeda,
Hatfield, UK at sea level, and Vmax ST at 5200 m). The
analogue flow and CO2 signals were digitized using an
analogueedigital converter (instruNet iNET-100B, Amplicon
Liveline Ltd, Brighton, UK) and displayed using software
supplied with the converter on a laptop PC. HCVR was
measured at SL and days 2 and 8 at 5200 m (HA2 and HA8
respectively).
Hypercapnic ventilatory response data were plotted
using Sigma Plot v8.0 (SPSS Inc. Chicago, Illinois, USA) and
the slope of the linear section of the curve (S ) calculated
using least squares linear regression.
Acute mountain sickness
Acute mountain sickness (AMS) was assessed each day using
the Lake Louise scoring system.19
Completeness of the data
Three male and 2 female subjects withdrew from the study
at 5200 m because of severe AMS. One male subject
persistently hyperventilated during HCVR testing at 5200 m.
These subjects have been excluded from all analyses.
Hypercapnic ventilatory response was only measured in
male subjects as menstrual cycle variability in ventilatory
control complicates the interpretation of results in
females.20
Statistical analysis
Data were analyzed for normality using QeQ plots and the
KolmogoroveSmirnov Test. Serial measures were tested for
statistical significance using one way analysis of variance
(ANOVA) for repeated measures with significant post hoc
differences being analyzed using a Tukey test. HCVR was
compared with citric acid cough threshold (CACT) using
correlation plots and the Bonferroni correction for multiple
comparisons was applied. Analyses were performed using
SPSS vs. 11.5 software and Sigma Plot v8.0 (SPSS Inc. Chicago, Illinois, USA).
Results
Citric acid cough threshold
Data were available for 20 subjects. Data were positively
skewed. Transformation, by taking natural logarithms,
resulted in data at each point passing the Kolmogorove
Smirnov test. Citric acid cough threshold fell from
a geometric mean value at SL of 3.7 g/dl (95% CIs 23.2e
64.3) to 2.5 g/dl (95% CIs 1.6e20.0) on HA1 (geometric
mean difference 3.1, 95% CIs 1.4e6.9, p Z 0.002). It
remained significantly reduced compared to SL at HA3
(2.8 g/dl, 95% CIs 11.1e23.8, geometric mean difference
2.4, 95% CIs 1.08e5.23, p Z 0.026) and HA9 (2.7 g/dl, 95%
CIs 9.5e24.1, geometric mean difference 2.55, 95% CIs
1.2e5.6, p Z 0.014). The trend towards a reduction at HA5
(3.3 g/dl, 95% CIs 16.0e43.4) compared to SL did not meet
statistical significance (geometric mean difference 1.46,
95% CIs 0.67e3.22, p Z 0.57). We found no difference
between male and female subjects. These data are shown
in Fig. 1.
A.A.R. Thompson et al.
Citric acid cough threshold (g l-1)
1184
100
Male
Female
80
60
*
*
HA1
HA3
*
40
20
0
SL
HA5
HA9
Figure 1 Change in citric acid cough threshold (g l1) with
time at 5200 m for male and female subjects. Error bars
represent SE. SL: Sea level. HA1, HA3, HA5, HA9: days 1, 3, 5
and 9 respectively at 5200 m. *p < 0.05 for pooled mean cf sea
level control.
14.52 l min1 kPa CO1
(95% CIs 12.34e16.70) at SL, to
2
32.11 l min1 kPa CO1
(95% CIs 22.73e41.49) at HA2
2
(difference between means 17.59, p < 0.001) and
35.64 l min1 kPa1 (95% CIs 27.93e43.34) at HA8 (difference
between means 21.20, p < 0.001). There was no significant
difference between HA2 and HA8.
The PE0CO for a ventilation of 15 l min1 has been
2
calculated from the regression equation for each subject,
this being more meaningful than the intercept with the
ordinate, when PE0CO Z0. The PE0CO at a ventilation of
2
2
15 l min1 decreased from a mean value of 7.45 kPa (95%
CIs 6.97e7.93) at SL, to 5.07 kPa (95% CIs 4.91e5.23) at
HA2 (difference between means 2.71, p < 0.001) and
4.75 kPa (95% CIs 4.59e4.92) at HA8 (difference between
means 2.39, p < 0.001). There was no significant difference
between HA2 and HA8.
An example of a HCVR plot from one subject is shown in
Fig. 2 and HCVR data from individual subjects are summarized in Table 1.
Correlation between CACT and HCVR
There was no significant correlation between the SL slope
of HCVR and the natural logarithm of CACT. There was
a significant but weak negative correlation between the
slope of HCVR and the natural log of CACT when data were
pooled from all altitudes (r Z 0.382, p Z 0.028). This is
illustrated in Fig. 3. There was a significant positive
correlation between the SL slope of HCVR and the change in
the natural log of CACT from SL to HA1 (r Z 0.678,
p Z 0.022). This is shown in Fig. 4. There was no difference
in HCVR between any groups of high and low CACT, irrespective of the arbitrary division used.
There was no correlation between AMS score or SpO2 and
CACT.
Discussion
Hypercapnic ventilatory response
Data were available from 12 subjects and were normally
distributed. The slope, S, increased from a mean value of
We have demonstrated a decrease in citric acid cough
threshold (CACT) on ascent to 5200 m which remained
decreased after 9 days at altitude. This fall in CACT with
High-altitude cough
1185
70
Ventilation (litres / min)
60
50
40
30
20
10
0
4
5
6
7
8
9
10
ETCO2 (kPa)
SL
Slope = 17.99 l.min-1 .kPa CO2-1
CO2 at 15 l.min-1 = 8.08 kPa
r = 0.989, p<0.001
HA2
Slope = 25.91 l.min-1 .kPa CO2-1
CO2 at 15 l.min-1 = 4.86 kPa
r = 0.923, p<0.001
Figure 2
HA8
Slope = 41.40 l.min-1 .kPa
CO2-1
CO2 at 15 l.min-1 = 4.52 kPa
r = 0.990, p<0.001
regression line
95% confidence intervals
Sample HCVR plot from subject 11.
decreasing barometric pressure is consistent with previous
findings at a similar altitude4 and in a hypobaric chamber
study to the barometric equivalent of the summit of Mount
Everest.6 We have also demonstrated an increase in the
hypercapnic ventilatory response (HCVR) on ascent to
5200 m which was sustained throughout the stay at 5200 m,
consistent with previous reports.21 We did not demonstrate
Table 1
Subject
1
3
5
7
11
13
14
15
16
21
24
25
Mean
U 95% CI
L 95% CI
a convincing relationship between CACT and HCVR or
between the changes in CACT and HCVR with ascent to
altitude.
We chose to investigate the relationship between CACT
and HCVR because of previous work which showed that
subjects who did not cough in response to inhaled hypotonic saline at sea level had a lower HCVR compared with
those who did cough, suggesting that cough sensitivity and
HCVR might be mediated by a common mechanism, and
because HCVR undergoes profound changes on ascent to
and with acclimatization to altitude.8 The central control
of cough is complex and poorly understood.22e24 We
hypothesized that a subject’s underlying central ventilatory
control as represented by HCVR, or the change in that
ventilatory control on ascent to altitude, could influence or
parallel their susceptibility to cough at altitude. Although
the hypoxic ventilatory response (HVR), which is mediated
via the peripheral chemoreceptors in the carotid bodies
also increases with acclimatization to altitude, it is unclear
whether the increased sensitivity of HVR is due to changes
in peripheral chemoreceptor sensitivity or changes in the
central integrating mechanism.25 One reported post-hoc
analysis, which must be interpreted with caution, also
suggested a relationship between CACT and the ventilatory
response to early inspired pulses of carbon dioxide.13
The only previous report of a relationship between HCVR
and cough sensitivity used hypotonic saline as a challenge.
We chose to use citric acid as our cough challenge to
maintain consistency with previous studies at altitude,4,6
because hypotonic saline can potentially cause bronchoconstriction26 and because the hypotonic saline challenge is
difficult to standardize as it is very dependent upon nebulizer output.16 Changes in nebulizer output secondary to
reduced barometric pressure are a possible false cause of
a change in citric acid cough threshold on ascent to
Summary of hypercapnic ventilatory response (HCVR) data in 12 subjects.
Sea Level
HA2
HA8
S
CO2 V15
r
S
CO2 V15
r
S
CO2 V15
r
16.93
17.09
12.08
10.98
17.99
15.08
16.09
9.89
15.80
7.87
18.11
16.32
14.52
16.7
12.34
6.04
6.83
8.03
8.37
8.08
7.16
7.74
8.42
7.28
6.97
6.63
7.90
7.45
7.93
6.97
0.944*
0.958*
0.866*
0.837*
0.989*
0.938*
0.925*
0.932*
0.956*
0.946*
0.948*
0.909*
42.11
56.98
20.45
16.56
25.91
29.80
35.00
13.69
33.11
16.46
57.19
38.09
32.11y
41.49
22.73
5.07
5.04
4.74
5.48
4.86
5.23
5.40
5.24
4.72
5.23
5.03
4.80
5.07y
5.23
4.91
0.952*
0.970*
0.958*
0.888*
0.923*
0.965*
0.938*
0.934*
0.897*
0.970*
0.968*
0.935*
38.82
39.61
28.20
27.89
41.40
55.20
31.63
14.92
30.89
20.87
53.57
44.62
35.64y
43.34
27.93
4.84
4.64
4.71
5.05
4.52
4.52
4.68
4.99
4.67
4.60
5.32
4.42
4.75y
4.92
4.59
0.984*
0.969*
0.973*
0.837*
0.990*
0.938*
0.951*
0.908*
0.958*
0.968*
0.979*
0.992*
S: slope of HCVR curve (l min1 kPa CO1
2 ).
CO2 V15: End tidal CO2 (kPa) at ventilation of 15 l min1.
r: correlation coefficient for linear regression model.
U 95% CI and L 95% CI: upper and lower 95% confidence intervals of the mean, respectively.
*p < 0.001. yp < 0.001 cf sea level control.
1186
6
SL S vs SL ln CACT
HA2 S vs HA1 ln CACT
HA8 S vs HA9 ln CACT
Regression line
95% confidence intervals
natural log CACT
5
4
3
2
1
0
10
20
30
40
50
60
HCVR slope (S) / litres.min-1 kPa.CO2-1
Figure 3 Linear regression line of the natural logarithm of
the citric acid cough threshold (ln CACT) plotted against the
slope (S ) of the hypercapnic ventilatory response (HCVR) for
pooled data from all altitudes (SL: sea level; HA1, HA2, HA8
and HA9 the first, second, eighth and ninth days at 5200 m
respectively). The regression equation describing the line is
ln CACT Z 3.704 (0.0275 S );
correlation
coefficient
r Z 0.382, p Z 0.028. See Discussion for interpretation.
Change in natural log CACT from SL to HA1
altitude. Barry assessed the output of three types of
nebulizers in a hypobaric chamber at sea level and altitudes
equivalent to 4200 and 5300 m.27 While the output from the
ultrasonic device used in this study fell by only 23% at
4200 m and 13% at 5300 m, the output from both
a conventional jet nebulizer and a breath enhanced nebulizer fell by over 50% at the simulated altitudes compared
with sea level. In a further study serum salbutamol levels
were measured in human subjects following ultrasonic
1
0
-1
-2
-3
-4
6
8
10
12
14
16
18
20
Sea Level HCVR slope (S) / litres.min-1 kPa.CO2-1
Figure 4 Linear regression line of the change in the natural
logarithm of the citric acid cough threshold (ln CACT) between
sea level (SL) and the first day at 5200 m (HA1) plotted against
the sea level slope (S ) of the hypercapnic ventilatory response
(HCVR). The regression equation describing the line is: change
in ln CACT Z 4.184 þ (0.211 S ); correlation coefficient
r Z 0.678, p Z 0.022. See Discussion for interpretation.
nebulizer administration of salbutamol at sea level and
a simulated altitude of 5000 m in a hypobaric chamber.27
There was no difference in serum salbutamol levels
between these altitudes. These findings argue against
changes in nebulizer output producing a false change in
citric acid cough threshold on ascent to altitude.
There was a significant correlation between the sea
level slope, S, of HCVR and the change in CACT from sea
level to the first day at 5200 m (Fig. 4). This would suggest
that those subjects with a low initial HCVR had minimal or
no change, or even an increase, in CACT on ascent to
5200 m, while those with a brisk initial HCVR had the more
usual fall in CACT indicating increased sensitivity to citric
acid on ascent to altitude. However the relationship is
weak and significantly skewed by one outlying data point
(S Z 8 l min1 kPa CO1
2 ). Removing this outlier renders the
correlation insignificant and we believe it to be spurious.
There was a significant, but weak (r Z 0.382), negative
correlation between the slope of HCVR and the natural log
of CACT when data were pooled from all altitudes suggesting that those subjects with the lowest HCVR were the
most resistant to citric acid, while those with the highest
HCVR and who had the lowest cough threshold were most
sensitive to citric acid (Fig. 3). While this relationship
appears to be intuitively correct if the central control of
cough and respiration are linked, it depends primarily on
the low values of HCVR which were measured at sea level.
As such it is equally explicable as a simple altitude effect
on HCVR.
Our findings substantiate the work of Banner found no
correlation between HCVR and cough threshold in those
subjects who did cough in response to hypotonic saline.11
Taking Banner’s work together with the present study it seems
unlikely that the changes seen in CACT on ascent to altitude
are related to respiratory control as reflected by HCVR.
A weakness of the study design is the lack of a control
group unexposed to hypobaric hypoxia and challenged at
sea level with successive citric acid exposures. A short-term
tachyphylaxis occurs when tests are performed within hours
of each other.28 Apart from this, we think it is unlikely that
repeated exposure to citric acid will have a longer lasting
effect on cough receptors. Cough threshold tests on 4
normal and 8 asthmatic subjects, repeated on 4 occasions
within 10 days showed that baseline cough threshold did
not vary by more than two dosage increments in any
subject.29 We believe the reproducibility of the CACT is
robust enough to allow valid comparison between our sea
level and altitude data, despite the time interval between
initial testing and testing at altitude. Tests conducted 1 day
or 2 weeks apart are reproducible within one doubling of
the citric acid concentration.30,31
Citric acid stimulates airway sensory nerve receptors
which constitute a heterogeneous group activated by
a number of stimuli including gaseous or aerosolized irritants; inflammatory and immunological mediators such as
bradykinin; edema and atelectasis.32 It has recently been
suggested that the term altitude-related cough may cover at
least two conditions: a cough which can occur at lower
altitudes, which is related to exercise and possibly trauma or
infection of the respiratory tract and a cough which is only
a clinical problem at higher altitudes.7 None of the subjects
in the current study showed signs of respiratory tract
High-altitude cough
infection and the ascent to high altitude by vehicle did not
involve exercise, excluding the first mechanism as a cause
for the change in CACT seen in this study. If changes in the
central control of cough can be tentatively excluded, this
leaves subclinical edema stimulating pulmonary afferents as
an explanation for the changes seen in CACT in this study.
Edema stimulates both rapidly adapting receptors and Cfibres in animal models33e35 and two mechanisms have been
shown to be capable of causing subclinical pulmonary
edema at high altitude. Hypoxic pulmonary vasoconstriction increases pulmonary artery pressure. The vasoconstriction is uneven, reducing blood flow in some areas but
leading to overperfusion of others and exposing those
capillaries to high transmural pressures.36 If pulmonary
capillary pressure is sufficiently elevated, this will result in
clinical pulmonary edema. In haemodynamic studies in
humans at 4559 m a minimum pulmonary capillary pressure
of 19 mm Hg was necessary for the development of clinical
high-altitude pulmonary edema.37 However in a rabbit
model increased activity in airway rapidly adapting receptors was demonstrated with increases in left atrial
pressure of as little as 5 mm Hg.38 The pulmonary capillary
pressure resulting from these modest increases in left atrial
pressure is not known and although the mechanisms
elevating pulmonary capillary pressure are very different in
these two models, small degrees of hypoxic pulmonary
vasoconstriction could result in a raised pulmonary capillary pressure and sensitization of rapidly adapting receptors before any clinical pulmonary edema becomes evident.
As pulmonary artery pressure increases with exercise39 this
mechanism would also explain the observation that cough
at altitude can often be precipitated by exercise.
The second possible mechanism which could result in
subclinical edema sufficient to stimulate pulmonary afferents is an alteration in transepithelial fluid transport. There
is increasing evidence that this mechanism, which is
necessary for the resolution of pulmonary edema,40 is
deranged in hypoxia.41e43 An etiological role for subclinical
pulmonary edema in altitude-related cough should be
investigated further by intervention studies using agents
known to lower pulmonary artery pressure or increase
transepithelial fluid clearance.
In conclusion we have failed to demonstrate
a convincing relationship between the change in CACT
which occurs on ascent to high altitude and changes in the
central control of ventilation as represented by HCVR. In
the absence of signs of respiratory tract infection this
suggests that the fall in CACT seen in this study, and in
previous studies in hypobaric hypoxia, may be due to early
subclinical pulmonary edema stimulating airway sensory
nerve endings. Further studies are required to investigate
this mechanism.
Conflict of interest
No known conflicts of interest.
Acknowledgements
The authors would like to thank Datex-Ohmeda Ltd.,
Amplicon Liveline Ltd. and Sensormedics for the loan of
1187
equipment and technical support. We also thank all the
expedition members, especially Dr S. Patel, J. Read, Y. D.
J. Bennett and J. Balfour, who helped carry out the
research.
References
1. Steele P. Medicine on Mount Everest 1971. Lancet 1971;2:32e9.
2. Litch JA, Tuggy M. Cough induced stress fracture and
arthropathy of the ribs at extreme altitude. Int J Sports Med
1998;19:220e2.
3. Tasker J. Everest the cruel way. London: Eyre Methuen Ltd.;
1981.
4. Barry PW, Mason NP, Riordan M, et al. Cough frequency and
cough-receptor sensitivity are increased in man at altitude.
Clin Sci (Lond) 1997;93:181e6.
5. Houston CS, Sutton JR, Cymerman A, et al. Operation
Everest II: man at extreme altitude. J Appl Physiol 1987;63:
877e82.
6. Mason NP, Barry PW, Despiau G, et al. Cough frequency and
cough receptor sensitivity to citric acid challenge during
a simulated ascent to extreme altitude. Eur Respir J 1999;13:
508e13.
7. Mason NP, Barry PW. Altitude-related cough. Pulm Pharmacol
Ther 2007;20:388e95.
8. Ward MP, Milledge JS, West JB. Ventilatory response to hypoxia
and carbon dioxide. In: High altitude medicine and physiology.
London: Arnold; 2000. p. 50e64.
9. Douglas NJ. Control of breathing during sleep. Clin Sci (Lond)
1984;67:465e71.
10. Gutstein H, Akil H. Opioid analgesics. In: Brunton LL, editor.
Goodman and Gilman’s the pharmacological basis of therapeutics. New York: McGraw-Hill; 2006. p. 578e9.
11. Banner AS. Relationship between cough due to hypotonic
aerosol and the ventilatory response to CO2 in normal subjects.
Am Rev Respir Dis 1988;137:647e50.
12. Nishino T, Sugimori K, Hiraga K, et al. Influence of CPAP on
reflex responses to tracheal irritation in anesthetized humans.
J Appl Physiol 1989;67:954e8.
13. Barry PW, Mason NP, Nickol A, et al. Cough receptor sensitivity
and dynamic ventilatory response to carbon dioxide in man
acclimatised to high altitude [Abstract]. J Physiol 1996;497:
29e30.
14. Pounsford JC, Saunders KB. Diurnal variation and adaptation of
the cough response to citric acid in normal subjects. Thorax
1985;40:657e61.
15. Hsu JY, Stone RA, Logan-Sinclair RB, et al. Coughing frequency
in patients with persistent cough: assessment using a 24 hour
ambulatory recorder. Eur Respir J 1994;7:1246e53.
16. Morice AH, Fontana GA, Belvisi MG, et al. ERS guidelines on the
assessment of cough. Eur Respir J 2007;29:1256e76.
17. Pedersen OF, Miller MR, Sigsgaard T, et al. Portable peak flow
meters: physical characteristics, influence of temperature,
altitude, and humidity. Eur Respir J 1994;7:991e7.
18. Read DJ. A clinical method for assessing the ventilatory response
to carbon dioxide. Australas Ann Med 1967;16:20e32.
19. Roach RC, Bärtsch P, Hackett PH, et al. The Lake Louise acute
mountain sickness scoring system. In: Sutton JR, Houston CS,
Coates G, editors. Hypoxia and mountain medicine. Burlington, VT: Queen City Printers; 1993. p. 272e4.
20. Schoene RB, Robertson HT, Pierson DJ, et al. Respiratory drives
and exercise in menstrual cycles of athletic and nonathletic
women. J Appl Physiol 1981;50:1300e5.
21. Kellogg RH. The role of CO2 in altitude acclimatization. In:
Cunningham DJC, Lloyd BB, editors. The regulation of human
respiration. Oxford: Blackwell Scientific Publications; 1963. p.
379e94.
1188
22. Bolser DC, Davenport PW. Functional organization of the
central cough generation mechanism. Pulm Pharmacol Ther
2002;15:221e5.
23. Bonham AC, Sekizawa S, Chen CY, et al. Plasticity of brainstem
mechanisms of cough. Respir Physiol Neurobiol 2006;152:312e9.
24. Bonham AC, Chen CY, Sekizawa S, et al. Plasticity in the
nucleus tractus solitarius and its influence on lung and airway
reflexes. J Appl Physiol 2006;101:322e7.
25. Smith C, Dempsey JA. Control of breathing at high altitude. In:
Hornbein TF, editor. High altitude, an exploration of human
adaptation. New York: Marcel Dekker; 2001. p. 139e73.
26. Eschenbacher WL, Boushey HA, Sheppard D. Alteration in
osmolarity of inhaled aerosols cause bronchoconstriction and
cough, but absence of a permeant anion causes cough alone.
27. Barry PW. The output of nebulisers at high altitude [abstract].
High Alt Med Biol 2000;2:114.
28. Morice AH, Higgins KS, Yeo WW. Adaptation of cough reflex
with different types of stimulation. Eur Respir J 1992;5:841e7.
29. Pounsford JC, Birch MJ, Saunders KB. Effect of bronchodilators
on the cough response to inhaled citric acid in normal and
asthmatic subjects. Thorax 1985;40:662e7.
30. Barber CM, Curran AD, Bradshaw LM, et al. Reproducibility and
validity of a Yan-style portable citric acid cough challenge.
Pulm Pharmacol Ther 2005;18:177e80.
31. Dicpinigaitis PV. Short- and long-term reproducibility of capsaicin
cough challenge testing. Pulm Pharmacol Ther 2003;16:61e5.
32. Sant’Ambrogio G, Widdicombe J. Reflexes from airway rapidly
adapting receptors. Respir Physiol 2001;125:33e45.
33. Zhang Z, Bonham AC. Lung congestion augments the responses
of cells in the rapidly adapting receptor pathway to cigarette
smoke in rabbit. J Physiol 1995;484(Pt 1):189e200.
34. Roberts AM, Kaufman MP, Baker DG, et al. Reflex tracheal
contraction induced by stimulation of bronchial C-fibers in
dogs. J Appl Physiol 1981;51:485e93.
35. Gunawardena S, Bravo E, Kappagoda CT. Rapidly adapting
receptors in a rabbit model of mitral regurgitation. J Physiol
1999;521(Pt 3):739e48.
36. Bartsch P, Mairbaurl H, Maggiorini M, et al. Physiological
aspects of high-altitude pulmonary edema. J Appl Physiol
2005;98:1101e10.
37. Maggiorini M, Melot C, Pierre S, et al. High-altitude pulmonary
edema is initially caused by an increase in capillary pressure.
Circulation 2001;103:2078e83.
38. Hargreaves M, Ravi K, Kappagoda CT. Responses of slowly and
rapidly adapting receptors in the airways of rabbits to changes
in the Starling forces. J Physiol 1991;432:81e97.
39. Reeves JT, Dempsey JA, Grover RF. Pulmonary circulation
during exercise. In: Weir EK, Reeves JT, editors. Pulmonary
vascular physiology and physiopathology. New York: Marcel
Dekker; 1989. p. 107e33.
40. Matthay MA, Wiener-Kronish JP. Intact epithelial barrier function is critical for the resolution of alveolar edema in humans.
41. Clerici C, Matthay MA. Hypoxia regulates gene expression of
alveolar epithelial transport proteins. J Appl Physiol 2000;88:
1890e6.
42. Mairbaurl H, Mayer K, Kim KJ, et al. Hypoxia decreases active
Na transport across primary rat alveolar epithelial cell
monolayers. Am J Physiol Lung Cell Mol Physiol 2002;282:
L659e65.
43. Mason NP, Petersen M, Melot C, et al. Serial changes in nasal
potential difference and lung electrical impedance tomography at high altitude. J Appl Physiol 2003;94:2043e50.
ACKNOWLEDGMENTS
I would like to thank the following people without whom this thesis would not have been
possible:
Professor Robert Naeije for the generous welcome into his department; for sharing his
knowledge, encouraging me to further my understanding of respiratory physiology and for the
opportunity to pursue my research in my own way.
Marie-Therese Gautier for her kindness and hospitality throughout my stay in Brussels.
Professor Christian Melot for his support, wisdom and invaluable statistical advice.
Dr. Peter Barry of Leicester Royal Infirmary for his friendship over many years; for
encouraging my interest in altitude-related cough and for sharing many of the adventures
through which the results presented in this thesis were obtained.
The late Prof. John Widdicombe of St George’s Hospital Medical School, London for his
kindness and encouragement to investigate the mechanism of cough at altitude.
Prof. Brian Brown of the University of Sheffield for patiently helping me to understand
electrical impedance tomography.
Finally I thank my wife Emma for enriching my life by her constant love, patience, support
and encouragement. Without her life would be less rich and this thesis would have remained
unfinished.

Mechanisms of Altitude-Related Cough

Transcription

Similar documents

The Shreveport Times

GW-9400 Quick Operation Guide - G

ESI-500 GENESIS Brochure / Manual

From The Headache Mountains To The Pyramid Lab: Research At

Introduction Mount Elbrus, the highest mountain in Europe, is

Climatotherapy and medical effects at high altitude Sulden am Ortler

Helping a Choking Adult

RespiCare

Pleural fluid