cardiovascular control in space and on earth: the

ACTA BIOMEDICA LOVANIENSIA 397
Katholieke Universiteit te Leuven
Faculteit Geneeskunde
Departement Hart- en Vaatziekten
Afdeling Experimentele Cardiologie
Bart VERHEYDEN
CARDIOVASCULAR CONTROL IN SPACE
AND ON EARTH:
THE CHALLENGE OF GRAVITY
LEUVEN UNIVERSITY PRESS
Thesis submitted in partial fulfillment of the requirements for the degree
of *Doctor in de Medische Wetenschappen+
8 2007 by Leuven University Press / Presses Universitaires de Louvain / Universitaire Pers
Leuven.
Minderbroedersstraat 4 - bus 5602, B-3000 Leuven (Belgium)
All rights reserved. Except in those cases expressly determined by law, no part of this
publication may be multiplied, saved in an automated data file or made public in any way
whatsoever without the express prior written consent of the publishers.
ISBN 978 90 5867 618 4
D/2007/1869/38
NUR: 876
« Pour penser il faut manger »
Teilhard de Chardin (French philosopher 1881 – 1955)
…
« Pour BIEN penser il faut BIEN manger »
André E. Aubert (Belgian scientist 1943 – …)
ACKNOWLEDGEMENTS
This work would not have been possible without the support of many people, who
contributed to the conception, execution and completion of the underlying manuscript.
I respectfully present this thesis to Prof. Marc Vervenne, Rector of the Katholieke
Universiteit Leuven, to Prof. Bernard Himpens, Dean of the Faculty of Medicine, and
to all the honorable members of the Jury.
My deepest gratitude goes to Prof. André Aubert for his courtesy to present as
promoter of this work. André, you gave me the opportunity to discover the intriguing
field of biomedical engineering. Your enthusiasm and confidence have always been a
great support to me.
I have also appreciated very much the invaluable support of Dr. Frank Beckers who
taught me how to deal with the many pitfalls of cardiovascular signal processing.
With Frank on my side I never had to worry about computer problems. He was always
helping me to improve myself as well as the quality of this work.
I would like to thank Prof. Frans Van de Werf, Chairman of the Department of
Cardiology; and Prof. Karin Sipido, Head of the Division of Experimental
Cardiology, for giving me the freedom to work under their wings. Further, I wish to
thank Prof. Hugo Ector, Prof. Tony Reybrouck and Dr. Wouter Wieling, not only for
their general supportiveness, but also for the opportunity they gave me to perform
clinical research in their syncope unit.
I am very grateful to the astronauts and cosmonauts of the ESA-Souyz missions for
their time and efforts to participate in our experiments: Frank De Winne, Yuri
Lonchakov, Sergey Zalyotine, Pedro Duque and André Kuipers. This work was
funded by PRODEX and managed by ESA in collaboration with the Belgian Federal
Science Policy Office.
I thank all my personal colleagues, as well as the people of our Lab, for the pleasant
cooperation the last years.
Last but certainly not least I want to thank my wife Eva who always kept encouraging
me although my lack of time for her. Also to my dearest friends and family I promise
to catch up the time that we have lost these past years.
Leuven, June 2007
i
TABLE OF CONTENTS
List of abbreviations……………………………………………………….…............v
Preface………………………………………………………………………............vii
Chapter 1: General introduction................................................. 1
1.1
Neurocardiovascular regulation .......................................................................... 1
1.1.1
1.1.2
1.1.3
1.1.4
1.1.5
1.1.6
1.2
Regulation of heart rate............................................................................... 1
Regulation of vascular tone ........................................................................ 3
Hormonal control of the circulation............................................................ 4
Special features of venous control .............................................................. 6
Cardiovascular baroreflex control............................................................... 6
Central nervous system substrate................................................................ 9
Cardiovascular dynamics in orthostasis ........................................................... 11
1.2.1
1.2.2
Hemodynamic effect of orthostasis .......................................................... 11
Cardiovascular response to orthostasis ..................................................... 13
1.3
Gravity and cerebral perfusion: the siphon controversy ................................ 17
1.4
Cardiovascular variability ................................................................................. 20
1.4.1
1.4.2
1.4.3
1.4.4
1.5
Respiratory frequency oscillations............................................................ 20
Mayer wave frequency oscillations .......................................................... 21
Mathematical modeling of cardiovascular control ................................... 23
Conclusion: mathematics linked to physiology ........................................ 28
Mathematical modeling of variations in aortic flow........................................ 31
Chapter 2: Influence of microgravity exposure on
cardiovascular control mechanisms .......................................... 35
2.1
Introduction......................................................................................................... 35
2.1.1
2.1.2
2.1.3
2.1.4
Cardiovascular response to initial microgravity ....................................... 36
Cardiovascular adaptation to sustained microgravity ............................... 37
Return to Gravity: orthostatic intolerance................................................. 38
Study objectives ........................................................................................ 39
ii
2.2
-Table of contents-
Spectral characteristics of heart rate fluctuations during parabolic flight ... 41
2.2.1
2.2.2
2.2.3
2.2.4
2.2.5
Introduction............................................................................................... 41
Methods..................................................................................................... 42
Results....................................................................................................... 47
Discussion ................................................................................................. 54
Conclusion ................................................................................................ 57
2.3
Dynamic cardiovascular control during 60 min of thermoneutral head-out
of water immersion ......................................................................................................... 58
2.3.1
2.3.2
2.3.3
2.3.4
2.3.5
Introduction............................................................................................... 58
Methods..................................................................................................... 58
Results....................................................................................................... 61
Discussion ................................................................................................. 65
Conclusion ................................................................................................ 69
2.4
Respiratory modulation of cardiovascular rhythms before and after
short-duration human spaceflight ................................................................................. 70
2.4.1
2.4.2
2.4.3
2.4.4
2.4.5
Introduction............................................................................................... 70
Methods..................................................................................................... 71
Results....................................................................................................... 74
Discussion ................................................................................................. 84
Conclusion ................................................................................................ 87
Chapter 3: Tilt table testing in patients with neurally
mediated syncope ........................................................................ 89
3.1
Introduction......................................................................................................... 89
3.1.1
3.1.2
3.1.3
Tilt table test in the evaluation of vasovagal syncope .............................. 90
Repeated tilt table testing.......................................................................... 90
Study objectives ........................................................................................ 91
3.2
Impact of age on the vasovagal response provoked by sublingual
nitroglycerine in routine tilt testing............................................................................... 92
3.2.1
3.2.2
3.2.3
3.2.4
3.2.5
Introduction............................................................................................... 92
Methods..................................................................................................... 92
Results....................................................................................................... 95
Discussion ............................................................................................... 101
Conclusion .............................................................................................. 104
-Table of contents-
iii
3.3
Tilt training therapy increases vasoconstrictor reserve in patients with
neurally mediated syncope and a positive head-up tilt test....................................... 105
3.3.1
3.3.2
3.3.3
3.3.4
3.3.5
Introduction............................................................................................. 105
Methods................................................................................................... 106
Results..................................................................................................... 108
Discussion ............................................................................................... 113
Conclusion .............................................................................................. 116
Chapter 4: Concluding remarks and perspectives ................ 119
4.1
Cardiovascular variability and circulatory control mechanisms................. 119
4.2
Orthostatic blood pressure control: the challenge of gravity ....................... 120
4.3
Baroreflex function and orthostatic intolerance ............................................ 121
Summary……………………………………………………………………………123
Samenvatting……………………………………………………………………......129
References…………………………………………………………………………..133
Curriculum vitae and publications
v
ABBREVIATIONS
ACE:
ANS:
bpm:
BPV:
BRS:
CO:
CVLM:
CVP:
Cw:
DAP:
DC:
ECG:
FFT:
Gz:
HF:
HR:
HRV:
HUT:
IBI:
ISS:
LF:
MAP:
MSNA:
NPB:
NTG:
NTS:
nu:
PP:
PSD:
Rp:
RRI:
RSA:
RVLM:
SAP:
SPB:
SV:
SVR:
TP:
VLF:
VVS:
WI:
Z0:
angiotensin converting enzyme
autonomic nervous system
beats per minute
blood pressure variability
baroreflex sensitivity
cardiac output
caudal ventrolateral medulla
central venous pressure
arterial compliance
diastolic arterial pressure
direct current
electrocardiogram
fast Fourier transform
gravity
high frequency
heart rate
heart rate variability
head-up tilt
interbeat interval
international space station
low frequency
mean arterial pressure
muscle sympathetic nerve activity
normal paced breathing
nitroglycerine
nucleus tractus solitarius
normalized unit
pulse pressure
power spectral density
total peripheral resistance
R-R interval
respiratory sinus arrhythmia
rostral ventrolateral medulla
systolic arterial pressure
slow paced breathing
stroke volume
systemic vascular resistance
total power
very low frequency
vasovagal syncope
water immersion
aortic characteristic impedance
vii
PREFACE
Although the adoption of the upright posture represents one of the most defining
moments in human development, it nonetheless provides a unique challenge to the
cardiovascular system. In humans on Earth, circulating blood is subjected to gravity.
On standing up, blood shifts toward regions below the heart, resulting in a reduced
venous return to the heart. Appropriate autonomic reflexes are aimed to prevent
systemic blood pressure from falling; unchecked this can lead to inadequate brain
perfusion and loss of consciousness.
It is due to gravity that postural changes result in fluid shifts: in microgravity blood
volume is chronically redistributed toward the thorax and head, giving appearance of
‘puffy faces’ and ‘chicken legs’. Various microgravity-induced cardiovascular
adaptations may contribute to the genesis of post-spaceflight orthostatic intolerance.
This implies a diminished ability to assume the upright position without experiencing
problems of lightheadness, dizziness, and/or total loss of consciousness. The
symptoms can be triggered most rapidly by means of a head-up tilt test in those most
prone to it. Clinical use of a head-up tilt test is to confirm the diagnosis in patients
with a typical history of vasovagal syncope. Repeated tilt testing (tilt training)
prevents syncope recurrence. Sublingual nitroglycerine can be administered to shorten
the tilt duration, because it enhances venous pooling of blood in the upright posture.
This work deals with the impact of gravity on multiple aspects of circulatory control
in humans. The first chapter provides an introduction on baroreflex control of blood
pressure upon standing. It further gives an overview of methods that can be used to
infer measures of beat-to-beat circulatory control from non-invasive blood pressure
recordings at the finger. Chapter 2 describes the cardiovascular response to
(simulated) microgravity in healthy subjects, as well as the circulatory response to
standing before and after 10 days spaceflight in 5 cosmonauts. Chapter 3 describes
orthostatic blood pressure control in patients who are prone to head-up tilt induced
syncope. A first study examines the effect of sublingual nitroglycerine on baroreflex
control of blood pressure in patients with distinct age-related collapse patterns during
syncope. A second study discusses possible underlying mechanisms by which tilt
training therapy improves symptoms associated with syncope. Finally, in a last
viii
-Preface-
chapter, concluding remarks are described about the influence of gravity on
circulatory control.
-CHAPTER I-
1
1 General introduction
1.1 Neurocardiovascular regulation
To survive in the world, all species must possess the ability to make moment by
moment alterations in physiological parameters allowing their internal environment to
remain stable despite dramatic changes in their external environment. This
homeostasis is achieved by nervous and endocrine reflexes, which are coordinated by
the brain.
1.1.1
Regulation of heart rate
Heart rate is controlled by sympathetic and parasympathetic autonomic nerves, which
innervate the sinoatrial (SA) node (Figure 1.1).
Parasympathetic
efferent pathway
Sympathetic
efferent pathway
Figure 1.1: Innervation of the cardiovascular system by sympathetic fibres (right)
and parasympathetic fibres (left).
Sympathetic fibres richly innervate the ventricular muscle as well as the atria and
pacemaker-conduction system. Parasympathetic fibres primarily supply the SA node
and the atrioventricular (AV) node. The atria are also innervated by parasympathetic
efferents, whereas the ventricular myocardium is only sparsely innervated by
parasympathetic fibers. Unlike the sympathetic system, long preganglionic fibres
2
-CHAPTER I-
synapse with the postganglionic parasympathetic neurons within the plexi of the
epicardium. Short postganglionic fibres innervate the SA and AV nodes. Both sets of
autonomic nerves are tonically active but parasympathetic activity predominates at
rest.
1.1.1.1 Effects of sympathetic stimulation
The sympathetic fibres release the neurotransmitter noradrenaline. Noradrenaline
binds to β1-adrenoceptors on the cardiac cell membrane. The heart also possesses αand β2-adrenoceptors, but the β1-receptors predominate. Activation of the β1-receptors
is followed over several beats by an increase in heart rate (chronotropic effect). This
is achieved by increasing the decay rate of the pacemaker potential, so the threshold is
reached sooner (Figure 1.2).
Figure 1.2: Effects of
sympathetic and parasympathetic stimulation
on the slope of the
pacemaker potential of
a sinoatrial nodal cell.
Stimulation of β1-receptors further induces an increased AV node conduction velocity
(dromotropic effect), shortening of the myocyte action potential duration, increased
contractile force (inotropic effect), and increased rate of relaxation (lusitropic effect).
The hormone adrenaline acts similarly. Adrenaline is secreted into the blood stream
by the medulla of the adrenal gland in response to preganglionic sympathetic fibre
activity. Adrenaline and noradrenaline are known as the catecholamines. The action
of noradrenaline is terminated partly by diffusion into the bloodstream, which washes
it away; and partly by reuptake into the sympathetic nerve. Since these processes are
relatively slow, tachycardia elicited by sympathetic stimulation is relatively sluggish
in onset and decays relatively slowly when stimulation ceases.
-CHAPTER I-
3
1.1.1.2 Effects of parasympathetic stimulation
Firing of parasympathetic postganglionic fibres releases the neurotransmitter
acetylcholine from the nerve terminal. Acetylcholine acts by binding to muscarinic
M2-recepters on the myocyte membrane. Parasympathetic stimulation produces
bradycardia through two electrophysiological actions. The main effect is to slow
down the increased rate of diastolic depolarization (Figure 1.2). In addition, the
membrane is quickly hyperpolarized by activating acetylcholine-sensitive K+channels. As a result of these two effects the potential takes longer to reach the
threshold and heart rate decreases. The bradycardia elicited by parasympathetic
stimulation has a very short latency and resolves quickly when fibre activity ceases.
Thus, changes in parasympathetic activity account for rapid changes in heart rate,
such as the slowing of heart rate with expiration (respiratory sinus arrhythmia). The
fast ‘on’ effect is due to the hyper-polarization pathway activated by acetylcholine.
The fast ‘off’ effect is due to the rapid removal of acetylcholine by an enzyme,
cholinesterase.
1.1.2
Regulation of vascular tone
The active tension of smooth muscle in the tunica media, i.e. the vascular tone,
controls the diameter of a blood vessel. An increase in tone causes vasoconstriction.
Conversely, a reduction in tone causes vasodilatation, because the wall is distended by
the internal blood pressure as the media relaxes. The regulation of vascular tone can
be viewed as a three tier hierarchy. First, the most basic form of regulation is
autoregulation. Resistance vessels react to changes in blood pressure so that blood
flow varies little with pressure. Autoregulation is mainly due to the Bayliss myogenic
response (1). Second, auto-regulated flow is increased or decreased by intrinsic
regulatory chemicals produced within the tissue. The chief ones are the vasodilators
of functional hyperaemia (CO2, lactate, adenosine, K+, phosphate), endothelial
secretions (nitric oxide, endothelin, prostacycline) and autocoids (histamine,
bradykinin, serotonin, thromboxane). Finally, the highest level of control is extrinsic
regulation from outside the tissue by vasomotor nerves and circulating hormones.
This brings vascular regulation under the control of the brain.
1.1.2.1 Sympathetic vasoconstrictor nerves
The pathway controlling the sympathetic vasoconstrictor fibres begins in the medulla
of the brainstem. From here, descending excitatory and inhibitory bulbospinal fibres
-CHAPTER I-
4
pass down the spinal cord. These fibres synapse with sympathetic preganglionic
neurons in the intermediolateral columns of the gray matter, in the thoracicolumbar
segments T1 to L3. The interplay of excitatory and inhibitory inputs from bulbospinal
fibres determines the activity of sympathetic preganglionic neurons. The
postganglionic fibre activity is controlled by thoracic spinal preganglionic neurons.
Preganglionic fibres are mostly cholinergic and postganglionic receptors are nicotinic.
About 80% of the noradrenaline that is released by sympathetic fibres is transported
back into the axon, terminating its action and restocking the axon. Much of the
circulating plasma noradrenaline originates from the ‘spillover’ from sympathetic
terminals.
Sympathetic vasoconstrictor noradrenergic fibres are tonically active and raise
vascular tone through α1- and α2-adrenoreceptor activation. Sympathetic regulation of
resistance vessels stabilizes blood pressure, regulates local blood flow, and influences
fluid partitioning between plasma and interstitium. Sympathetic fibres also regulate
splanchnic and cutaneous venous tone, and thereby influence central blood volume
and cardiac filling pressure. Some preganglionic neurons do not synapse until they
reach the adrenal medulla or a distant ganglion (Figure 1.1).
1.1.2.2 Sympathetic and parasympathetic vasodilator nerves
In certain species and tissues there is a limited distribution of sympathetic cholinergic
fibres that cause vasodilatation. This sympathetic vasodilatation is transient and the
system plays no part in the reflex control of blood pressure. Sympathetic cholinergic
innervation of resistance vessels is unlikely in humans. Parasympathetic dilator nerves
occur as cranial and sacral outflow. The cranial outflow innervates the salivary
glands, pancreas and gut. The sacral outflow innervates the genitalia (erectile tissue),
bladder and colon.
1.1.3
Hormonal control of the circulation
Several hormones influence the heart and circulation acutely, although they are
generally less important than direct neural control mechanisms under physiological
conditions.
1.1.3.1 Circulating catecholamines
Adrenaline and noradrenaline are secreted by the adrenal medulla in response to a rise
in preganglionic sympathetic activity via the splanchnic nerve (Figure 1.1). Although
-CHAPTER I-
5
the gland secretes both adrenaline and noradrenaline, adrenaline accounts for ±90% of
the secreted catecholamine. The metabolic effects of adrenaline are at least as
important as its cardiovascular effects. Adrenaline stimulates glycogenolysis in the
liver and lipolysis in adipose tissue, which releases glucose in the blood stream. The
cardiovascular effects of circulating catecholamines include tachycardia and increased
contractility due to stimulation of β1-adrenoreceptors, as well as arterial and venous
vasoconstriction. As an exception to the rule that catecholamines cause
vasoconstriction, adrenaline causes vasodilatation in the skeletal muscle, myocardium
and liver. This is due to an abundance of β2-adrenoreceptors in these tissues.
Noradrenaline causes solely vasoconstriction, because it has a higher affinity for αreceptors than β-receptors. Since skeletal muscle is the single most abundant tissue in
the body, the overall effects of the two catecholamines on the systemic circulation
differ considerably.
1.1.3.2 Vasopressin
Vasopressin is produced in the hypothalamus and released from the posterior pituitary
in response to hypertonicity and hypotension. The main role of vasopressin at normal
plasma concentration is the regulation of water excretion by the kidneys, as indicated
by its common name ‘anti-diuretic hormone’ (ADH). High concentrations of
vasopressin elicit a pronounced vasoconstriction in most tissues. Cerebral and
coronary vessels, however, respond to vasopressin with a nitric oxide-mediated
dilatation. Vasopressin thus causes a redistribution of circulating blood in favor of the
brain and heart.
1.1.3.3 The renin-angiotensin-aldosterone system
The production of angiotensin begins with the secretion of renin into the bloodstream
of the kidney in response to reduced renal arterial pressure, a low NaCl load and renal
sympathetic activity (Figure 1.3). Renin acts on angiotensinogen to cleave of
angiotensin I. Angiotensin I is then cleaved further in the lungs by an enzyme
situated in the surface of endothelial cells (angiotensin converting enzyme, ACE) to
form angiotensin II. Angiotensin II stimulates the adrenal cortex to secrete
aldosterone, a steroidal hormone. Aldosterone promotes renal salt and water retention
to maintain plasma volume, and indirectly blood pressure. A second role of
angiotensin II is vasoconstriction by peripheral (vascular smooth muscle) and central
sympathetic (brainstem) actions.
-CHAPTER I-
6
1.1.3.4 Atrial natriuretic peptide
Atrial natriuretic peptide is secreted by the atria in response to distention. It reduces
plasma volume by a mild diuretic action and by enhancing microvascular filtration
and increased permeability.
Figure
1.3:
Renin-angiotensinaldosterone system. Renal artery (RA)
and renal vein (RV). Central effects
are stimulation of sympathetic
outflow, reduction in sensitivity of
baroreceptor reflex and stimulation
of thirst. Adapted from ref (2)
1.1.4
Special features of venous control
The control of peripheral veins (capacitance vessels) is important because venous tone
governs the distribution of blood between the periphery and thorax, and thereby
regulates cardiac filling pressure and, indirectly, stroke volume. Splanchnic veins
contain 20% of total blood volume at rest. They are well innervated by sympathetic
constrictor nerves and possess α-adrenoreceptors. Skeletal veins are poorly
innervated. Their volume is nevertheless indirectly affected by sympathetic vasomotor
activity because arteriolar constriction reduces downstream pressure. The veins of the
skin are richly innervated by sympathetic noradrenergic fibres.
1.1.5
Cardiovascular baroreflex control
The heart and blood vessels are controlled by sympathetic and parasympathetic
nerves. The nerve activity is regulated by the brain, which is guided by sensory
information from peripheral receptors. In terms of blood pressure homeostasis, the
most important groups of sensors are the high-pressure receptors in the walls of
-CHAPTER I-
7
systemic arteries (arterial baroreceptors) and the low-pressure receptors in the heart
(cardiopulmonary receptors) (3). Their afferent fibers transmit information about
arterial pressure and cardiac filling to the brainstem, where it is integrated with
information from muscle receptors, arterial chemoreceptors and other sensors. The
integration of information and computation of an appropriate cardiovascular response
involve up-and-down traffic between the brainstem, hypothalamus, cerebellum and
cortex. Pre-sympathetic and parasympathetic outflows from the brainstem are then
adjusted to initiate the desired cardiovascular response (Figure 1.4).
Figure 1.4: Overview of reflex control of the circulation, excluding endocrine
aspects. Terms ‘inhibitory’ and ‘excitatory’ refer to the net effect on cardiac output
and blood pressure; inhibitory reflexes are depressor, excitatory reflexes are pressor.
Adapted from ref (4)
1.1.5.1 The arterial baroreceptor reflex
One of the most important cardiovascular reflexes is the arterial baroreceptor reflex,
which stabilizes arterial blood pressure and thereby safeguards the blood flow to the
brain. This blood pressure control mechanism involves complex afferent signal
processing by the central nervous system and subsequent efferent modulation of
cardiac and vascular targets. Normal moment-to-moment regulation of arterial
pressure is controlled predominantly by stretch-activated baroreceptors located in the
aortic arch and carotid sinus. In the case of a blood pressure increase, baroreceptors
are stimulated not only by the magnitude of the pressure (static sensitivity) but also by
its rate of rise (dynamic sensitivity). As a result of the dynamic sensitivity, the
8
-CHAPTER I-
baroreceptors characteristically fire a burst of action potentials in systole and fall
silent in diastole (Figure 1.5).
Figure 1.5: Baroreceptor responses to
alterations in mean and phasic arterial
blood pressures. The phasic discharge
rate of the baroreceptor nerve increases
as mean arterial pressure increases. At
very high pressures, this variation is no
longer evident, indicating receptor
saturation.
The afferent baroreceptor impulses are transmitted to the nucleus tractus solitarius in
the medulla via the vagus (aortic arch) and glossopharyngeal (carotid sinus) nerves.
Their inputs activate polysynaptic central pathways that enhance the vagal
parasympathetic output to the heart and inhibit the sympathetic output to the heart and
vasculature, giving reflex cardiac slowing and vasodilatation in an attempt to bring
blood pressure back to within normal levels. The reverse sequence of events occurs
when baroreceptors are deactivated with an ensuing fall in afferent baroreceptor
traffic. The buffering process against acute changes in blood pressure is very rapid.
The latency between baroreceptor stimulation and bradycardia is less than 0.5 s, and
the latency between the onset of vascular dilatation is ±1.5 s.
1.1.5.2 The cardiopulmonary baroreceptor reflex
Next to the arterial baroreceptors, the heart and pulmonary arteries are also innervated
by afferent fibers running to the cardiovascular control centers of the medulla.
Myelinated veno-atrial stretch receptors monitor atrial distention and evoke reflex
tachycardia and diuresis, which reduce cardiac distention and extracellular fluid
volume. The reflex was first demonstrated by Bainbridge who discovered that the
rapid infusion of a large volume of saline into the venous system causes a transient
tachycardia. This reflex tachycardia may serve to shift blood out of the congested
venous system into the arterial system and may be due in part to the activation of
pacemaker stretch-activated ion channels. Non-myelinated mechanoreceptors in the
-CHAPTER I-
9
left ventricle, atria and pulmonary artery elicit a depressor reflex (bradycardia and
vasodilatation) that is activated by maneuvers that affect cardiac filling
(cardiopulmonary
reflex).
Cardiac
de-afferentation
studies
show
that
the
cardiopulmonary afferents generally have a net tonic inhibitory effect on heart rate
and peripheral vascular tone, similar to that of arterial baroreceptors. Other cardiac
receptors have been described, although there is little evidence that they are directly
involved in arterial pressure regulation.
1.1.6
Central nervous system substrate
The central neural basis for the integration of cardiovascular reflexes is not as well
defined as the peripheral mechanisms. Investigation of central nervous system control
of cardiovascular function have focused primarily on brain-stem structures involved
in the baroreflex and the medullary nuclei mediating this fundamental reflex.
However, the medulla is by no means the only region involved in cardiovascular
regulation. In addition, the hypothalamus limbic system, cerebellum and cerebral
cortex all play a role. The nucleus tractus solitarius (NTS) receives and integrates
cardiovascular receptor traffic and the output is relayed to other parts of the medulla,
hypothalamus and cerebellum.
Two main routes link the baroreceptor input and the vagal motor neurons that control
heart rate (Figure 1.6). One route, of short latency, remains within the medulla and
passes from the NTS to the cardiac vagal motor nuclei of the nucleus ambiguus. The
other route, of longer latency, passes from the NTS up to the hypothalamic depressor
center and from there to the nucleus ambiguus. In addition, during inspiration,
influences of the brainstem respiratory neurons to the vagal nuclei hyperpolarizes the
cardiac motor neurons. The hyper-polarization causes a loss of vagal neuronal
responsiveness to the baroreflex during each inspiration (gating of the baroreflex).
The inhibition of vagal activity causes tachycardia in synchrony with inspiration
(sinus arrhythmia).
The inhibitory pathway from the NTS to the spinal sympathetic neurons is more
complex and less well understood than the vagal pathway (Figure 1.7). Several
intermediate pathways, some involving higher regions of the brain, mediate inhibition
of the presympathetic outflow from the rostral ventrolateral medulla (RVLM). One
pathway passes from the NTS to the caudal ventrolateral medulla (CVLM), which
then inhibits the RVLM. There is also a direct, descending inhibitory influence from
10
-CHAPTER I-
the raphe nuclei of the brainstem to the spinal sympathetic neurons. The net effect of
the NTS projection is that many RVLM neurons fall silent during the pulse. The
baroreflex thus inhibits the descending excitatory drive from the RVLM to the spinal
preganglionic sympathetic neurons.
Figure 1.6: Central pathways by which the baroreflex regulates vagal
parasympathetic drive to the heart. Dashed lines denote inhibitory pathways. Adapted
from ref (4)
Figure 1.7: Simplified central pathways governing sympathetic drive (red neurons) to
heart and vessels. Dashed lines denote inhibitory pathways. Adapted from ref (4)
-CHAPTER I-
11
1.2 Cardiovascular dynamics in orthostasis
Although the adoption of the upright posture represents one of the defining moments
in human development, it nonetheless provides a unique challenge to the system of
blood pressure control. When we talk about the pressure within blood vessels, venous
or arterial, we are dealing with three different concepts (and the interaction of these
concepts must be taken into account): the mean systemic filling pressure, which is
related to the volume in the vessel and the wall properties; the dynamic pressure,
related to the velocity of the blood flow and the resistance; and the hydrostatic
pressure, which is related to gravity (5). The importance of the latter was remarked by
Hill and Barnard as early as 1897 when they stated: “The expression ‘mean pressure’
cannot be justly used in any discussions on hemodynamics, for a uniform hydrostatic
mean pressure in the vascular system cannot be obtained” (6).
1.2.1
Hemodynamic effect of orthostasis
Gravity affects the fluid distribution in the human body (Figure 1.8). In the supine
position, approximately 25% to 30% of the blood volume is located in the thorax. On
standing up, there is a downward displacement of roughly 300 to 800 ml of blood to
the splanchnic, pelvic and leg vasculature (7). Orthostatic pooling of venous blood
begins immediately, and the total transfer is almost complete within 3-5 min
depending on the investigated body region. This represents a volume drop of between
25% and 30%.
Standing also produces a substantial increase in the transmural capillary pressure
present in the lower extremities, which causes a rise in fluid filtration into tissue
spaces. Continued filtration into the tissue further reduces the circulating volume,
although fluid is gained from the tissue above the venous hydrostatic indifference
point (8), which is defined as the axial reference within the column of venous blood
where pressure is not altered by postural reorientation (8). This process reaches a
steady state after ±30 minutes of upright posture and can produce a decline in plasma
volume of up to 10%.
As a consequence of blood pooling and the superimposed decline in plasma volume,
the return of venous blood to the heart is reduced and central venous pressure (CVP)
falls. There is a decrease in end-diastolic filling of the right ventricle and, because the
12
-CHAPTER I-
heart can only pump the blood that it receives, this leads to a reduction in stroke
volume and a fall in cardiac output (9).
Figure 1.8: Distribution of pressures in upright humans. Pressure scales on the right
are arterial pressures. Heart level is zero pressure reference level. Water-filled tubes
at the left show measured venous pressure. Veins collapse at the zero pressure
reference level. The hydrostatic indifference point (HIP) is at diaphragmatic level. ρ
= blood viscosity; g = gravity; h = vertical distance from the heart. ρgh =
gravitational potential energy Adapted from ref (10)
Maintaining arterial pressure is of vital importance for the perfusion of the brain: the
brain needs a considerable part of the total cardiac output (±750 ml/min out of a total
cardiac output of ±5 l/min, with a wide range depending on body size and
constitution). Standing up places the brain in a most disadvantageous position: in the
upright position, the cerebral arteries are positioned some 30 cm above the heart,
-CHAPTER I-
13
whereas ±70% of the total blood volume shifts below that level (11). Considering the
circulatory demands of the brain, fast and efficient response to gravity-induced fluid
shifts is crucial.
1.2.2
Cardiovascular response to orthostasis
The human body has a remarkable ability to maintain a stable blood pressure in the
presence of ever-changing forces that constantly shift and redistribute the circulating
blood volume. The mechanism of this successful response involves 1) the rapid
adaptation of smooth muscles of the arterial and venous vessels to changing vascular
diameters; 2) autonomic reflex mechanisms to adjust cardiac output and vascular
tone; 3) the skeletal muscle pump of the lower body; 4) neuro-hormones.
The cardiovascular response to orthostatic stress may be divided arbitrarily into
immediate (0-30 s), stabilized (30 s to 20 min), and prolonged (20 min tot several
hours) reflexes. Rapid short-term adjustments to orthostatic stress are mediated
exclusively by neural pathways of the ANS. During prolonged orthostasis, additional
adjustments are mediated by the humoral limb of the neuroendocrine system (i.e.
renin-angiotensin-aldosterone system). The exact response to postural change differs
with standing (active process) compared with responses seen during head-up tilt
(passive process).
1.2.2.1 Immediate cardiovascular response to orthostasis (Figure 1.9)
As soon as the upper part of the body is raised, the transmural pressure in the carotid
arteries becomes more negative, lowering the effective pressure at the baroreceptor
areas because of gravity. The fast-conducting carotid vagal fibers with a very short
latent period produce cardiac acceleration within 1 to 1.5 seconds. Although this
initial cardiac acceleration is vagal in origin, sympathetic effects may not be totally
excluded (12). In addition, because of a rapid drop in cardiac filling pressure, stroke
volume falls markedly and causes arterial pressure to decrease after 3 to 5 s. This
rapid decrease in blood pressure further activates arterial baroreceptors, prompting a
further reflex rise in heart rate (13). The more slowly responding sympathetic
vasoconstrictors then cause the restoration and overshoot of arterial pressure, with the
resulting reflex decrease in heart rate. Interestingly, the rises in heart rate and vascular
resistance upon standing are not always preceded by a reduction in arterial pressure
(14). Possible explanations for this are, firstly, that arterial baroreceptors respond to
mechanical deformation and not pressure, and small reductions in effective blood
-CHAPTER I-
14
volume are known to trigger baroreflex adjustments of arterial pressure (15); and
secondly, another pathway leading to an increase in the vascular resistance is via the
cardiopulmonary reflex (16), which is sensitive to changes in venous pressure.
Figure 1.9: Schematic presentation of the immediate cardiovascular response to
standing
1.2.2.2 Stabilized cardiovascular response to orthostasis (Figure 1.10)
The total amount of dislocated thoracic blood is very dependent on the net lower body
vascular compliance, which in turn depends on 1) vascular wall compliance, 2) the
transmural pressure applied to vascular walls by surrounding tissues, and 3)
antigravity muscles and the functional state of venous valves. The most important
defense against a critical reduction in central blood volume after continued orthostasis
is that of muscle activity; in everyday life extensive pooling is limited by activation of
the skeletal muscle pump (17). In addition, progressive reduction in venous return,
and
thus
cardiac
filling,
reduces
the
stretch
on
cardiac
low-pressure
-CHAPTER I-
15
mechanoreceptors. As their firing rates decrease, there is a change in medullary input,
which triggers a general increase in sympathetic outflow. Clinically, this response
manifests as a further increase in heart rate and a gradual diastolic pressure increase,
which is mediated by constriction of the systemic resistance vessels. In addition, there
is a focal axon reflex (the venoarteriolar axon reflex) that can constrict flow to the
skin, muscle, and adipose tissue. This may contribute up to 50% of the increase in
limb vascular resistance (18).
Figure 1.10: Schematic presentation of the cardio-vascular response to continued
standing
1.2.2.3 Response to prolonged orthostasis
Continued head-up tilt further activates a series of neurohumoral changes, the exact
extent of which depend on the patient’s volume status; the greater the volume
depletion, the greater the degree of activation of the renin-angiotensin-aldosterone
system (and vasopressin).
1.2.2.4 Active stand versus head-up tilt (Figure 1.11)
The circulatory changes seen during active standing are somewhat different from
those seen during passive tilt. Standing is much more an active process that is
16
-CHAPTER I-
accompanied by contractions of muscles of both the leg and abdomen, which
produces a compression of both capacitance and resistance vessels and results in an
elevation in peripheral vascular resistance. This increase is sufficient to cause a
transient increase in both right atrial pressure and cardiac output, which in turn causes
an activation of the low-pressure receptors of the heart. This provokes an increase in
neural traffic to the brain, with a subsequent decrease in peripheral vascular
resistance, which can fall as much as 40% (7). This can allow a fall in mean arterial
pressure of up to 20 mmHg that can last for up to 6 to 8 seconds. The decline in
pressure is then compensated for by the same mechanisms as during passive head-up
tilt. The transient fall in arterial pressure is likely to explain the feelings of
lightheadness that even healthy humans sometimes experience shortly after standing
up.
Figure 1.11: Hemodynamic responses to passive tilting and active standing in 8
subjects (19). Systolic, mean and diastolic arterial pressures (SBP, MBP, and DBP,
respectively) and heart rate (HR) are expressed as absolute changes from control.
Stoke volume (SV), cardiac output (CO), and total peripheral resistance (TPR) are
expressed as % changes from control.
-CHAPTER I-
17
1.3 Gravity and cerebral perfusion: the siphon controversy
On standing up gravity tends to pull blood away from the head towards the lower
parts of the body and superimposes a hydrostatic pressure gradient on the circulatory
system. Stability of cephalic blood flow therefore depends on the vertical blood
column above the heart and blood pooling below.
Whether gravity challenges blood supply to the brain in the upright position is a
much-disputed topic in physiology. The circulatory system is best described as a
closed system; for the brain, blood is driven from the thoracic aorta via the filled
cerebral veins where they leave the skull to the right atrium. The mechanical
advantage of a closed system is similar to the operation of a siphon loop. A siphon in
the circulation suggests that blood flow to the head is independent of the height of the
vascular loop (20). This implies that no work is done on the blood to increase its
gravitational potential energy because pressure gradients are equal and opposite in
direction in the ascending and descending limbs of the loop (21). The only
requirement for the siphon principle is a continuous fluid circuit. Nevertheless, the
application of siphon principles to the cerebral circulation has engendered a surprising
amount of controversy (22-27).
Both empirical and theoretical hydraulic models have been used to argue against the
operation of a siphon principle in the brain circulation (25). Hydraulic models,
however, are often based on an open system, which is not a simple analog of the
closed circulation. Within open systems, fluids accelerate down the descending limb,
resulting in a vascular waterfall. In contrast, blood flow from the brain does not ‘fall’
to the right atrium (24). Even more troubling is the failure of hydraulic models to
reconcile with the well-established observation that the upright human brain contains
negative pressure in its veins and sinuses (27). The principle of a siphon stands alone
in accounting for this negative pressure.
Gisolf et al. (26) calculated that the total resistance of the brain is more than 85-fold
the resistance of the venous outflow pathway in standing men. Therefore, the brain
itself could be suggested to prevent a siphon in the cerebral blood flow. The brain
may be better regarded as a throttle (or baffle), disrupting the continuity that is
required for a siphon (Figure 1.12) (26).
18
-CHAPTER I-
Figure 1.12: Illustration of a siphon to and from the brain (left) and the throttle
concept (right). In both diagrams the left, ascending limb represents the internal
carotid arteries; the right, descending limbs represent the internal jugular veins and
the vertebral venous plexus (the two interwoven lines). For the siphon, brain
perfusion pressure is determined by the central arterial and venous pressure
difference, regardless of the hydrostatic pressure gradient between heart and brain
level in standing men. In the throttle model, brain perfusion pressure is determined by
arterial pressure at brain level only, not by a height-corrected negative venous
pressure at brain level. Redrawn from (5).
The principle of a siphon should apply to any closed circulatory system, and thus
comparative physiology provides an alternative approach to investigate this
fundamental hemodynamic concept. The impact of gravity on the cerebral perfusion is
particularly striking in long neck animals, such as the giraffe. Simply by virtue of its
stature, giraffes operate a cardiovascular system under gravitational pressures that are
higher than any living vertebrate. The collapsed jugular veins of quietly standing
giraffes render a siphon mechanism unlikely (28). In addition, the high aortic pressure
in giraffes (mean 210 mmHg) is often provided as evidence that the heart must
overcome gravitational pressures in the neck arteries (25). However, with the head
raised 1.5 m above heart level, the aortic blood pressure in the giraffe should have
increased up to 280 mmHg as predicated by gravity alone (24). This suggests that the
giraffe heart does not defeat the total gravitational pressure related to the weight of the
blood above the heart, and thus a siphon might be operative (24).
Comparative analysis with a single group of animals can be a useful approach to
inferring physiological or morphological adaptations. Snakes are particularly
instructive because they have evolved in aquatic, terrestrial and arboreal habitats in
-CHAPTER I-
19
which gravity has vastly different influences on the cardiovascular system. It has been
demonstrated that the snake heart moves relatively closer to the head from aquatic
through terrestrial and arboreal species, minimizing the work for pumping blood
against gravity if applicable (29). The fact that natural selection would favor a heart
position that reduces cardiac work supports the notion that the siphon principle does
not apply in these long-bodied reptiles (30;31). Furthermore, snake resting blood
pressure also appears to be related to its behavior and habitat: aquatic species have a
much lower pressure compared with non-climbing terrestrial species; arboreal species
have the highest blood pressure. It should be noted, however, that direct correlation
analysis between species using simple statistical tests can be misleading because one
must take into account the genetic relatedness between species (32). Such analysis of
the original snake data set still needs to be conducted.
In conclusion, biologists debate about the role of a siphon in the brain circulation.
Whereas some have concluded that a siphon might be operative in giraffes, many
others dispute this. Although the giraffe is a unique model for studying adaptations to
cope with gravity, at the same time, it is a confusing model for understanding human
physiology. Aside from the obvious difference in heart-to-head distance, humans lack
the multitude of jugular valves making conclusions about giraffe physiology difficult
to apply directly to understand human physiology.
-CHAPTER I-
20
1.4 Cardiovascular variability
Heart rate and blood pressure continuously fluctuate over time. Rather than being
“undesirable noise”, heart rate variability (HRV) and blood pressure variability (BPV)
reflect the activity of cardiovascular control mechanisms, representing a rich source of
information on their performance in health and disease. A variety of animal and
human research has established two clear frequency bands in heart rate and blood
pressure with autonomic involvement (33). The first, occurring over several cardiac
cycles, is respiration related. Both heart rate and the entire arterial pressure waveform
rise and fall at the frequency of respiration. In humans, these oscillations represent
both autonomic neural fluctuations and mechanically induced central blood volume
changes in synchrony with respiration (34). The contributions of these two
mechanisms to both heart rate and arterial pressure fluctuations remain equivocal. The
second rhythm occurs over an approximate 10 s cycle, and in arterial pressure, has
been termed Mayer waves. This rhythm has been attributed to various sources, but
most recently has been presumed to reflect reflex mediated fluctuations in
sympathetic outflow to the vasculature and in parasympathetic and sympathetic
outflows to the heart (34). Although fluctuations at the respiratory and Mayer wave
frequencies have been studied most, there is also significant variability at longer time
scales. However, it remains unclear whether the variabilities below ~0.04 Hz
represent primary cardiovascular oscillations or are epiphenomena of long-term
cardiovascular recordings (34).
1.4.1
Respiratory frequency oscillations
Respiratory oscillations in blood pressure can be ascribed to the cyclic variation in
intrathoracic pressure, with breathing mechanically perturbing venous return, cardiac
output, and thus blood pressure (35;36). Respiratory frequency cardiac interval
oscillations, termed respiratory sinus arrhythmia (RSA), could be generated by a
number of scenarios coupling haemodynamic and reflex responses.
One explanation has been to see it as a baroreflex phenomenon: blood pressure
changing mechanically by the respiratory act, and heart rate following at the speed of
vagal control, within the same beat (37-39). By this reasoning, RSA arises from a
baroreflex mechanism that should counteract stroke volume fluctuations and reduce
arterial pressure fluctuations (35). However, elimination of RSA by parasympathetic
-CHAPTER I-
21
blockade (40) or fixed rate atrial pacing (41) reduces pressure oscillations at the
respiratory frequency in supine position, but increases them in the 40 deg tilt position.
These state differences may be explained by the shift in cross-spectral phase relation
between systolic pressure and R–R interval. In the supine position, the R–R interval is
in phase with or slightly leads arterial pressure changes (37), and in the upright
position, follows pressure changes (41). Thus, respiratory sinus arrhythmia can
actually contribute to respiratory arterial pressure fluctuations and does not always
represent simple baroreflex buffering.
An alternative explanation would place the origin of RSA in the central nervous
system, where co-activity of the vagus nerve with respiratory activity would drive
heart rate in the same frequency as respiration (42). From this, it would seem that
respiratory sinus arrhythmia arises from an efferent vagal oscillation that contributes
to arterial pressure fluctuations (41;43). There also appears to be a purely mechanical
proportion of respiratory sinus arrhythmia: after combined sympathetic and
parasympathetic blockade, and after cardiac transplantation, substantial (2-8% of
normal) respiratory sinus arrhythmia persists, due to the mechanical effects of
respiration in the sinus node (44;45).
The view that RSA is a vagal phenomenon is widely held, and is supported by
substantial published data (46-51). In healthy subjects under strictly controlled
conditions the amplitude of RSA is shown to be very sensitive to changes in vagal
tone resulting from vagomimetic and vagolytic effects of atropine (52;53). Though,
multiple physiological studies have been questioning RSA as a valid and reliable
cardiac parasympathetic index (45;54;55). It appears that sympathetic influences
significantly impact RSA (45), challenging the notion that RSA represents a purely
vagal mechanism (Figure 1.13).
1.4.2
Mayer wave frequency oscillations
Unlike respiratory frequency oscillations, Mayer wave fluctuations appear
spontaneously and inconsistently across a broad frequency range, as low as 0.04 Hz
and up to 0.15 Hz, although generally close to 0.10 Hz (52). The physiological basis
of low-frequency (LF) oscillations is still and unsettled matter. Although it is
generally accepted that the slow oscillation in blood pressure does involve the action
of the sympathetic nervous system on the vasculature (56-58), the debate is whether
arterial baroreceptors are involved in its generation.
22
-CHAPTER I-
Figure 1.13: Effects of cardioselective β-adrenergic blockade (atenolol) and double
autonomic blockade (atenolol + atropine) on respiratory sinus arrhythmia during
controlled frequency breathing from 0.25 to 0.05 Hz in the 40 deg tilt position n = 10.
β-blockade enhances respiratory sinus arrhythmia at all breathing frequencies, not
just at higher breathing frequencies. This undermines the contention that respiratory
sinus arrhythmia is a purely vagal phenomenon. redrawn from ref (45).
Based on the observed sympathetic discharge patterns in single neurons located in the
brainstem in cats after section of sinoaortic and vagal nerves, Montano et al. (59)
proposed an inherent ability of the central nervous system to generate Mayer wave
frequency oscillations. This “central oscillator theory” is only endorsed by a relatively
small number of research groups (59-61).
A more dominant hypothesis is often termed the “baroreflex feedback theory”. In this
view, a change in blood pressure, e.g., due to respiration, is sensed by arterial
baroreceptors, and accordingly the central nervous system adjusts the heart rate both
by the fast vagal action and the slower sympathetic action (62). The baroreflex also
adjusts sympathetic outflow to the vasculature and therefore peripheral resistance,
leading to a change in blood pressure in an attempt to buffer the initial change in
blood pressure (37). The critical point is that the combination of a series of time
delays present among baroreceptors, the central nervous system, sympathetic outflow,
and the response of the vasculature means that the input change in blood pressure
results in an output change in vascular resistance that is slightly shifted in time, and
instead of buffering the initial change in blood pressure, it leads to the development of
its own change in blood pressure. Using a range of time delays and known properties
-CHAPTER I-
23
of the heart and vasculature, de Boer et al. (37) were able to construct a model that
accounted for oscillations at 0.1 Hz in the human.
The spectral estimate of Mayer wave amplitude is commonly used to measure
vascular sympathetic activity (59;60;63-65) and the cross-spectral magnitude of
pressure and R-R interval to measure baroreflex gain (66-70), reflecting the heart rate
response to spontaneous pressure oscillations. These simple interpretations can be
argued because of the complex interactions that appear to underlie Mayer wave
fluctuations. First, subjects with characteristically different levels of muscle
sympathetic outflow show no difference in Mayer wave amplitude (71). However, it
is often overlooked that the pattern of sympathetic activities to various target organs is
not uniform, but is adjusted in a differential manner to different end organs (33).
Second, oscillatory lower body negative pressure at 0.10 Hz increases both low
frequency blood pressure and R–R interval oscillations, but with considerable
inconsistencies in their spectral correlation (i.e. coherence) (72). Thus, short-term
fluctuations in cardiac interval do not appear to be linked inextricably to those in
arterial pressure. Finally, it has been suggested that the arterial baroreflex link
between Mayer wave oscillations in pressure and cardiac interval may be evident only
when sympathetic outflow is augmented (e.g. head-up tilt) (41).
1.4.3
Mathematical modeling of cardiovascular control
Although the complex interaction between central and reflex mechanisms of neural
cardiovascular regulation is not yet resolved, continuous progress in technology and
in linear and nonlinear mathematical models of cardiovascular variability allows
important insights into the dynamic features of cardiovascular regulation. This is
evidenced by thousands of papers published at a regular pace over a few decades,
further clarifying the physiology of cardiovascular regulation, and also exploring the
pathophysiological and clinical relevance of its changes in several diseases.
Various algorithms have been developed to model cardiovascular variability. Linear
analysis tools are designed to quantify regular oscillatory patterns. Special techniques
deducted from chaos theory have been developed to quantify non-linear components
in cardiovascular time series. For a description of the most common techniques to
quantify non-linear behavior we refer to the PhD dissertation of Frank Beckers (73).
The following paragraphs provide a short overview of the most important linear
-CHAPTER I-
24
analysis tools that have been adressed in the present study. These methods can be
divided into:
1) Monovariate analysis techniques: R-R interval and arterial blood pressure
oscillations are studied on its own, without the analysis of other
parameters/signals.
2) Multivariate analysis techniques: R-R interval variations are studied in
conjunction with variations in arterial blood pressure to investigate dynamic
characteristics of the baroreflex mechanism.
For a more detailed description on the standards for the measurement and calculation
of linear cardiovascular variability indices we refer to the Task Force of the European
Society of Cardiology and the North American Society of Pacing and
electrophysiology (52).
1.4.3.1 Monovariate analysis in time domain
Time domain analysis has been employed to characterize variability in cardiac cycle
length and is based on simple descriptive statistics, such as the standard deviation
(SD) of R-R interval (52). The most commonly used measures to estimate respiration
synchronous variations in time domain include rMSSD, the square root of the mean
squared successive differences and pNN50, the percentage of successive interval
differences greater than 50 ms. They provide a general marker of cardiac-vagal
modulation (52), however, this approach does not provide any information on the time
structure or the periodicity of the data.
1.4.3.2 Monovariate analysis in frequency domain
The use of spectral analysis implies that the event series can be represented by a sum
of sinusoidal components, of different amplitude, frequency and phase values. Welldefined fluctuations can be identified in distinct frequency bands, which have been
attributed to the influence of vagal and/or sympathetic outflows. Various spectral
methods for the calculation of power spectra density have been applied since the
1960's; they may be generally classified as non-parametric and parametric. In the
following chapters, the non-parametric approach using Fast Fourier Transform (FFT)
is used to avoid statistical uncertainty related to the choice of the model order, which
is inherent to the parametric PSD estimates (74).
Traditional spectral analysis of heart rate variability (HRV) and blood pressure
variability (BPV) results in a spectrum with three major components, defined as very
-CHAPTER I-
25
low-frequency (VLF: 0.003 – 0.04 Hz), low-frequency (LF: 0.04 – 0.15 Hz) and highfrequency (HF: 0.15 – 0.4 Hz) (Figure 1.14). The distribution of the power and the
central frequency of these components are not fixed but vary in relation to changes in
autonomic modulation of heart rate and blood pressure (52).
Figure 1.14: Upper panel: The R-R interval (thick dashed line) as a function of time
and the respiration signal (thin dashed line). The time axis has been limited to 50 s so
that we can see that the RRI oscillates with respiration. Lower panel: Power spectral
density (PSD) of the RRI (bold line) and the spectral density of the respiration signal
(dashed line), which present a single peak at 0.25 Hz (15 breath/min), the imposed
breathing frequency.
It is important to note that a large proportion of the VLF component is due to nonharmonic noise (direct current - DC), rendering VLF assessment from short-term
recordings a dubious measure that should be avoided. The LF and HF components are
evaluated in terms of frequency (Hz) and power. This power is assessed by the area of
each component and, therefore, squared units are used for its absolute value. In
addition, an appraisal of the fractional distribution of power, independent of the
absolute values of variance, can be obtained with computation of normalized units
(nu). They are obtained by dividing the power of a given component by the total
power from which the VLF has been subtracted. This procedure focuses on the
possible reciprocal link between LF and HF components but remains somewhat
controversial (75;76).
26
-CHAPTER I-
1.4.3.3 Multivariate analysis in time domain
Since the autonomic nervous system simultaneously modulate blood pressure and
heart rate, a quantification of the coupling between these variables is often used to get
information on the efficacy of cardiovascular control. The efficacy of the baroreflex
function is commonly estimated by evaluating the sensitivity of the baroreflex control
of heart rate (BRS), i.e. quantifying the capability of the baroreflex to modulate heart
rate in an attempt to buffer a unitary change in blood pressure. A significant
advancement in the assessment of baroreflex function is represented by the
development of techniques that quantify the “spontaneous” sensitivity of the
baroreflex control of heart rate. This is done at blood pressure levels where the
baroreflex usually works in real life, with no need of external interventions either on
the baroreceptor areas or on the cardiovascular parameters under evaluation, as with
conventional laboratory maneuvers (77). Hereafter, some features of the presently
employed BRS techniques will be described, in the time (78;79) and the frequency
domain (66).
The sequence technique is based on the automatic scanning of beat-by-beat systolic
blood pressure (SAP) and R-R interval (RRI) series searching for spontaneous
sequences of three or more consecutive beats in which a progressive rise in SAP is
followed by a progressive increase in RRI or by a progressive decrease in SAP and
shortening in RRI. The slope of the regression line between SAP and RRI values
forming the sequence is estimated and taken as an index of the sensitivity of arterial
baroreflex modulation of heart rate (Figure 1.15) (78). This method is suitable to
provide a detailed profile of BRS modulation over time.
The beat-to-beat x-BRS approach is based on the estimation of the cross-correlation
between 10-sec segments of SAP and RRI data. For any SAP segment, the crosscorrelation is repeatedly estimated by considering sliding 10-sec segments of RRI
with a time delay with respect to SAP up to 5 sec. The SAP and RRI segments
showing the maximal correlation coefficient are selected and the slope of the
regression line obtained from the corresponding SAP and RRI values is taken as an
index of BRS, provided the probability of being a random regression is lower than 1%
(79). This method yields several values per minute (good time resolution) with a low
within-patient variance and a measured baroreflex delay.
-CHAPTER I-
27
Figure 1.15: Scheme of the sequence technique. Adapted from ref (80)
1.4.3.4 Multivariate analysis in frequency domain
Transfer function analysis is based on the splitting of beat-by-beat SAP and RRI
series into short data segments, each lasting a few minutes (usually 2-5), followed by
estimation of the SAP spectrum and the cross-spectrum between SAP and RRI. The
transfer function H(f) between the output RRI (y) and the input SAP (x) is then
evaluated as the ratio between the cross-spectrum Pxy(f) and the SAP autospectrum
Pxx(f):
H(f) =
Pxy(f)
Pxx(f)
The modulus of the transfer function gain G(f) in the LF and HF frequency regions
are taken as BRS estimates (66;67); the transfer function phase Ø(f) represents the
baroreflex time delay,
G(f) = H(f)
⎛ I (H(f)) ⎞
Ø(f) = tan -1 ⎜
⎟
⎝ R (H(f)) ⎠
where I is the imaginary and R the real part of the complex function H(f).
The degree of linear correlation between SAP- and RRI-variations is usually
quantified by the coherence function:
2
pxy(f)
C(f) =
Pxx(f)Pyy(f)
-CHAPTER I-
28
With the hypothesis that SAP-RRI coherence may reflect the degree of baroreflexinduced coupling between SAP and RRI, the modulus in the LF and HF regions are
taken as indexes of BRS whenever the squared modulus of the RRI-SAP coherence
significantly exceeds 0. The corresponding minimal coherence allowing reliable
transfer function estimates depends on the parameters of the estimation procedure
(40). Figure 1.16 shows typical coherence squared modulus and phase between
systolic blood pressure and R-R interval in a healthy subject at rest.
Figure 1.16: Squared modulus and phase of
the coherence function between systolic
blood pressure and R-R interval, from a 10min recording in a young healthy subject at
rest. A negative phase implies the systolic
pressure oscillation to lead the R-R interval
oscillation.
Typically, the modulus of coherence between systolic blood pressure and R-R interval
shows two peaks: around the respiratory frequency and at 0.1 Hz. These peaks
indicate that respiratory oscillations and LF powers are linearly correlated. By
contrast, coherence is remarkably low at lower frequencies, suggesting that
oscillations of longer periods are uncoupled, or coupled through non-linear
mechanisms (81).
1.4.4
Conclusion: mathematics linked to physiology
The current knowledge about the physiological substrate of frequency-specific
cardiovascular oscillations comes from three types of studies: 1) animal studies
showing univocal changes in HRV or BPV after blockade, amplification or selective
interference with autonomic cardiovascular regulation; 2) human studies in which
-CHAPTER I-
29
manipulation of autonomic cardiovascular control through drug administration or
laboratory stimulations induced consequent changes in HRV or BPV; and 3) studies
focusing on changes in HRV and/or BPV in patients affected by diseases where the
ANS was primarily or indirectly affected.
When considering cardiac interval spectral powers, the HF-component is a
satisfactory, although partly incomplete, measure of vagal cardiac control (52). The
interpretation of the LF component of HRV has been more controversial (76;82). It
may reflect the combined effect of both sympathetic and parasympathetic activities to
the sinus node, but occurrence of resonance in the baroreflex loop may also play a
role (83). There is ample evidence for a substantial sympathetic contribution to the LF
R-R interval oscillations (63;84-88), though, other factors such as baroreceptor
unloading, adrenergic receptor sensitivity and adrenergic transduction processes
clearly influence the end-organ responsiveness to sympathetic activity, rendering LFpower a more integrated index of sympathetic activity. In circumstances saturating
sympathetic tone, such as exercise (89) or advanced heart failure (90), the potential of
physiological modulation is lost, which explains the reduction of the LF component.
In these circumstances where the sinus node appears to have a drastically diminished
responsiveness (91), appraisal of the fractional distribution of power by computing
normalized units may be helpful to describe the system as a sympathovagal balance
(82). This implies that if one side of the balance decreases, the other side increases
proportionally suggesting a centrally mediated push-pull mechanism (82). An
advantage is that fluctuations in total power are excluded. However the normalization
procedure loses every direct physiological correlate to LF and HF separately. An
increase in e.g. LF expressed in normalized units can indicate an increase in absolute
value of LF power, or a decrease in absolute value of HF power (76). In addition,
some physiological interventions provoke parallel, rather than reciprocal, changes of
vagal and sympathetic nerve activity (92). Also there is no physiological evidence that
fluctuations of sympathetic and vagal origin constantly interact.
When considering blood pressure spectral powers, HF fluctuations depend on the
mechanical effects of respiration, being largely unmodified in patients with
denervated donor hearts (44). Conversely LF powers are predominantly caused by
fluctuations in the vasomotor tone and systemic vascular resistance and are influenced
by neural, humoral, and endothelial factors and by thermoregulation (65). LF powers
of BPV increase or decrease with stimuli or conditions that, respectively, increase or
30
-CHAPTER I-
decrease sympathetic cardiovascular influences, such as head-up tilt or mental stress
in the former case, sleep or α-adrenergic blockade in the latter case (65;82). However,
the specificity of SAP LF powers in reflecting sympathetic activation is limited
because these components are also affected by resonance in the baroreflex loop (83).
These observations, on one side, suggest caution in regarding LF powers as specific
markers of sympathetic cardiovascular drive, but, on the other side, further emphasize
their dependence on autonomic cardiovascular modulation.
The ability of cardiovascular variability to reflect autonomic cardiovascular control is
improved by use of multivariate models for its assessment. The mutual interaction
between variabilities in heart rate and arterial blood pressure can be used to
investigate “spontaneous” cardiac baroreflex sensitivity (BRS). Methods of cardiac
spontaneous BRS reflect changes of baroreflex control associated to modulations of
autonomic activity during daily life, or to the occurrence of autonomic impairments.
A number of papers support the pathophysiological and clinical relevance of
spontaneous BRS estimates. Their ability to explore the baroreflex function was
demonstrated by animal studies where surgical denervation of arterial baroreceptors
was followed by disappearance of significant links between blood pressure and heart
rate fluctuations in the above models (93;94). It was also demonstrated in humans by
relating spontaneous BRS estimates with those measured by injection of vasoactive
drugs (95;96). Although quantitatively different, as expected, BRS estimates provided
by spontaneous cardiovascular variability and by laboratory methods displayed high
correlation in most instances, confirming their ability to provide complementary
information on baroreflex modulation of heart rate (77).
-CHAPTER I-
31
1.5 Mathematical modeling of variations in aortic flow
There is growing interest in monitoring physiological parameters non-invasively, both
in research and in clinical situations. Blood pressure can now be measured noninvasively and continuously in the finger, which is based on the volume-clamp
method of Peñàz (97). Application of pulse wave analysis to the measurement of
finger arterial pressure offers a non-invasive and continuous recording of stroke
volume. The analysis is based on models of the arterial system that assume that both
the aortic dimension and the elastic properties remain constant (98), although these
are known to change when the distending pressure of the aorta is changed (99). A
three-element model of the arterial input impedance (Modelfow) has been advanced
that takes into account the non-linear aortic pressure-area relationship (Figure 1.17)
(100).
Figure 1.17: Diagram of modeling flow from measurements of arterial pressure. Left
panel: non-invasive finger arterial pressure as input to the model for one heartbeat.
Middle panel: three-element model of the aortic input impedance used to compute
flow from pressure. Z0, characteristic impedance of the proximal aorta; Cw'
'Windkessel' compliance of the arterial system; Rp, total systemic peripheral
resistance. The Z0 and Cw elements have non-linear, pressure-dependent properties
indicated by the stylized f symbol. The peripheral resistance element Rp, varies with
time, as symbolized by the arrow. P (t), arterial pressure waveform; Q (t), blood flow
as a function of time; Pw (I) Windkessel pressure. Right panel: the computed output of
the model, i.e. aortic flow as function of time. Adapted from ref (9)
The Modelflow method computes stroke volume from the arterial pressure wave, with
continuous nonlinear corrections for variations in aortic diameter, compliance and
impedance during the arterial pulsation (99). A non-linear three-element model of the
aortic input impedance is used (Figure 1.17). The first element in the model is the
aortic characteristic impedance (Z0); this describes the relationship between pulsatile
flow and pressure at the entrance to the aorta. The rise in pressure will depend on the
instantaneous flow, on the cross-sectional area of the aorta and on the aortic
32
-CHAPTER I-
compliance. Hence Z0 represents the aortic opposition to pulsatile inflow from the
contracting left ventricle. The second model element is the arterial compliance (Cw);
this describes how much the aortic pressure rises for a given volume of blood, and
represents the aortic opposition to an increase in blood volume. The third element in
the model is peripheral vascular resistance (Rp). This is a measure of the ease of
constant blood drainage from the Windkessel into the peripheral vascular beds.
The estimation of Z0 and Cw is based on the arctangent model of aortic mechanics
(100), which nonlinearly relates the thoracic aortic cross-sectional area A(p) to the
blood pressure P:
1
⎡
⎛ P - P0 ⎞ ⎤
A(p) = Amax ⎢0.5 + arctan ⎜
⎟⎥
Π
⎝ P1 ⎠ ⎦
⎣
With Amax, the maximal aortic diameter during ejection, P0 the position of the
inflection point on the pressure axis at 0.5 Amax, and P1 the steepness of the curve at
0.75 Amax. P0, P1 and Amax are the age-and gender-dependent parameters in the
equation (100). From this relationship, the aortic compliance per unit area, C’, is
derived as:
C' =
dA( p )
dP
The arterial compliance C is obtained as C = L x C’, where L is the aortic length
which depends on the patient height and weight. Characteristic impedance Z0 is
calculated as:
Z0 =
ρ
AC '
where ρ is the blood density. Instantaneous values of Z0 and C are further used in the
model simulation, resulting in the computation of an aortic flow waveform. The third
element, Rp, is calculated for each beat by the model simulation and updated.
Integrating the aortic flow waveform per beat provides left-ventricular stroke volume.
Cardiac output is computed by multiplying stroke volume by heart rate.
This modelflow method assumes a normal human aorta and proper functioning of the
aortic valve. It also uses the premise that the transmural aortic pressure is not affected,
for example, by extreme pulmonary hyperinflation or increased intra-abdominal
pressure. The maximal aortic diameter during ejection is the parameter included in the
model that does not regress with age and its variability is considerable, explaining
-CHAPTER I-
33
why computed cardiac output does not reflect the true cardiac output without
calibration (101-103). If accurate values are required, a calibration against a standard
method is needed, such as thermodilution (9;104;105) or the Fick principle (106).
However, validations are limited in so far as they are based on averaging procedures
in steady-state conditions and do not reflect the beat-to-beat fluctuations, which may
be considerable (35).
Relative changes in cardiac output can be tracked with good precision (107) during
various experimental protocols including postural stress (9;108) and static (109) and
dynamic exercise (110;111). However, changes in the aortic transmural pressure upon
standing up may lead to an offset in computed stroke volume compared to supine (9).
In conclusion, the Modelflow method cannot provide accurate absolute values of
cardiac output, but relative changes in cardiac output can be tracked with good
precision within the same body posture (9;107).
-CHAPTER II-
35
2 Influence of microgravity exposure on cardiovascular
control mechanisms
2.1
Introduction
The human body has developed over thousands of years in the presence of gravity.
Gravity importantly affects the fluid distribution in man: on standing up, blood
volume is shifted toward the splanchnic, pelvic and leg vasculature. Postural changes
in microgravity do not result in any fluid shift other than that resulting from muscle
contraction. In microgravity, blood is chronically shifted towards the thorax and head,
giving an appearance of ‘puffy faces and chicken legs’. A schematic representation
(Figure 2.1) of the expected distributions of tissue fluid and blood pressure in
microgravity was proposed by Hargens et al. (112).
Figure 2.1. Expected distributions of tissue fluid (shading) and mean arterial
pressure (numerical values in mmHg) at head, heart, and feet during preflight
standing posture on Earth (left), microgravity (middle), and postflight on Earth
(right). During microgravity, all gravitational blood pressure gradients disappear.
Postflight orthostatic intolerance results from inadequate cerebral perfusion, but
vestibular factors may also be involved. Adapted from ref (112).
The fluid redistribution in (simulated) microgravity produces immediate and longterm reactions to establish a new level of circulatory homeostasis (113). The
-CHAPTER II-
36
immediate reactions develop in response to cardiovascular changes induced by the
increased central blood volume, while the long-term reactions develop as the complex
and multifactorial response to the fluid regulating reflexes. Cardiovascular adaptation
mechanisms importantly contribute to the genesis of post-spaceflight orthostatic
intolerance, which is characterized by a diminished ability to assume the standing
posture back on Earth. The cardiovascular consequence of microgravity exposure can
be best appreciated by separately focusing on the immediate and long-term
cardiovascular adaptation mechanisms (114).
2.1.1
Cardiovascular response to initial microgravity
Based on studies using head-down tilt and water immersion models of microgravity,
investigators predicted that loss of gravitational pressure gradients and a shift of blood
and tissue fluids from the legs toward the thorax would increase central venous
pressure, thus initiating a sequence of renal and hormonal responses. Such responses
were expected to increase urine production and reduce blood volume within the first
day of microgravity exposure. However, Skylab and more recent Spacelab and Mir
studies documented a surprising discrepancy between a reduction in central venous
pressure and elevated cardiac filling volumes early in space (115-117). In addition,
evidence has been provided of an initial reduction in blood volume without
considerable diuresis the first days in space (115;118;119). These observations have
been explained by findings from parabolic flight studies: an increased cardiac
transmural pressure is brought about by a proportional greater fall of intrathoracic
pressure compared with the fall in central venous pressure (117). Increased cardiac
transmural pressure and intrathoracic blood volume facilitate cardiac filling. Thus,
despite a reduction in central venous pressure, the mechanical consequences of
entering microgravity lead to cardiac distention in the first 24 hours of spaceflight
(117;120;121).
Increased cardiac filling early in spaceflight is confirmed by elevated stroke volume
and cardiac output (122;123). This higher stroke volume triggers a fast reflex reaction
through the baroreflex which decreases heart rate. A concomitant reduction in
sympathetic outflow leads to a drop in total peripheral resistance (123). Right atrial
stretching further enhances the secretion of atrial natriuretic peptide (124) leading to
an increased vascular permeability, which together with an increased transmural
-CHAPTER II-
37
pressure facilitates extravasation of fluid and sodium (125). In addition, the expended
central blood volume is sensed as a ‘fluid-volume overload’ inhibiting the reninangiotensin system (126). The resulting plasma volume loss (500 – 800 ml)
corresponds approximately to the blood volume that is transferred to the peripheral
venous system upon assuming the standing position on Earth. This reduction in blood
volume takes place rapidly within 1 to 3 days in space (118;127).
It should be noticed that some plasma volume loss may precede launch because
astronauts commonly experience 2 or more hours in supine legs-elevated posture prior
to launch (115). Space motion sickness and diminished fluid intake in the first days of
microgravity exposure may also contribute to plasma volume loss (112).
2.1.2
Cardiovascular adaptation to sustained microgravity
Chronic gravity unloading together with the ongoing deficit in effective blood volume
are the key events in the chronic adaptation to microgravity. Following the initial
increase in left ventricular volume, cardiac adaptations to spaceflight include a
diminished
cardiac
compliance
(128;129),
decreased
cardiac
dimensions
(120;121;130) and cardiac atrophy (130), and deteriorated left ventricular chamber
performance (131). However, current evidence suggests that the risk of diminished
cardiac function during spaceflight is negligible in the presence of the current
effective exercise spaceflight countermeasures (132).
Vascular adaptations to spaceflight include vascular smooth muscle atrophy (133;134)
and an increased distention and capacity in the leg veins (127). Echo-Doppler studies
have shown that there is a blood-flow redistribution in the body (135). This may be
due to the fact that blood vessels in the legs are chronically exposed to lower than
normal upright blood pressure during spaceflight, while vessels between the heart and
head are exposed to higher than normal blood pressure (Figure 2.1). Assuming that
blood vessels respond appropriately to local mechanical stress conditions, it is
expected that the thick-walled arteries in the feet would experience smooth muscle
atrophy while the thin-walled arteries of the head would undergo hypertrophy during
extended exposure to microgravity (112). Diminished vascular function and/or lower
vasoconstrictive reserve secondary to hypovolemia may represent a significant
cardiovascular risk of spaceflight (132).
The plasma volume reduction in space has been linked to an increased sympathetic
neural outflow in order to preserve mean arterial blood pressure. This is reflected by
38
-CHAPTER II-
higher circulating catecholamine levels (136-138) and higher muscle sympathetic
nerve activity (138). Also the fluid-retaining renin-angiotensin-aldosteron system is
activated in space (119). The sympathetic baroreflex response is preserved in space
whereas the cardiac-vagal baroreflex response is found to be diminished (139).
Finally, while a vagal predominance of heart rate control has been suggested by some
heart rate variability studies during short-term spaceflight (140;141), reports on longterm space missions rather indicate diminished cardiac-vagal control (142;143).
Altered baroreflex function after microgravity exposure likely results from autonomic
neural plasticity that develops secondary to changes from autonomic sensory input
(144).
2.1.3
Return to Gravity: orthostatic intolerance
Especially after return to Earth, spaceflight-induced changes in the human body may
cause physiological problems for the astronauts. The inability to stand for long
periods with the risk of fainting, also known as ‘orthostatic intolerance’, can be
considered as one of the most problematic from an operational point of view (Figure
2.1). The degree of orthostatic intolerance varies a lot from subject to subject. Despite
the large amount of studies devoted to solving the problem of orthostatic intolerance,
or to elucidate the mechanisms behind it, many aspects are still unclear.
Orthostatic intolerance is present when an excessive postural decrease in cardiac
filling and stroke volume and/or inadequate compensatory neurohumoral responses
leads to failure to maintain adequate brain perfusion in the upright position. At this
moment, the most likely cause of orthostatic intolerance after spaceflight is a
limitation of compensatory elevation in vascular resistance upon standing (145;146).
In addition, hypoadrenergic responsiveness has been supported by lower or equal
values of plasma noradrenaline levels postflight compared to preflight in orthostatic
intolerant subjects (147). At the same time changes in the baroreflex might contribute
to orthostatic intolerance (143;148-150). Reduced vagal cardiac efferent neural
outflow in combination with reduced cardiovagal baroreflex gain have been described
in relation with orthostatic intolerance (143;150). On the contrary, astronauts that are
able to complete a stand test show increased levels of plasma noradrenaline after
spaceflight (146;147). The sympathetic dominance after spaceflight might result from
the decreased stroke volume (SV) and cardiac output (CO) upon standing (123;151-
-CHAPTER II-
39
153), suggesting a tight coupling (signaling) between stroke volume and sympathetic
nerve activity (152).
Although orthostatic intolerance usually disappears quickly after spaceflight, the
adaptation process after return to Earth may take several weeks, the exact duration of
which is still unknown (140;141;143).
2.1.4
Study objectives
This chapter deals with the effects of changing gravity levels on cardiovascular
control mechanisms in healthy subjects.
In a first part, we set out to determine the effect of acute gravity changes,
induced by parabolic flight, on cardiac-vagal (baroreflex) control. During parabolic
flight, acute changes in the vertical acceleration (Gz) elicit variations in the
hydrostatic pressure gradients inside the cardiovascular system. Stabilization of blood
pressure during different gravity periods of parabolic flight is maintained by changes
in autonomic nervous system (ANS) activity.
In a second part, we investigate the effect of 60 min of thermoneutral (34.5°C)
head-out of water immersion (WI) on integrated circulatory control in healthy
voluntary subjects. Head-out of water is a well established means of simulating
microgravity on Earth. This is because during WI, not only does buoyancy reduces
body weight, but hydrostatic pressure induces a thoracic fluid shift which increases
the central blood volume and stroke volume, thus, mimicking the hemodynamic
response to microgravity. Up till now, only little attention has been directed towards
the initial time course of cardiovascular control during WI.
In a third part, we aimed to assess the effect of short-duration (10-days)
spaceflight on circulatory neural control mechanisms in 5 cosmonauts. Experiments
were performed during 3 scientific ESA-Soyuz missions to the International Space
Station (ISS) (Odissea, Cervantes and Delta). Multiple aspects of post-spaceflight
cardiovascular deconditioning have already been addressed, though, many features
remain unclear. It is unknown how long the cardiovascular system needs to recover
from short-duration spaceflights as most postflight investigations only extended
between a few days to two weeks. Furthermore, it remains unclear whether a post-
40
-CHAPTER II-
spaceflight reduction in vagal-cardiac reflex control elicits arterial blood pressure
instability at respiratory frequencies.
-CHAPTER II-
2.2
41
Spectral characteristics of heart rate fluctuations during parabolic
flight
2.2.1
Introduction
Changing gravity conditions have an important impact on almost all of the human
body’s systems, particularly the cardiovascular system (114). Assuming the upright
position for example, results in an autonomic reflex-response consisting of
hemodynamic and cardiac rhythmic alteration that prevent blood from pooling in the
legs and arterial blood pressure from falling (154). Pronounced autonomic reflexresponses may not always be without risk, especially when they occur very rapidly as
demonstrated by the so called push–pull effect in military aviators (155).
Parabolic flight is used to create short successive periods (20–25 s) of changing
gravity in a range between 0 and 1.8 Gz (1 Gz: 9.81 m/s2). The resultant gravity force
vector always remains perpendicular to the deck of the aircraft allowing altered body
fluid hydrostatic pressure gradients in the longitudinal axis of the erected human body
(156). This causes venous blood shifts with alterations in the thoracic blood volume
(157), cardiac filling (117), ejection properties (158) and carotid arterial distention
pressure (159). Autonomic baroreceptor reflex adjustments of heart rate and
peripheral vascular resistance prevent arterial blood pressure to vary with these
gravitational challenges (157;159). Iwase et al. (160) examined changes in muscle
sympathetic nerve activity during parabolic flight in quietly seated human subjects. It
appears that the average sympathetic neural outflow is increased during transient
hypergravity, while sympathetic withdrawal occurs during microgravity induced by
parabolic flight. Accordingly, the reduction in heart rate during transient microgravity
appears to result from a parasympathetic predominance, as indicated by time domain
analysis of heart rate variability (HRV) (161-163).
In the present study, we hypothesized that the dynamic behavior of parasympathetic
neural outflow could be better unraveled using frequency domain analysis of HRV
during changing gravity levels of parabolic flight. Because the parasympathetic
system reacts rapidly to rhythmic respiratory discharges, neural oscillations are passed
on to the heart causing respiratory sinus arrhythmia (42). The sympathetic system is
too sluggish to mediate respiratory oscillations (164); therefore, high frequency (0.16
– 0.4 Hz) oscillations of heart rate are mainly attributed to parasympathetic
-CHAPTER II-
42
mechanisms (48). Power spectral analysis is the most appropriate technique to
evaluate oscillatory components of HRV at distinct frequency bands (52). However,
this type of analysis is difficult to apply for signals of very short duration because of
the conflict between time and frequency resolution (165). The purpose of this study
was to develop a specific method of power spectral analysis that allows establishing
respiratory oscillations of heart rate during ultra short (±20 s) gravity periods of
parabolic flight (166).
2.2.2
Methods
2.2.2.1 Subjects
Fourteen healthy non-smoking male volunteers between 22 and 44 years of age (mean
± SD: 28 ± 7 year), and free from any cardiovascular, metabolic or neural pathology
were selected for this study (stature: 176 ± 8 cm; mass: 72 ± 5 kg; BMI: 23 ± 2).
Special flight medico-physical examination was performed at ‘‘the Medical center of
the Belgian Air Force, Brussels’’1 month before the flight campaign in order to pass
FAA III tests. Informed consent to participate in the study was obtained from all
subjects. To eliminate the effects of pharmacological agents that might alter
cardiovascular ANS control, neither general medications nor medications for the
control of motion sickness were taken before or during flights. The study was
approved from an ethical point of view by the Comité Consultatif de Protection des
Personnes dans la Recherche Biomédicale, the Ethical Committee of the Faculty of
Medicine, K.U. Leuven, Belgium and the European Space Agency (ESA) medical
board.
2.2.2.2 Equipment and protocol
This study was conducted during the 29th and the 32nd ESA parabolic flight campaign
performed in November 2000 and April 2002. The flights were organized by ESA and
NOVESPACE at the Société Girondine d’Equipments, de Réparation, et de
Maintenance Aéronautique (SOGERMA) center in Bordeaux, France. An AIRBUS
A300 aircraft was used to perform the parabolic flight profiles. Flights were managed
on three consecutive days. Figure 2.2 depicts the succession of parabolas in each
flight together with the parabolic flight profile. Each flight session lasted 2.5 – 3 h and
incorporated one test-parabola followed by 30 parabolas. At least 1 min rest was
allowed between two parabolas. A minimum of 5 min was spent between groups of
-CHAPTER II-
43
five parabolas. Instantaneous gravity was measured using an aircraft Gzaccelerometer. Five consecutive parabola phases (±20 s) were manually separated
offline, based on the Gz-forces: Phase I, normogravity (1 Gz) before each parabola;
Phase II, hypergravity (±1.8 Gz) at the ascending leg of the parabola; Phase III,
microgravity (±0 Gz) at the top of the parabola; Phase IV, a second period of
hypergravity (±1.8 Gz) at the descending leg of the parabola; Phase V, again
normogravity (1 Gz) after each parabola.
All subjects participated in at least two flights. To reduce the effect of gravity-related
stimulation of vestibular-sympathetic reflexes (167) we asked our subjects to maintain
their head position during parabolic flight profiles. One subject suffered from severe
travel sickness for two consecutive flights and was excluded from the study. Six
subjects had nausea during their first flight. In order to reduce possible side-effects of
nausea and/or other feelings of discomfort (163), only data from the second flight of
these subjects were analyzed. A total of 13 subjects were left for further analysis. To
prevent free floating during microgravity, subjects were attached with a belt around
the chest in supine position and around the feet in standing subjects. The standing
subjects had one hand fixed on a handle attached to the ceiling of the plane. The
supine subjects were perpendicular to the longitudinal axis of the plane. The initial
position of the subjects was chosen at random and after 15 parabolas positions were
changed. To allow position changing and adaptation to the new condition, there was a
15-minute time break between parabolas 15 and 16 (Figure 2.2).
ECG electrodes were applied to the chest wall of all subjects prior to the flight. ECG
was monitored with a lead II derivation. Leads were connected to an ECG amplifier.
The analogue output was connected to an external A/D converter (DATAQ
Instruments Inc., Akron, OH, USA) and sampled at 1000 Hz per channel and stored in
a laptop computer. Respiratory rate was evaluated using an abdominal pressure sensor
(MR10, Smiths Medical, Turnhout, Belgium). To avoid influences by the respiratory
pattern (168), al subjects were instructed to breathe normally and not to change the
respiratory pattern during gravity transitions. Baseline data were collected (1 Gz) for
10-minutes in standing and supine position in the airplane one day before the first
flight, at the same hour as take off.
44
-CHAPTER II-
Figure 2.2: Schematic presentation of the parabolic flight profile; parabola phases
are separated based on the instantaneous gravity level. The succession of parabolas
in each flight session is shown at the bottom.
2.2.2.3 Data analysis
After peak detection in the QRS-complexes (169), a file (tachogram) consisting of
consecutive RR intervals was created for every distinct gravity phase. Visual
inspection was performed for artifacts and ectopic beats. In case of ectopic beats, two
linear filters could be applied to correct for data points outside a limit interval (165).
Signals with artifacts were eliminated from further analysis (34 time series in total).
Each RR tachogram was resampled with a sample rate of 2 Hz using a third order
cubic spline approximation. Data sets that covered less than 18 s were eliminated.
Table 2.1 gives the number of gravity phases that are preserved for further analysis.
-CHAPTER II-
45
Table 2.1: Characteristics (number and duration) of stable parabolic gravity phases
used for spectral analysis
Phase
Supine
Standing
Number (n)
Duration (s)
Number (n)
Duration (s)
I
233
20.7 ± 1.5
166
20.3 ± 0.7
II
41
19.2 ± 0.8
32
18.8 ± 0.6
III
216
20.2 ± 0.9
198
20.3 ± 0.8
IV
130
21.7 ± 1.3
94
20.4 ± 1.6
V
214
20.7 ± 1.5
166
20.5 ± 1.1
Data are presented as the mean value ± SD. Number = number of parabola phases that
are included for spectral analysis; Duration = timeframe of parabola phases
In order to overcome methodological problems related to power spectral analysis of
the short duration stable gravity periods, additional signal processing of the
tachograms was required:
1. At the onset of parabola phase III, transient phenomena occur due to the
transition from 1.8 Gz to 0 Gz (Figure 2.3). In order to prevent artificial high
frequency contribution due to sudden heart rate transients, the initial first two
heart beats at the onset of phase III were eliminated.
2. Linear trend removal was performed by subtracting the slopes of the linear
interpolate in all tachograms of distinct gravity phases to prevent DC and
artificial low frequency contribution (170).
3. Tapering at both ends in all tachograms of distinct gravity phases was
performed with a Hamming window [(A = 0.08 + 0.46(1- cos2лt/T) for t=0 to
T, with A = window amplitude and T = time duration of the gravity phase] to
prevent spectral leakage.
4. Stationarity was assessed according to the definition that statistical properties
of a series [x(t)] do not change with time. This rule was implemented by the
following algorithm. For each [x(t)] the standard deviation [SDtot] was
obtained. A running mean <xi> was obtained from a sliding window of 6
points. The window is shifted by one point until all <xi> have been assessed.
The outcome of the following test indicated whether the signal was stationary:
│<xi> - <xi+1>│ < [SDtot]. This procedure proved that signal conditions were
46
-CHAPTER IIvalid for performing Fourier Transfrorm (165;171). All RR series that did not
meet the stationarity criterion were eliminated.
5. Short time frames (±20s) give rise to a time-frequency resolution conflict, due
to the Heisenberg principle. Though the paradox remains (an imposed lower
frequency limit), frequency resolution can be artificially enhanced with a well
known mathematical technique of zero padding. It consists in adding an equal
number of zeros (Nz) before and after the data set to obtain 256 equally spaced
(∆t = 0.5 seconds) data points (N). Accordingly, power spectral density (PSD:
ms2/Hz) is calculated by digital Fast Fourier Transform (FFT). The resulting
spectral output virtually expands towards 128 elements [(N + Nz)/2].
Consequently, sample spacing in the PSD becomes 0.0078 Hz [∆f = 1/(∆t.N);
N = number of points (256); ∆t = time resolution (0.5 s)] without creating
uncontrolled/artificial frequency components (165;170). The maximum
frequency was: fmax=∆f x N/2 or 1 Hz, which due to the transfer function of the
window was lowered to 0.5 Hz (165).
Figure 2.3: Simultaneous evolution of ECG and heart rate (HR) with gravity (Gz) in
a representative standing subject during one parabola. Gravity transitions are
marked by the light shaded area. The first two heart beats during stable 0 Gz are
marked by the dark shaded area. Heart rate time series from the blank area’s
corresponding with the parabola phases (I – V), are preserved for further analysis.
A low frequency (LF) component, starting from the lowest measurable frequency (±
0.05 Hz) up to 0.15 Hz was distinguished from a high frequency (HF) component,
ranging from 0.16 Hz up to 0.4 Hz. Total power (TP) and powers in the LF and HF
component were computed from the PSD by integrating between the appropriate
-CHAPTER II-
47
limits. Because of the important changes in heart rate between parabola phases and
the rather poor frequency distinguishing capacity, due to the short duration of each
tachogram, we used normalized units, i.e. the proportions of LF and HF powers in the
PSD [LFnu = LF/(LF+HF); HFnu = HF/(LF+HF)], to track changes in the distribution
of powers among successive periods of stable gravity during parabolic flight.
Additionally, the ratio of LF and HF powers was calculated (LF/HF). According to
the above described methodology, all 10-minutes baseline measures were processed
using a sliding window of 20 s without overlap. The mean values of the resulting 30
spectra that were obtained from each 10-minute recording were compared to those
obtained by a standard calculation algorithm using a sliding window of 128 s and 50%
overlap (165) in standing and supine positions. All analysis software has been
developed in house using Labview 6.1 (National Instruments, Austin, TX, USA) for
windows.
2.2.2.4 Statistical analysis
Statistical analysis was performed with SPSS version 8.0 for windows (Scientific
Packages for Social Sciences, Inc., Chicago, IL, USA). Observed variables were
tested for normality (Kolmogorov-Smirnov test). If they did not pass, a natural log
transformation was performed before further statistical testing. The effect of gravity
on heart rate variability was investigated for each body posture using one-way
ANOVA for repeated measures. In order to compare stable hypergravity (phase II and
IV, ±1.8 Gz) with stable microgravity (phase III, ±0 Gz) and stable normal gravity
(phase I, ±1 Gz), both phases II and IV of parabolic flight were pooled into one
subgroup. Using this stratification, polynomial contrast analysis was performed to
assess whether variables could be described as a function of the gravity level (0 Gz, 1
Gz and 1.8 Gz). Differences between standing and supine postures were assessed by
2-tailed paired sample t-test. Unless stated otherwise, results are expressed as mean ±
SD and p < 0.05 was chosen as the level of significance.
2.2.3
Results
There were no statistical differences among different parabolas for all subjects in each
position or parabola phase. There was no effect due to the number of parabolas
(accumulation effect) for any of the observed variables. Accordingly, we studied all
parabolas for all subjects in the same ANOVA.
48
-CHAPTER II-
2.2.3.1 Power spectral analysis of ultra short data segments
The influence of initial signal processing on the resulting power spectral density
(PSD: ms2/Hz) function of ultra short data segments is demonstrated by means of a
simulated RR tachogram (Figure 2.4) and a physiological correlate (Figure 2.5).
Figure 2.4: simulated RR oscillation
Figure 2.5: real tachogram
Figure 2.4 shows a time series, created by the sum of two sinusoidal waveforms of
0.1Hz and 0.3 Hz and equal amplitude. A tendency of simulated RR to increase from
± 800 ms in the beginning towards ± 950 ms after 20 s can be observed. At the left
-CHAPTER II-
49
side of Figure 2.4, the original simulated time series is shown together with its PSD
function. The right side shows the same signals in time and frequency domain after
DC and trend removal, applying a Hamming window function and zero padding. It is
obvious that with this procedure the proportion of spectral components is better
preserved. Because of the artificially enhanced frequency resolution, the variance of
each FFT epoch is reduced, spectral leakage is suppressed and selectivity is improved.
At the same time, it causes smoothing of the power spectrum.
Figure 2.5 shows the modified spectral procedure in a representative RR series
obtained from a standing subject at phase I of parabolic flight. The distribution of
powers is characterized by two frequency components around 0.1 Hz and 0.3 Hz,
similar to short-term heart rate recordings of several minutes (172). The lowest
measurable cyclic component remains limited, due to the length of the data recording;
i.e., a time window of e.g. ±20 s (T). This allows a theoretical lower limit oscillatory
frequency of ±0.05 Hz (1/T). However, practically it is advised to have at least two
oscillations within the time window (171).
2.2.3.2 Parabolic flight phases
Table 2.1 shows the number and duration of all stable parabola phases that were
preserved for further analysis in standing and supine subjects. After elimination of the
first two heart beats from the RR tachograms obtained at phase III of parabolic flight,
the duration of the remaining part for spectral analysis approximated 20 s. The
number of recordings during phase II and phase IV was smaller compared to the other
parabola phases due to increased prevalence of unstable gravity levels during the pullup and pull-out phases of the parabolic trajectory. Gravity transitions between phase
II and phase III of parabolic flight (mean ± SD: 3.2 ± 0.4 s) were significantly shorter
(P < 0.001) compared to the gravity transitions between other parabola phases (phase
I–II: 5.9 ± 1.3 s; phase III–IV: 5.4 ± 1.3 s; phase IV–IV: 6.7 ± 3.2 s).
2.2.3.3 Postural differences during 10 min baseline and during parabolic flight
Table 2.2 shows the results of 10-min baseline standing and supine measurements in
all subjects, calculated by two different algorithms: 1) 20-s sliding windows without
overlap and 2) 128-s sliding windows with 50% overlap. Briefly, upon assuming the
upright posture, mean RR decreased (thus HR increased) together with a reduction in
the proportion of HF power in the PSD (HFnu). The standing proportion of LF power
in the PSD (LFnu) and the LF/HF ratio were increased compared to supine.
-CHAPTER II-
50
Table 2.2: Power spectral variables calculated with sliding windows of 20s and 128s
during 10-min baseline recordings in standing and supine positions
Time frame
Variable
20s
Mean RR (ms)
Supine
t-test
767 ± 70
992 ± 97
p < 0.001
2
346 ± 321
347 ± 199
p = 0.15
2
254 ± 243
167 ± 89
p = 0.56
2
33 ± 35
128 ± 96
p < 0.001
TP (ms )
LF (ms )
HF (ms )
128s
Standing
LF/HF
10.87 ± 4.84
2.5 ± 1.84
p < 0.001
LFnu
0.75 ± 0.07
0.49 ± 0.13
p < 0.001
HFnu
0.18 ± 0.07
0.42 ± 0.12
p < 0.001
Mean RR (ms)
771 ± 67
997 ± 97
p < 0.001
2
2691 ± 2256
3223 ± 1511
p = 0.13
2
1853 ± 1811
1275 ± 729
p = 0.43
2
187 ± 191
807 ± 605
p < 0.001
LF/HF
11.53 ± 4.69
2.58 ± 1.99
p < 0.001
LFnu
0.85 ± 0.06
0.57 ± 0.14
p < 0.001
HFnu
0.1 ± 0.05
0.35 ± 0.12
p < 0.001
TP (ms )
LF (ms )
HF (ms )
Data are presented as the mean value ± SD. HF = high frequency; LF = low
frequency; TP = total power; nu = normalized units. t-test: paired sample t-test
between standing and supine subjects.
The observed postural differences were independent of the methodology applied, and
could be reproduced during the stable 1 Gz phases (phase I and V) of parabolic flight
(Table 2.3). During the 0 Gz phases (phase III), significant differences disappeared
between standing and supine values of mean RR, LF/HF ratio, LFnu and HFnu (Table
2.3). Furthermore, Figure 2.6 shows that postural differences in mean RR and the
LF/HF ratio were larger during the 1.8 Gz phases (phases II and IV) of parabolic
flight. Polynomial contrast analysis revealed a linear relationship between the gravity
level of parabolic flight and the difference between standing and supine values of
mean RR (p < 0.001) and the LF/HF ratio (p < 0.05).
-CHAPTER II-
51
Figure 2.6: Mean RR, normalized LF and HF powers (LFnu, HFnu) and the ratio of
LF and HF powers (LF/HF) at gravity levels (0Gz, 1Gz, 1.8Gz) obtained during
parabolic flight in standing versus supine positions. Results are presented as the
median, 25 and 75 percentiles, minimum and maximum.
470 ± 216
2
0.14 ± 0.10†
0.75 ± 0.09†
0.18 ± 0.09†
LFnu
HFnu
3.08 ± 1.86*
0.58 ± 0.21*
0.33 ± 0.21*
2.8 ± 2.33*
0.49 ± 0.15*
0.41 ± 0.16*
LF/HF
LFnu
HFnu
0.35 ± 0.13
0.56 ± 0.13
3.34 ± 2.8
86 ± 96*
158 ± 97*
294 ± 204*
912 ± 172‡
0.31 ± 0.13‡
0.59 ± 0.14‡
5.34 ± 5.71
206 ± 198‡
268 ± 141
601 ± 382
930 ± 153‡
III
0.32 ± 0.11*
0.58 ± 0.11*
4.63 ± 3.08
163 ± 199*
323 ± 407
585 ± 687
920 ± 160‡*
0.17 ± 0.13†
0.76 ± 0.15†
12.07 ± 34.72
36 ± 29†
224 ± 228
331 ± 299†
699 ± 187†‡
IV
0.41 ± 0.15*
0.5 ± 0.14*
3.26 ± 2.62*
187 ± 246*
215 ± 156
477 ± 420
942 ± 183*
0.21 ± 0.10†
0.72 ± 0.10†
9.09 ± 5.3
83 ± 50†
353 ± 190
552 ± 282
806 ± 173†‡
V
0.175
0.160
0.186
0.070
0.504
0.256
0.005
0.001
0.001
0.167
0.002
0.072
0.033
0.001
p-value
ANOVA
Data are presented as the mean value ± SD. HF = high frequency; LF = low frequency; TP = total power; nu =
normalized units. † p < 0.05 compared to phase III; ‡ p < 0.05 compared to phase I; * p < 0.05 compared to
standing.
212 ± 270*
467 ± 1026
779 ± 1413
212 ± 288*
223 ± 161
503 ± 440
HF (ms )
2
LF (ms )
2
TP (ms )
2
Mean RR (ms)
962 ± 207†*
0.8 ± 0.13†
9.51 ± 4.05
LF/HF
961 ± 187†*
16.06 ± 15.69
64 ± 51†
HF (ms )
Supine
13 ± 12+‡
330 ± 158
2
149 ± 220
197 ± 250†
664 ± 145†‡
II
LF (ms )
2
TP (ms )
762 ± 160†
Mean RR (ms)
Standing
I
52
-CHAPTER II-
Table 2.3: Power spectral variables during parabola gravity phases (I-V) averaged
for 13 subjects in standing and supine positions
-CHAPTER II-
53
2.2.3.4 Differences between parabolic flight phases
Except for mean RR, no differences were found among the parabola phases in the
supine position, whereas in the standing position, significant differences were shown
for mean RR, TP, HF, LFnu and HFnu (Table 2.3). Pairwise comparison among the
different gravity phases showed that, in standing subjects, mean RR was significantly
higher at phase III compared to all the other phases. At phases II and IV, significantly
lower values for mean RR were found compared to phase I of parabolic flight.
Additionally, mean RR was higher during phase V compared to phase I of parabolic
flight. A representative example of the variation of power spectral density in a
standing subject during different gravity phases of parabolic flight is presented in
Figure 2.7. An increase of powers located at the HF component of PSD is shown
during phase III. During phases II and IV, powers at the HF component are obviously
suppressed.
Figure 2.7: Representative example of the evolution of the Power Spectral Density
(PSD) function (ms2/Hz) among parabola phases in a standing subject. The
respiratory frequency in this example was ~0.4 Hz.
Table 2.3 shows that averaged for all standing subjects, normalized HF power (HFnu)
and HF power expressed in absolute values (HF) were significantly higher at phase III
compared to the other phases of parabolic flight. During phase II, HF power expressed
in absolute values was significantly lower compared to phase I. Further, normalized
LF power (LFnu) was significantly lower at phase III compared to the other phases
and no statistical significant differences were found for both LFnu and HFnu between
-CHAPTER II-
54
phase I, and phases II and IV of parabolic flight. Finally, total power (TP) was higher
during phase III compared to phases II and IV of parabolic flight. However, these
differences disappeared after correction for heart rate, which is a significant covariant
of TP among parabola phases.
No significant differences were observed in the mean respiratory rate between
subjects, and no changes were observed between consecutive parabola phases in both
standing and supine positions (range from 11 – 22 breaths/min or 0.18 Hz – 0.36 Hz).
2.2.4
Discussion
2.2.4.1 Critique of methods
Various methods for power spectral analysis of HRV have been used under a wide
range of physiological and pathophysiological circumstances since its introduction in
1981 (48). One important requirement for these methods is the stationarity
requirement for the signals during the analyzed time window. Therefore, continuous
data of dynamic processes are often split artificially into sequential shorter
(overlapping) segments. The shorter the length of each segment, the higher the time
resolution and the more likely the stationarity of data within the segment. The well
known conflict between time and frequency resolution hampers, however, the
assessment of rhythmic components of heart rate over ultra short periods of time (82).
This might become substantially important in situations where the state of autonomic
cardiovascular control is characterized by transient phenomena and/or rapid
alterations. In this study, we adopted a specific algorithm that allows inferring rapid
alterations of parasympathetic induced cyclic variations of heart rate with a time
resolution approximating 20 s.
2.2.4.2 Application of methods: the effect of body posture
It is well known that, during gradual head-up tilt, the spectral profile of HRV
represents a progressive shift of powers toward lower frequencies (173). This has
been related to progressive baroreceptor unloading leading to diminished
parasympathetic influences on HR with a generally increased sympathetic outflow
(154). Our data in Table 2.2 indicate that the redistribution of spectral powers upon
posture change can be assessed by using power spectral analysis of ultra short data
segments. Although a 20-second time frame puts a limit on the lowest measurable
-CHAPTER II-
55
heart rate oscillation, the spectral HRV-profile appears well preserved during steady
state conditions of 10 min recording in the standing or supine positions (Table 2.2).
During parabolic flight, neural control of HR is a dynamic process comprising
continuous stimulation of baroreceptor reflexes in association with varying
hydrostatic and intrathoracic pressures (117;159). Hydrostatic stimulation of
baroreceptor reflexes largely depends on the blood pressure gradients along the
longitudinal axis of the human body, which is a posture-dependent phenomenon
(116;159;174). In standing subjects, body fluid hydrostatic pressure gradients are
cancelled out during 0 Gz periods, while they are augmented during the 1.8 Gz
periods of parabolic flight (156;175). Changing gravity periods of parabolic flight will
therefore affect cardiac chronotropy significantly over time in standing subjects (176).
Accordingly, postural differences in mean HR during 1 Gz are more pronounced
during 1.8 Gz, while being abolished during 0 Gz periods of parabolic flight (Figure
2.6). We were able to show that postural differences in the spectral distribution of
HRV are also abolished during the 0 Gz phases (loss of hydrostatic pressure
gradients) and amplified during the 1.8 Gz phases (increased hydrostatic pressure
gradients). We therefore felt confident in our data to reflect dynamic parasympathetic
influences on HR among distinct gravity phases of parabolic flight. It is important to
note that such small time windows prohibit any judgment on the dynamic behavior of
sympathetic induced rhythmic heart rate oscillations, as these generally respond more
slowly (177).
2.2.4.3 Parasympathetic heart rate modulation during parabolic flight
Specific time delays of autonomic efferent activities (178) may affect the ability of the
two branches of the ANS to modulate heart rate among parabola gravity phases.
Cardiac chronotropic effects of parasympathetic stimulation have a very short latency
period and also resolve very quickly when fiber activity ceases (179); the maximum
response to a single parasympathetic stimulus occurs within the same heart beat (180).
Therefore, parasympathetic nervous system control of heart rate is able to affect the
HF fluctuations in heart rate during periods of stable gravity in parabolic flight.
According to this, it is important to note that breathing frequencies did not drop below
the lower limit of the HF component (0.18 Hz vs. 0.15 Hz). Our results in standing
subjects (Table 2.3) confirm previously published observations of enhanced
parasympathetic activity causing heart rate to slow down following a decrease in Gz
during parabolic flight (162;163). During an increase in Gz, parasympathetic activity
-CHAPTER II-
56
is suppressed causing heart rate to increase. Remark that the alterations in
parasympathetic activity between different gravity phases of parabolic flight are better
assessed by HF in proportional units, as the influence of variations in total power are
eliminated (181).
Sympathetic heart rate responses have a great latency period, decay rather slowly and
there may be little change in cardiac chronotropy for less than 10 seconds (177). It is
therefore impossible to capture the full periodicity of sympathetically mediated heart
rate oscillations (6.5 s – 25 s) in the remaining part of the stable gravity periods of
parabolic flight. The latter prevents us of doing any judgment on the behavior of the
sympathetic nervous system. Previous studies recorded the arterial blood pressure
during parabolic flight (159;176). These studies suggested unloading of baroreceptors
during hypergravity, leading to an increase in sympathetic neural outflow. Using
direct recordings of muscle sympathetic nerve activity, Iwase et al. (160) confirmed
that hypergravity provokes a reflex activation of the sympathetic nervous system
during parabolic flight. On the contrary, baroreceptor stimulation during the
microgravity periods leads to sympathetic withdrawal. In addition to our results, the
results from Iwase et al (160) support the concept of a sympatho-vagal interaction
(82), according to a central push-pull pattern of neural organization (182;183). In this
respect, our result in standing subjects could be interpreted as a shift towards
sympathetic predominance in heart rate control following an increase in Gz, against a
shift towards parasympathetic predominance following a decrease in Gz.
2.2.4.4 Autonomic interaction effects during parabolic flight
Variable body fluid hydrostatic pressure gradients among different gravity phases of
parabolic flight induce variable pressure loads at both sites of the baroreflex loop
(176).
Consequently,
changes
in
central
venous
blood
pressure
(static
cardiopulmonary volume reflex) in combination with variable pressure loads in the
arterial system (dynamic arterial baroreflex) will induce peripheral interaction effects
between sympathetic and parasympathetic efferent activities (157). The overall effect
of this interaction is demonstrated by the net behavior of heart rate during parabolic
flight: a rise in heart rate following an increase in Gz is generally slower than a heart
rate drop following a decrease in Gz (Figure 2.3) (157).
-CHAPTER II2.2.5
57
Conclusion
In conclusion, this study shows that frequency domain analysis of HRV is feasible for
ultra-short time series. The present data illustrate that spectral transformation of RR
time series obtained from distinct gravity periods (± 20s) of parabolic flight allows
isolating parasympathetic induced rhythmic components of heart rate. During 1 Gzconditions, we have a normal upright situation with less parasympathetic activity
causing heart rate to be higher compared to supine. Due to greater hydrostatic
pressure gradients, this effect is augmented during the 1.8 Gz-conditions. In contrast,
during 0 Gz-conditions, enhanced parasympathetic control in the upright position
eliminates the difference in cardiac chronotropy compared to supine.
Related publications:
Verheyden B, Beckers F, Aubert AE. Spectral characteristics of heart rate
fluctuations during parabolic flight. Eur J Appl Physiol 2005;1-12.
Aubert AE, Beckers F, Verheyden B, Pletser V. What happens to the human heart in
space? Parabolic flights provide some answers. ESA-Bulletin. 2004; 119: 30-38.
Verheyden B, Beckers F, Aubert AE. Frequency analysis of cardiovascular variability
during parabolic flight. J Gravit Physiology. 2003; 10: 85-86
Beckers F, Seps B, Ramaekers D, Verheyden B, Aubert AE. Parasympathetic heart
rate modulation during parabolic flights. Eur J Appl Physiol 2003; 90(1-2): 83-91
-CHAPTER II-
58
2.3
Dynamic cardiovascular control during 60 min of thermoneutral
head-out of water immersion
2.3.1
Introduction
Thermoneutral (34.5°C) water immersion (WI) has attracted wide interest in the past
decades for studying fluid and electrolyte homeostasis in humans. Head-out of WI
increases the central blood volume by redistributing venous blood and extracellular
fluids from the lower to the upper part of the body (184-190). The concomitant rise in
venous return induces distention of the heart and adjacent vessels mimicking the
effect of an increased transmural central venous pressure during weightlessness
(117;159;191). Accordingly, the WI model can be used to simulate the initial
cardiovascular response to microgravity under standardized conditions in humans on
Earth (192-196). In both conditions, central blood volume expansion elicits
simultaneous stimulation of cardiopulmonary (low-pressure) and aortic (highpressure) baroreceptors. The result is an integrated baroreflex response with
bradycardia and vasodilatation (123;197-200).
Up till now, only little attention has been directed towards the initial time course of
integrated circulatory control during WI. There is mounting evidence that WI elicits a
progressive transfer of fluid from the interstitial compartment of the legs to the
intravascular space, leading to a peak rise in plasma volume of about 16 % (185;186).
The subsequent renal response acts to minimize a further plasma volume elevation,
rather than to bring about a reduction back to baseline (185). We therefore
hypothesized that integrated baroreflex control during WI comprises an initial
dynamic process that further stabilizes up to 60 min of WI. The purpose of this study
was to determine hemodynamic characteristics and baroreflex control of heart rate and
systemic vascular resistance during 1 hour of thermoneutral WI in healthy young
subjects. We further sought to assess pre- and post-immersion baroreflex responses to
a 10-min standing test.
2.3.2
Methods
2.3.2.1 Study population
Twenty healthy young subjects (10 females) participated in the immersion study.
They exhibited the following characteristics: female age range 22-23 yr; height 166 ±
-CHAPTER II-
59
5 cm; mass 61 ± 6 kg; male age range 19-23 yr; height 183 ± 6 cm; mass 76 ± 12 kg.
On average, men were taller and heavier than women (p < 0.001). Ten age- end
gender-matched healthy young subjects underwent the same protocol during a seated
time control study. This control group consisted of 5 male subjects; age range 22-25
yr; height 180 ± 9 cm; mass 75 ± 10 kg; 5 female subjects; age range 22-24 yr; height
173 ± 7 cm; mass 59 ± 3 kg. The differences in weight and mass between the two
groups were not statistically significant. All subjects had a negative history of
cardiovascular, neurological or kidney disease. No medication was used that might
affect the circulation or circulatory control. No complications occurred. All subjects
were informed about the procedure. The protocol was approved by the local Medical
Ethical committee.
2.3.2.2 Study design
Data collection was performed in a temperature controlled laboratory (25-27°C) in the
morning before 1 PM. All subjects were instructed to abstain from alcohol and
caffeine and to avoid strenuous exercise from the day before experimentation. Eating
and drinking were not allowed for 6 h before the start of the experiment. At the start
of the experiment and after emptying the bladder, all subjects ingested 330 ml of
water and were placed in the supine position for a 15-min hemodynamic equilibration
period. In subjects referred to the immersion experiment, this was followed by: 1) a
10 min baseline standing recording, 2) a 10 min baseline seated recording, 3) 60 min
of seated thermoneutral WI to the xiphoid, and 4) a 10 min post-immersion standing
recording. WI was carried out in a pre-filled tank with tap water of 34.5 - 35.0 °C.
The water temperature was continuously evaluated by a mercury thermometer.
Subjects were made to sit upright in a chair suspended from the ceiling in a vertical
hoist. WI was performed by lowering the chair with the subject into the water. At the
end of WI, subjects stepped out of the immersion tank and dried with a towel to
prevent cooling. In those subjects referred to the time control experiment, apart from
the standing tests, the same procedures were undertaken with subjects sitting upright
in an empty immersion tank for 60 min.
2.3.2.3 Data acquisition
Beat-to-beat arterial blood pressure was measured non-invasively by means of a
servo-controlled
photoplethysmograph
(Portapress,
TNO,
Amsterdam,
The
Netherlands) placed on the midphalanx of the right middle finger (97). The hand was
positioned at heart level to prevent hydrostatic pressure differences, and held in place
60
-CHAPTER II-
using an arm support. Respiratory rate was evaluated using an abdominal pressure
sensor (MR10, Graseby Medical Limited, Hertfordshire, UK). Finger blood pressure
and the respiratory frequency were digitized at 1000 Hz using an external A/D
converter (DATAQ Instruments Inc., Akron, OH, USA) and stored on a laptop
computer. The equipment was attached in all subjects during the 15-min period of
baseline supine rest prior to the experiment.
2.3.2.4 Hemodynamic analysis
Mean arterial pressure (MAP) was the true integral of the arterial pressure wave over
one beat divided by the corresponding beat interval. Pulse pressure (PP) was
calculated as the difference between systolic (SAP) and diastolic arterial pressure
(DAP). Heart rate (HR) was computed as the inverse of the inter-beat interval (IBI),
and expressed as beats per minute. Beat-to-beat changes in stroke volume (SV) were
estimated by modeling flow from the finger arterial pressure waveform (Modelflow,
TNO Biomedical Instrumentation) (9;99;201). The patient’s gender, age, height and
weight were inserted in the model to determine Modelflow parameters using
previously established equations (100). Cardiac output (CO) was assessed by the
product of SV and HR. Systemic vascular resistance (SVR) was obtained through a
self-adapting process in which an initial value is assumed for the first beat. The ratio
of MAP to CO for this first beat defines a new resistance value which is used in the
model for the next beat. Within 5 beats from the start, the model resistance stabilizes
to actual SVR and further allows close tracking of changes in real SVR (105). Beatto-beat hemodynamic values were computed and averaged per 5-min periods during 1
h of WI or seated time control. Stroke volume, CO and SVR were set at 100% in the
baseline seated position and variations were further expressed as percentages (%) of
this baseline.
2.3.2.5 Baroreflex sensitivity
Beat-to-beat IBI and SAP time series were interpolated, resampled at 2 Hz, and
divided at 90 % overlapping segments of 128 s. Each segment was detrended,
Hanning windowed, and fast Fourier transformed (165). The obtained spectral
resolution was 0.0078 Hz. Power spectral density (PSD in ms2/hz for tachograms and
mmHg2/hz for systograms) was then computed in the low-frequency band (0.04 to
0.15 Hz) for each time window.
-CHAPTER II-
61
The BRS was estimated in time domain using the cross-correlation method (79). The
SAP and IBI time series were resampled at 1 Hz. In a 10-s window, the correlation
and regression slope between SAP and IBI were computed. Delays of 0- to 5-s
increments in IBI were computed, and the delay with the highest positive coefficient
of correlation was selected. The slope between SAP and IBI was recorded as a BRS
estimate if the correlation was significant at p = 0.01.
2.3.2.6 Statistical analysis
Statistical analysis was performed with SPSS version 11.5 for Windows (Scientific
Packages for Social Sciences, Inc., Chicago, IL, USA). Variables were tested for
normality with the Kolmogorov-Smirnov goodness of fit test. Spectral powers were
transformed by calculating the natural logarithm to achieve a normal distribution.
Responses to WI and post-immersion standing were analyzed using the nonparametric test for two related samples (Wilcoxon signed rank test) or a 2-tailed
paired t-test, where appropriate. Differences in Tau (time delay of BRS-computations)
between conditions were tested using the X2-test. Two-way repeated-measures
analysis of variance was used to compare hemodynamics patterns during WI and
seated time control. If the interaction-effect was significant, in each group, post hoc
contrast analysis was performed with Scheffé’s F test.
2.3.3
Results
2.3.3.1 Initial cardiovascular response to WI
Cardiovascular characteristics from the 10 min baseline recordings in the seated
position and during the first 10 min of WI are summarized in Table 2.4. Systolic,
diastolic and mean arterial blood pressures were not changed, whereas PP was
significantly increased compared to baseline. Accordingly, there was a drop in HR,
and although SV increased, CO and SVR were maintained during the first 10 min of
WI. Low-frequency powers of spontaneous IBI- and SAP-oscillations decreased,
whereas BRS increased after the hemodynamic changes induced by WI. Furthermore,
WI tended to shift the distribution of Tau (the 0- to 5-s optimal delay of IBI to SAP)
toward lower values (p = 0.27) (Figure 2.9 A). The respiratory frequency was not
changed during WI. During seated time control experiments no measurable changes
were observed within these periods (Figure 2.8).
-CHAPTER II-
62
Table 2.4: Cardiovascular data during 10 min periods before and at the start of 60
min water immersion (WI)
10 min seated
before WI
Hemodynamic data
SAP (mmHg)
DAP (mmHg)
MAP (mmHg)
PP (mmHg)
HR (bpm)
SV (%)
CO (%)
SVR (%)
120 ± 13
63 ± 12
82 ± 11
57 ± 8
77 ± 6
100
100
100
Spectral analysis
IBI LF power (ms2)
0 - 10 min
during WI
p-value
120
59
79
61
69
111
103
90
NS
NS
NS
0.047
0.001
0.022†
NS†
NS†
± 14
± 13
± 12
±9
±6
±5
±6
±6
2287 ± 971
1352 ± 904
0.001
16 ± 7
9 ±4
0.001
Respiration rate (breaths.min )
16 ± 2
17 ± 3
NS
Time-domain analysis
Time-domain BRS (ms/mmHg)
13 ± 3
20 ± 8
0.001
2
SAP LF power (mmHg )
-1
Data are presented as mean value ±SD. BRS = baroreflex sensitivity; CO = cardiac
output; DAP = diastolic arterial pressure; HR = heart rate; IBI = interbeat interval; LF
= low-frequency; MAP = mean arterial pressure; PP = pulse pressure; SAP = systolic
arterial pressure; SV = stroke volume; SVR = systemic vascular resistance. †
Wilcoxon signed rank test.
2.3.3.2 Hemodynamic trends during 60 min WI
Hemodynamic patterns during 1h of WI and seated time control are depicted in Figure
2.8. Shaded areas indicate baseline seated conditions before the intervention. During
the first 20 min of WI, HR increased gradually from 68 ± 7 toward 73 ± 7 beats per
min, but remained lower compared to baseline over the entire 60-min immersion
period (p = 0.002). Accordingly, the BRS recovered partially after 20 min of WI
(from 21 ± 11 to 16 ± 9 ms/mmHg) and remained elevated compared to baseline (p =
0.035). Furthermore, there was a sustained elevation in SV of 11 ± 5 % (p = 0.022)
while CO increased gradually by 8 ± 5 % within the first 20 min of WI (p = 0.037). At
the same time, SVR decreased progressively by 17 ± 6 % (p = 0.014) resulting in a
decline in MAP of 8 ± 4 mmHg (80 ± 2 vs. 74 ±13 mmHg; p = 0.034) after 20 min of
WI and until the end of the intervention (Figure 2.8). Hemodynamic patterns were
-CHAPTER II-
63
independent of gender and were stable during seated time control experiments (Figure
2.8).
2.3.3.3 Pre- and post-immersion standing response
Cardiovascular data from 10 min standing tests before and after 60 min of WI are
summarized in Table 2.5; no significant differences were observed between pre- and
post-immersion sessions. Accordingly, cardiovascular responses to standing were
comparable before and after WI.
Table 2.5: Cardiovascular data of 10-min standing tests preceding and following 60
min of water immersion (WI)
10 min stand
before WI
Hemodynamic data
SAP (mmHg)
DAP (mmHg)
MAP (mmHg)
PP (mmHg)
HR (bpm)
118
69
85
49
83
Spectral analysis
IBI LF power (ms2)
± 13
± 13
± 12
±6
±7
10 min stand
after WI
115
70
85
45
86
± 17
± 15
± 15
±9
± 10
p-value
NS
NS
NS
NS
NS
2341 ± 956
2787 ± 1126
NS
26 ± 11
32 ± 18
NS
Respiration rate (breaths.min )
16 ± 2
16 ± 2
NS
Time-domain analysis
Time-domain BRS (ms/mmHg)
9 ±2
9 ±4
NS
2
SAP LF power (mmHg )
-1
Data are presented as mean value ±SD. BRS = baroreflex sensitivity; DAP = diastolic
arterial pressure; HR = heart rate; IBI = interbeat interval; LF = low-frequency; MAP
= mean arterial pressure; PP = pulse pressure; SAP = systolic arterial pressure.
Figure 2.8: Hemodynamic trends during 60 min of water immersion to the xiphoid (solid circles), and during
corresponding seated time control (open circles). Values are presented as 5-min averages ± SEM. At 0 min (shaded
areas), all subjects were quietly seated at the start of the protocol. BRS = baroreflex sensitivity. CO = cardiac output.
HR = heart rate. MAP = mean arterial pressure. SV = stroke volume. SVR = systemic vascular resistance. * Withingroup difference (p < 0.05) compared to the last 5 min of WI
64
-CHAPTER II-
-CHAPTER II-
65
On average, the hemodynamic response to standing comprised a rise in HR of 8 ± 7
beats per min (p = 0.001) together with a reduction in PP of 9 ± 8 mmHg (p = 0.031).
Mean arterial blood pressure was well maintained upon standing, whereas DAP was 7
± 8 mmHg higher compared to the baseline recording in seated position (p = 0.022).
Furthermore, the low-frequency powers of IBI- and SAP-oscillations increased (both
p < 0.05), whereas the BRS decreased by 4 ± 4 ms/mmHg (p = 0.001) with posture
changes from seated to standing. Accordingly, in the standing position, there was a
redistribution of Tau toward higher values (p = 0.003) (Figure 2.9 A versus B). No
significant gender differences appeared in the cardiovascular response to standing
before and after WI.
Figure 2.9: Distribution of the 0- to 5-s optimal delays of IBI to SAP, selected to
estimate baroreflex sensitivity of heart rate. A: 10 min baseline seated versus WI. B:
pre- and post-immersion standing.
2.3.4
Discussion
The present findings demonstrate that thermoneutral WI leads to a rapid rise in SV,
and despite a lower HR, CO increases progressively compared to baseline seated
control. The subsequent reduction in SVR is suggested to account for a progressive
66
-CHAPTER II-
decline in MAP. Accordingly, spontaneous BRS is increased and the low-frequency
powers of IBI- and SAP-oscillations are decreased after the hemodynamic changes
induced by WI. Together, these findings indicate enhanced vagal-cardiac outflow and
diminished sympathetic influences on the heart and peripheral vasculature. It appears
that circulatory control during WI comprises a dynamic process that stabilizes after a
period of 20 min. The baroreflex response to standing was not altered after 60 min of
thermoneutral WI.
2.3.4.1 Initial cardiovascular response to WI (0-10 min)
The rapid rise in SV indicates central blood volume expansion from the very onset of
WI (Figure 2.8). This is further supported by previous echocardiographic indications
of distended heart chambers during WI (202). The result is a rapid rise in PP and a
drop in HR, suggesting vagal-cardiac chronotropic inhibition, due to pulsatile aortic
baroreceptor stimulation (174). In accordance, compared to baseline, we found that
the spontaneous cardiac BRS was significantly increased during WI (Table 2.4). Since
baroreflex modulation of heart rate results primarily from vagal-cardiac adjustments
(203) the increased BRS further suggests a rise in vagal-cardiac outflow during WI
(192;200).
Arterial blood pressure changes during WI depend in part on the portion of the body
covered by water because this determines the hydrostatically induced vascular
transmural pressure in the lower limbs (174;202). Indeed, progressive reductions in
leg-vascular transmural pressure with graded WI may ultimately abolish the ability of
resistance vessels to dilate in response to static and pulsatile thoracic baroreceptor
stimulation (204). Although in the present study, a reduction of SAP low-frequency
power suggests sympathetic vascular withdrawal (Table 2.4), reduced leg-vascular
transmural pressures likely account for the initially maintained SVR and MAP during
WI.
2.3.4.2 Sustained cardiovascular response to WI (60 min)
Previous studies have demonstrated a peak rise in plasma volume of about 16% due to
a transcapillary shift of extracellular fluids to the intravascular space during WI (185).
The accompanying elevation of central venous pressure stimulates cardiopulmonary
low-pressure receptors (186). This is indicated in the present study by a gradual
attenuation of SVR within the first 20 min of WI (Figure 2.8). Suppression of the
muscle sympathetic nerve activity during WI (205) may account for this reduction in
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67
SVR, thereby regulating systemic blood pressure; there was a gradual decline of MAP
below baseline seated control values. At the same time, a reflex-increase in HR with a
progressive reduction in BRS suggests static inhibition of aortic baroreceptor reflexes
buffering a further arterial pressure fall (174). It has been shown previously that
continued stimulation of low-pressure cardiopulmonary receptors overrides the
arterial baroreflex buffering (199). This is indicated by sustained reductions HR and
MAP until 60 min of WI (Figure 2.8) (174).
2.3.4.3 Post-immersion orthostatic blood pressure control
Assuming the upright posture is associated with the combined unloading of both lowpressure cardiopulmonary and high-pressure arterial baroreceptors. Orthostatic blood
pressure control involves baroreflex responses that are opposite in direction compared
to those observed during WI. Indeed, spontaneous cardiac BRS decreases with posture
changes from seated to standing, together with a shift toward higher values in the
distribution of IBI-to-SAP time delays (Figure 2.9 A versus B). An IBI-to-SAP lag of
~0 s can be expected for vagal control of HR, while increasing time lags result from
the combined effect of vagal and sympathetic baroreflex regulation of cardiac cycle
length (37). The increased IBI-to-SAP time-lag upon standing therefore indicates a
shift towards more sympathetic influence on dynamic heart rate control.
Adequate baroreflex function importantly relies on the amount of central blood
volume. It is generally accepted that central hypervolemia during WI induces an
increased rate of renal fluid and electrolyte excretion, which provokes progressive
dehydration (184). This in turn could elicit central hypovolemia, favoring impaired
dynamic regulation of HR and arterial blood pressure upon post-immersion standing
(194;206). In the present study, however, orthostatic blood pressure control was well
maintained during a 10 min standing test after 60 min of thermoneutral WI,
suggestive of a preserved circulating blood volume. This finding corresponds with the
hypothesis that diuretic responses to WI act to minimize elevations of plasma volume,
rather than to bring about a net reduction, even up to 12 hours of WI (185). Finally,
according to a previous report by Watenpaugh et al. (207), men and women appear to
respond largely similar to WI and neither group exhibited altered cardiovascular
responses to standing after 60 min of WI.
2.3.4.4 Perspectives
Our finding that CO is increased and the vasculature dilated during WI corresponds
with a recent report by Norsk et al. (123) of systemic vasodilatation at the onset of
68
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weightlessness and until at least one week into spaceflight. Further resemblance with
real microgravity exposure comprises increased vagal-cardiac baroreflex adjustments
of HR, which is probably due to increased pulsatile arterial baroreceptor stimulation
(139;140;166). Although physical features that drive the hemodynamic forces during
WI and real microgravity are quite different (191); the model of WI generally appears
to simulate spaceflight-induced integrated baroreflex adjustments of HR and vascular
resistance with sufficient accuracy.
Our data also have implications for understanding how gravity stresses patients with
heart failure. Gravity appears to be a constant burden for heart failure patients and
may even aggravate their condition. Conversely, when immersed in thermoneutral
water, the circulatory condition in patients with severe heart failure greatly improves
due to a rise in SV and a reduced SVR (208). It is important to note that, in the final
stage of heart failure, oedema re-absorption in lower limbs might result in pulmonary
oedema, which is an important complication that should be avoided.
Coruzzi P et al. (209) showed that WI identifies a subset of hypertensive patients
characterized not only by an impaired renal vasodilation response to WI but also by a
different humoral and neural cardiac autonomic profile in baseline conditions. These
findings suggest that WI may represent a useful additional tool in the detection of
hypertensive subjects, characterized by neurohumoral alterations.
2.3.4.5 Limitations
Because of the peripheral location of the finger in our blood pressure measurements, it
can be argued that local vasoconstriction could have led to erroneous estimations of
MAP. However, effects of these phenomena are abolished by a built-in calibration
algorithm (97). No data are available on total plasma volume and/or central venous
pressure during WI; therefore, we were unable to assess direct relationships between
fluid dynamics and baroreflex responses of HR and SVR during the initial time course
of WI. Finally, sympathetic outflow was estimated indirectly in this study from the
continuous behavior of HR and blood pressure, rather than from more direct methods
such as muscle sympathetic nerve activity recordings. We chose for non-invasive
measurements to avoid possible confounding influences of the applied method itself.
-CHAPTER II2.3.5
69
Conclusion
It is concluded that thermoneutral WI in healthy young subjects leads to a baroreflexmediated systemic vasodilatation and vagal-cardiac bradycardia, mimicking the
circulatory response to real microgravity. On the other hand, baroreflex control during
WI is a dynamic process that stabilizes after 20 min. Whether this may be explained
by a new equilibrium between transcapillary intravascular fluid shifts and the renal
diuretic response after 20 min of thermoneutral WI should be further investigated.
Related publications:
Verheyden B, Beckers F, Aubert AE. Heart rate variability during water immersion.
J Gravit Physiology. 2003; 10: 81-82
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70
2.4
Respiratory modulation of cardiovascular rhythms before and
after short-duration human spaceflight
2.4.1
Introduction
In astronauts returning from short-duration (1-2 wks) spaceflight, various
microgravity-induced cardiovascular adaptations are manifested by orthostatic
tachycardia in all, and hypotension in up to two-thirds of the astronauts (145).
Cardiovascular adaptations include an excessive reduction in stroke volume upon
standing (152), probably due to combined effects of hypovolemia (118), cardiac
remodelling and compromised diastolic function (130). Further evidence exists of
postflight diminished vascular control mechanisms (210), together with attenuated
carotid baroreceptor cardiac reflexes following exposure to simulated and real
microgravity (148;149). The overall result of these adaptations is a reduction in
vasoconstrictor reserve, which has been identified as an important contributor of
orthostatic intolerance after spaceflight (145;211).
Although symptoms of orthostatic intolerance disappear rather quickly the autonomic
control system recovers more slowly after spaceflight. There is mounting evidence
that blunted spontaneous cardiac baroreflex sensitivity (BRS) persists for at least two
weeks after landing (143;150). Accordingly, respiratory sinus arrhythmia (RSA), i.e.
the respiration-synchronous cyclic component of variability in cardiac cycle-length,
has not yet been fully recovered within this period (140). Postflight investigations
extending to time frames of more than two weeks are lacking; therefore, it is unclear
how long the alterations in dynamic heart rate control need to recover. Early groundbased experiments have suggested that 25 days of recovery should be sufficient to
return to preflight conditions (148).
In the autonomic intact state, mechanical effects of respiration on stroke volume are
opposed by changes in heart rate (HR) (36), either caused by the baroreflex (37;212)
or by central feed-forward effects of respiration on the vagal motor nucleus (42). This
mechanism is suggested to buffer variability in cardiac output (213), which in turn
appears to be the main source of respiration-synchronous fluctuations in arterial blood
pressure (35). There is mounting evidence from cholinergic blockade experiments that
a reduction in vagal-cardiac nerve traffic, and thus RSA, leads to enhanced respiratory
arterial pressure fluctuations (35). Targeting vagal cardiac outflow by electrical
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71
cardiac pacing showed that the buffering role of RSA only accounts when mechanical
effects of respiration are larger than those in supine humans (41). Whether reductions
in RSA after spaceflight can be associated with augmented respiratory blood pressure
dynamics in orthostatic tolerant astronauts has been argued (143;214).
In the present study, we used a paced breathing protocol to assess respiratory
modulation of cardiovascular rhythms in 5 cosmonauts, before short-duration
spaceflight (10 days), and after 1 and 25 days upon return to Earth. At the same time,
we set out to determine the baroreflex response to standing, using transfer function
BRS computations (215). With the above background we hypothesized that, if
diminished RSA after spaceflight is related to a vagal-cardiac neural deficit, the
postflight circulatory response to standing should be associated with higher
respiratory arterial pressure fluctuations compared to preflight.
2.4.2
Methods
2.4.2.1 Subjects
Five male cosmonauts who each took part in one of three different (10-11 day) ESA
Soyuz missions (Odissea, Cervantes, Delta) to the International Space Station (ISS)
were enrolled in this study. At preflight data collection, average cosmonaut age was
40 (SD 3) yr, height 180 (SD 4) cm, and weight 76 (SD 10) kg. Because of time
constraints, no routine physical exercise was performed on board the ISS to counter
post-flight orthostatic intolerance. Upon return, there was also no specific
rehabilitation program. Each subject was informed about the experimental procedures
and signed an informed consent form. The experimental protocol was approved by the
Ethics Committee of the local university and the ESA Medical Board. The study
complies with the Declaration of Helsinki.
2.4.2.2 Experimental design
Subjects refrained from alcohol and caffeine for at least 9 h before data collection.
Preflight (between 30 and 45 days before launch), as well as early postflight (R + 1;
one day after landing) data collection was performed in the Medical Building of
Gagarin Cosmonaut Training Center (Moscow, Russia) at ambient room temperature.
Late postflight (R + 25; between 25 and 28 days after landing) data collection was
performed in a temperature controlled laboratory (21-23°C) in the University Hospital
of Leuven, Belgium. Table 2.6 shows the days of data collection for pre- and
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72
postflight measurements in the individual cosmonauts together with their weight at the
time of measurement.
Table 2.6: Cosmonauts body weight on pre- and postflight sessions
Preflight
Early Postflight
Late Postflight
Cosmonaut
Day
Weight (kg)
Day
Weight (kg)
Day
Weight (kg)
I
II
III
IV
V
L - 30
L - 30
L - 45
L - 45
L - 45
88
64
68
81
81
R+1
R+1
R+1
R+1
R+1
84
58
65
79
78
R + 25
R + 25
R + 28
R + 25
R + 25
89
63
68
80
82
L – 30 and L – 45 represent 30 and 45 day before launch. R + 1, R + 25 and R + 28
represent 1, 25 and 28 days after landing.
Recording sessions took place in the morning before 1 PM. At the beginning of each
session, subjects rested quietly in the supine position and breathed at a comfortable
uncontrolled rate for about 15 minutes until hemodynamic equilibration. Subjects
were then instructed to pace their breathing to an audio stimulus with visual feedback. A sequence of tones was generated by a laptop computer and target breathing
sequences were displayed graphically in real time on the screen (216). The laptop
computer was positioned on an adjustable stand so that the subject could view the
screen comfortably throughout the entire protocol. Two breathing protocols were
performed in succession in which respiratory sequences were evenly spaced in time at
preset rates of 12 breaths.min-1 or 0.2 Hz (normal paced breathing, NPB) and 6
breaths.min-1 or 0.1 Hz (slow paced breathing, SPB) to ensure blood pressure
variations at these frequencies. Each breathing frequency was held for 3 minutes and
interspaced by a 1-min rest period (spontaneous breathing). Subjects were then moved
to the standing position and after a minimum of 10 minutes the breathing protocol was
repeated. Although respiratory intervals were controlled, subjects were able to
comfortably control depth of inspiration to preserve normal ventilation and to prevent
hypocapnia (217). Subjects were instructed by the authorities of the Russian human
spaceflight program to be seated for 3-5 min after supine recording and before
standing up. The same procedures were reproduced at pre- and postflight test sessions.
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73
2.4.2.3 Instrumentation
The electrocardiogram (ECG) was recorded and beat-by-beat arterial pressure was
estimated
with
a
servo-controlled
photoplethysmograph
(Portapress,
TNO,
Amsterdam, The Netherlands) placed on the midphalanx of the right middle finger
(97). The latter was positioned at heart level and held in place using an arm sling to
prevent hydrostatic pressure differences while standing. Finger cuff pressures were
calibrated against intermittent arm-cuff pressures (STBP-780, Colin, Komaki, Japan)
and used to track arterial blood pressure changes. Respiratory rate was evaluated
using an abdominal pressure sensor (MR10, Graseby Medical Limited, Hertfordshire,
UK). Finger blood pressure, ECG and respiration frequency were digitized at 1000 Hz
using an external A/D converter (DATAQ Instruments Inc., Akron, OH, USA) and
stored on a laptop computer.
2.4.2.4 Data processing
A file containing the R-R intervals (RRI) was created for each breathing frequency (3min period). Mean arterial pressure (MAP) was calculated as the true integral of the
arterial pressure wave divided by the corresponding beat interval from end-diastole to
end-diastole. Pulse pressure (PP) was calculated as the difference between systolic
(SAP) and diastolic arterial pressure (DAP). Two linear filters were applied to correct
for data points outside a limit interval (165). The resulting beat-to-beat hemodynamic
time series were interpolated using a third order cubic-spline approximation and were
resampled at 2 Hz to construct equidistant time series. Data sets comprising 128 s
(256 samples) and sliding in 16 s steps were subdivided. This process resulted in 4
segments of data. The DC component was removed by subtracting the mean value and
a Hanning window was applied. A non-parametric ‘run test’ of means and mean
square values was used to validate the stationarity of data within 5% of the confidence
limits (165). In the resulting time windows, power spectral density was averaged using
Fast Fourier transform. The spectral resolution for all estimates equalled 0.0078 Hz.
Respiratory powers were expressed as the area under the spectrum from 0.08 to 0.12
Hz (slow paced breathing) and from 0.18 to 0.22 Hz (normal paced breathing). During
normal paced breathing a second spontaneous rhythm occurring over an approximate
10 s cycle and resulting in a low frequency band (LF: 0.04-0.15 Hz) was obtained as
well. Power spectral units for RRI and arterial pressure fluctuations were squared
amplitudes. The transfer function gain, phase and squared coherence between SAP
and RRI were estimated by cross-spectral data derived with the same set of
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74
parameters used for power spectral analysis (215). All analysis software has been
developed in house using Labview 7.1 (National Instruments, Austin, TX, USA) for
Windows.
2.4.2.5 Statistical analysis
Statistical analysis was performed with SPSS version 8.0 for Windows (Scientific
Packages for Social Sciences, Inc., Chicago, IL, USA). Data are given as means ± SE
unless stated otherwise. Spectral data were logarithmically transformed before further
statistical testing. Hemodynamic measurements, spectral indices and cross-spectral
data were averaged per body position and breathing frequency and analyzed across
conditions using multiple repeated-measures ANOVA with pre- and postflight
sessions, body posture and breathing frequency as test variables. Differences between
pre- and postflight sessions were tested with Bonferroni-correction for repeated
measurements where appropriate. Pearson’s correlation coefficient (r) was computed
to evaluate the relationship between the mean RRI and RSA. P values < 0.05 were
considered statistically significant.
2.4.3
Results
2.4.3.1 Subjects
Cosmonaut body weight at pre- and postflight sessions is given in Table 2.6. Body
weight at early postflight sessions was significantly lower (p = 0.042) compared with
preflight [73 (SD 11) kg vs. 76 (SD 10) kg] and returned to preflight bodyweight at
late postflight sessions [76 (SD 11) kg]. All five cosmonauts completed the entire
pre- and postflight protocols and were able to remain upright for the entire test
duration. None suffered from problems of postflight orthostatic intolerance.
2.4.3.2 Hemodynamic data
Figure 2.10 shows the pre- and postflight recordings of HR and arterial blood pressure
in one representative cosmonaut. The upper panels show the results from slow paced
breathing (SPB: 0.1 Hz); results from normal paced breathing (NPB: 0.2 Hz) are
presented in the lower panels. Blood pressure was variable but did not drop upon
standing at both breathing frequencies. The corresponding averaged hemodynamic
data from all five cosmonauts are given in Table 2.7.
-CHAPTER II-
75
Figure 2.10: Supine and standing heart rate (HR) and arterial blood pressure (BP)
recordings in a representative cosmonaut before flight (preflight), early postflight
(R+1) and late postflight (R+25). Supine and standing recordings cover time periods
of 3 minutes. Upper panels show results from slow paced breathing (6 breaths.min-1).
Lower panels show results from normal paced breathing (12 breaths.min-1).
Comparing pre- to postflight conditions shows that, in both the standing and supine
postures, HR differed significantly between sessions; post-hoc analysis indicated an
increased HR at early postflight sessions compared with preflight (supine p = 0.015;
standing p = 0.010). The postflight changes in HR had returned to preflight levels
after 25 days upon return. Supine and standing systolic, diastolic and mean arterial
pressure did not differ between pre- and postflight conditions. Also pulse pressure was
not significantly different from preflight. On standing up, there was an increase in
HR, DAP and MAP, whereas PP decreased significantly (all p < 0.05). The increase
in HR upon standing was most pronounced at early postflight sessions (Figure 2.11).
93 ± 8
78 ± 8
128 ± 7
47 ± 2
87 ± 7
73 ± 6
122 ± 10
49 ± 5
MAP (mmHg)
DAP (mmHg)
SAP (mmHg)
PP (mmHg)
88 ± 5
70 ± 4
126 ± 7
53 ± 5
80 ± 3
63 ± 2
121 ± 3
59 ± 3
MAP (mmHg)
DAP (mmHg)
SAP (mmHg)
PP (mmHg)
54 ± 4
123 ± 7
69 ± 4
85 ± 5
62 ± 5
47 ± 4
126 ± 7
79 ± 4
91 ± 5
80 ± 6
Late Postflight
59 ± 2
122 ± 3
63 ± 2
81 ± 2
59 ± 7
53 ± 5
126 ± 7
73 ± 3
87 ± 4
81 ± 14
Preflight
58 ± 4
127 ± 4
68 ± 5
86 ± 5
68 ± 9
47 ± 2
132 ± 3
79 ± 5
96 ± 4
103 ± 9
Early Postflight
12 min-1 (0.2 Hz)
53 ± 3
121 ± 5
67 ± 3
83 ± 3
60 ± 4
45 ± 4
127 ± 6
82 ± 5
84 ± 5
85 ± 4
Late Postflight
Breathing was controlled at fixed frequencies of 6 (0.1 Hz) and 12 (0.2 Hz) breaths per min. Values are mean ±SE.
70 ± 8
62 ± 4
HR (bpm)
Supine
99 ± 6
Early Postflight
79 ± 12
Preflight
HR (bpm)
Standing
6 min-1 (0.1 Hz)
76
-CHAPTER II-
Table 2.7: Pre- and postflight hemodynamic data in standing and supine cosmonauts
-CHAPTER II-
77
Figure 2.11: Average heart rate response (∆ HR) to standing before and after
spaceflight. Data are presented of slow paced breathing and normal paced breathing
protocols at preflight, early postflight (R + 1) and late postflight (R + 25) sessions.
* p < 0.05 compared to preflight.
2.4.3.3 Spectral and transfer function analysis of paced breathing-induced
oscillations
All cosmonauts were able to track their respiratory rate with the visual targets closely
ensuring blood pressure and RRI variations at frequencies of 0.1 and 0.2 Hz. SPB
induced significant larger RRI (Figure 2.12) and MAP (Figure 2.13) variations
compared to NPB. On standing up there was a tendency of less respiratory R-R
interval variability (RSA) (p = 0.137), whereas respiration synchronous MAP
fluctuations significantly increased upon standing (p = 0.001). Comparing pre- to
postflight conditions, we found significant differences in RSA (Figure 2.12, upper and
middle panel), with the lowest values at early postflight standing (p = 0.001) and
supine (p = 0.005) conditions. Postflight differences in RSA did not persist until 25
days after landing. Unlike postflight changes in RSA, the magnitudes of paced
breathing-induced MAP fluctuations were not significantly different between pre- and
postflight sessions (Figure 2.13, upper and middle panel).
78
-CHAPTER II-
Figure 2.12: Average data of RRI spectral powers in standing and supine cosmonauts
before and after spaceflight. The respiratory RRI spectral powers are presented from
slow paced breathing (upper panel) and normal paced breathing (middle panel), as
well as powers from spontaneous low-frequency (0.04 Hz – 0.15 Hz) RRI oscillations
(lower panel) at preflight, early postflight (R+1) and late postflight (R+25) sessions.
* p < 0.05 compared to preflight.
-CHAPTER II-
79
Figure 2.13: Average data of MAP spectral powers in standing and supine
cosmonauts before and after spaceflight. The respiratory MAP spectral powers are
presented from slow paced breathing (upper panel) and normal paced breathing
(middle panel), as well as powers from spontaneous low-frequency (0.04 Hz – 0.15
Hz) MAP oscillations (lower panel) at preflight, early postflight (R+1) and late
postflight (R+25) sessions. * p<0.05 compared to preflight.
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80
Results of transfer function analysis are graphically presented in Figure 2.14. The
supine and standing SAP-RRI coherence was generally high at the respiratory
frequencies (shaded areas). Phase lags were consistently negative at SPB-induced
oscillations, indicating that pressure variability leads interval variability. This is
further denoted as the RRI-to-SAP time delay. The RRI-to-SAP time delay was close
to zero at the NPB-frequencies in the supine position but tended to become negative
upon standing (p = 0.09). Standing-up resulted in a significant smaller SAP-RRI
transfer function gain compared with supine at both slow and normal paced breathing
frequencies (Figure 2.15, upper panels). Comparing pre- to postflight conditions
shows that, in the standing position, the gain and the phase lag of paced breathing
induced SAP and RRI oscillations differed significantly between sessions; post-hoc
analysis indicated a lower transfer gain (p = 0.009) and larger RRI-to-SAP time delay
(p = 0.022) at early postflight sessions compared to preflight. The postflight changes
in transfer gain and phase lag had returned to preflight levels after 25 days upon
return. There were no differences in the SAP-RRI coherence between pre- and
postflight conditions.
2.4.3.4 Spectral and transfer function analysis of spontaneous low frequency
oscillations
Spectral estimates of spontaneous low frequency (0.04 – 0.15 Hz) rhythms are
obtained during normal paced breathing (0.2 Hz) by integrating the area between the
dashed lines (0.04 – 0.15 Hz) in the lower panel of Figure 2.14. Comparing pre- to
postflight conditions shows that, in the standing position, there was a significant
reduction in spontaneous low frequency RRI variability early postflight (Figure 2.12,
lower panel). Consequently, compared with supine, significant higher standing RRI
oscillations at preflight disappeared early postflight, and turned out to be higher again
after 25 days upon return. Standing further resulted in significant higher spontaneous
low frequency MAP fluctuations compared with supine (Figure 2.13, lower panel).
-CHAPTER II-
81
Figure 2.14: Results from SAP-RRI transfer function analysis in standing and supine
cosmonauts before and after spaceflight. Data are presented of slow paced breathing and
normal paced breathing protocols at preflight, early postflight (R + 1) and late postflight (R
+ 25) sessions. Shaded areas cover the respiratory frequencies. Areas between dashed lines
determine the range of spontaneous low-frequencies (0.04 Hz – 0.15 Hz) that occur during
normal paced breathing (lower panel).
82
-CHAPTER II-
Figure 2.15: Average results of SAP-RRI
transfer function gain and time delay in
standing and supine cosmonauts before
and after spaceflight. Results are obtained
at respiratory frequencies from slow
paced breathing (upper panel) and
normal paced breathing (middle panel)
protocols. In addition, the gain and delay
are also presented from spontaneous low
frequency rhythms (0.04 Hz - 0.15 Hz)
during normal paced breathing (lower
panel). Results are presented from
preflight, early postflight (R+1) and late
postflight (R+25) sessions. * p<0.05
compared to preflight
Figure 2.14 illustrates that the SAP-RRI coherence was generally high, and the phase
lag was consistently negative (RRI-to-SAP time delay) in the low-frequency range
(0.04 – 0.15 Hz). Upon standing, there was a significant reduction in low frequency
transfer function gain compared with supine (Figure 2.15, lower panel). At the same
time the RRI-to-SAP time delay was significantly larger in the standing position.
Comparing pre- to postflight conditions shows that, in the standing position, the low
frequency gain and the phase lag differed significantly between sessions; post-hoc
analysis indicated a lower transfer gain (p = 0.019) and larger RRI-to-SAP time delay
(p = 0.032) at early postflight sessions compared to preflight. Early postflight changes
in low frequency transfer gain and phase lag had returned to preflight levels after 25
-CHAPTER II-
83
days upon return. There were no differences in the SAP-RRI coherence between preand postflight conditions.
2.4.3.5 Relationship between mean RRI and RSA
Figure 2.16 shows the relationships between mean RRI and spectral powers
(logarithmically transformed) obtained from NPB-induced RRI oscillations (0.2 Hz)
and SPB-induced RRI oscillations (0.1 Hz). Reductions in the mean RRI are
significantly related to reductions in spectral powers of NPB-induced RRI oscillations
(r = 0.66; p < 0.001); however, this does not account for the relationship between
mean RRI and SPB-induced RRI oscillations (r = 0.47; p = NS).
Figure 2.16: Relationship between mean RRI and logarithmically transformed
respiratory sinus arrhythmia (RSA) obtained from normal paced breathing (upper
panel: NPB = 0.2 Hz) and slow paced breathing (lower panel: SPB = 0.1 Hz)
protocols. Data are presented as mean values and SE. Squared symbols = standing;
circular symbols = supine; solid symbols = preflight; open symbols = early
postflight; shaded symbols = late postflight
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84
2.4.4
Discussion
In this study, we used a simple paced-breathing protocol to investigate respiratory
modulation of human autonomic rhythms before 10-days sojourns in the ISS and after
1 and 25 days upon return to Earth. Our data in five orthostatic tolerant cosmonauts
indicate that, in spite of an increased HR and associated reduction in RSA, respiratory
mediated blood pressure dynamics are well preserved following short-duration
spaceflight. According to our hypothesis it appears that, instead of a vagal-cardiac
neural deficit, postflight reductions in RSA represent appropriate autonomic
adjustments that account for the altered hemodynamic conditions after spaceflight.
This seems at odds with a previous report of amplified respiration-synchronous blood
pressure fluctuations in three orthostatic tolerant astronauts who stayed in space for
nine months (143). Disparate findings from short- and long-duration spaceflight
studies emphasize the possible impact of different durations of microgravity exposure
on postflight autonomic adjustments.
We observed a postflight reduction in the SAP-RRI gain in standing position, which
together with the high coherence and consistent negative phase lag, supports the
premise
of
reduced
spontaneous
cardiac
BRS
compared
with
preflight
(143;149;150;214). Postflight reductions in RSA and baroreflex control of heart rate
did not persist until 25 days upon return to Earth. This is an important new finding as
previous studies consistently reported an incomplete autonomic recovery as long as
two weeks after landing (140;143;150).
2.4.4.1 Sympathetic implications on vagal-cardiac assessments after spaceflight
Parallel impairments of RSA and spontaneous cardiac BRS after spaceflight may be
causally related as both reflexes result primarily from vagal-cardiac adjustments
(203). Our findings therefore comply with multiple previous reports of diminished
vagal-cardiac outflow following short-duration spaceflight (139;140;150;152).
However, the reliability of these measures as a quantitative index of vagal-cardiac
nerve traffic has recently been argued (218;219). Instead of direct vagal tone, RSA
and BRS rather reflect the more dynamic features of vagal-cardiac modulation
(46;220). The latter is a frequency-specific phenomenon (217) that also relies on the
absolute level of sympathetic neural outflow (45). Taylor et al. (2001) demonstrated
that an increased sympathetic drive restricts RSA to lower levels at both rapid and
lower breathing frequencies. Postflight reductions in RSA and spontaneous cardiac
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85
BRS, therefore, may be partially ascribed to an augmented sympathetic neural
outflow, which is a common observation in orthostatic tolerant astronauts after
spaceflight (139;146;152). In the present study, an increased sympathetic effect on
dynamic HR control is suggested from a larger RRI-to-SAP time delay early
postflight (Figure 2.15) (214). Indeed, an RRI-to-SAP delay of ~ 0 s can be expected
for vagal HR control, whereas RRI-to-SAP time delays between 1 and 2 s are
suggested to result from the combined effect of vagal and sympathetic baroreflex
regulation of cardiac cycle length (37).
Interestingly, we found an inverse relationship between HR and RSA that only
accounts for NPB frequencies; postural differences in RSA are lacking at SPB
frequencies (Figure 2.16). A possible explanation for this inconsistency may be the
greater latency period of sympathetic-cardiac adjustments, allowing direct support of
RSA only if the breathing frequency becomes more slowly (217). In this regard,
rather than an epiphenomenon of increased HR, postflight reductions in RSA are
likely to reflect an underlying change in autonomic state probably due to a shift in the
regulatory operating set-point after spaceflight (221).
2.4.4.2 Cardiovascular alterations after short-duration spaceflight
Autonomic response changes after (simulated) microgravity are very consistent with
those observed during acute interventions that favour central hypovolemia, such as
acute hypovolemia with furosemide (206), head-up tilting (222) and lower-body
negative pressure (223). Studies using head-down bed rest (simulated microgravity)
deconditioning learned that attenuated vagal-cardiac modulation is mainly related to
reduced plasma volumes (206). This is supported by a later study demonstrating
normalization of post-bed rest vagal-cardiac modulation with restoration of plasma
volume (224). Congruent results from three Neurolab-reports (138;139;152) showed
that lower plasma volumes importantly contribute to the augmented sympathetic drive
in orthostatic tolerant astronauts after spaceflight. When considering our findings after
spaceflight in light of these studies, early postflight reductions in body weight (Table
2.6) may be partially explained by a reduction in plasma volume. Yet, other factors
like diminished nutrition and space motion sickness may have contributed to this
postflight reduction in body weight (114).
In addition to hypovolemia, other cardiovascular adaptations have been reported after
(simulated) microgravity including cardiac remodelling and/or compromised diastolic
function (130). The consequence is a prominent fall in stroke volume during
86
-CHAPTER II-
postflight orthostasis (128). In the present study, a greater HR response upon early
postflight standing (Figure 2.11) is likely to be related to a steeper fall in stroke
volume, as it was not preceded by a reduction in arterial pressure. Indeed, even small
reductions in effective blood volume are known to trigger reflex tachycardia (15),
which indicates that arterial baroreceptors respond to mechanical deformation rather
than pressure. The diminished spontaneous cardiac BRS can be ascribed to alterations
in autonomic function that trigger a higher HR after spaceflight. Conversely, impaired
baroreflex function may contribute to orthostatic intolerance in subjects showing
attenuated HR responses to standing following exposure to real and simulated
microgravity (148;149).
At this moment, the most likely explanation of orthostatic hypotension after
spaceflight is a diminished vasoconstrictor reserve capacity (145-147). This
mechanism explains the discrepancy in vascular function between orthostatic tolerant
and intolerant astronauts (210). The compromised ability to elevate vascular
resistance in presyncopal astronauts may be ascribed to systemic vascular remodelling
(133) and/or low α1-adrenergic receptor sensitivity (225). Low-frequency systolic
blood pressure spectral power is found to be decreased after bed rest, but increased
after acute hypovolemia (206), suggestive of impaired vasomotor function following
simulated microgravity deconditioning. In the present study, postflight arterial
pressure did not decrease in orthostasis and sympathetic vasomotor function was
preserved at preflight levels. Therefore, as expected in orthostatic tolerant
cosmonauts, we did not find evidence of a compromised ability to elevate vascular
resistance.
2.4.4.3 Postflight recovery after 25 days
In agreement with a previous ground-based experiment (148), our data show that
postflight alterations in circulatory control do not persist until 25 days after return
from short-duration spaceflight. This seems at odds with a preliminary study in 3
cosmonauts, reporting reduced high-frequency R-R interval spectral powers (0.15 Hz
– 0.4 Hz) up to 25 days after return to Earth (141). A major contribution for this
discrepancy, next to the limited number of subjects, may come from differing
methodologies: fixed breathing rates were imposed in the present study instead of
spontaneous breathing, which spreads out respiratory mediated R-R interval powers
over a broader frequency range (0.15 - 0.4 Hz). Although, the breathing frequency
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87
itself is highly reproducible if subjects do not talk (60), variability around the
respiratory centre frequency may occur (226) with a substantial percentage of breaths
at frequencies below 0.15 Hz (227). Therefore, respiration should be controlled if R-R
interval powers are to be interpretable (228). On the other hand, imposing fixed
breathing frequencies could disturb the net effect of vagal-cardiac respiratory
modulation under basal circumstances (63). Therefore, interpretation of respiratory RR interval oscillations from distinct breathing protocols must be founded on
knowledge and understanding of the respiratory patterns that shaped them (227).
2.4.4.4 Limitations
Most limitations of this study are related to life science space research, which are
generally associated with a low number of participants (especially in the post
Columbia accident era) and a wide range of parallel experiments. Although we have
imposed standardization of experiments between the different missions, we cannot
control differences in workload between these missions. Also, changes in sleep-wake
cycles, personal exercise regimens, quantity and quality of sleep, fluid intake and
nutrition before during and after the flight could not be controlled. We did not assess
end-tidal carbon dioxide partial pressure to exclude hyperventilation during the paced
breathing experiment. This could have confounded the data since baroreflex
modulation of heart rate is known to be attenuated in response to hyperventilationinduced blood pressure oscillations (229). However, it is well known that normal
ventilation is usually preserved when subjects are capable to comfortably control dept
of inspiration (217). Finally, none of the cosmonauts developed orthostatic
intolerance, which prohibited us from assessing failing adaptive mechanisms that may
contribute to orthostatic presyncope after spaceflight. Alternatively, the analysis of
autonomic response changes that assist in maintaining orthostatic stability is also
limited.
2.4.5
Conclusion
Using a simple paced breathing experiment, the present findings illustrate that, after
short-duration spaceflight, reduced respiratory modulation (RSA) and baroreflex
control (BRS) of heart rate does not persist until 25 days upon return to Earth. Despite
early postflight reductions in vagal-cardiac modulation, arterial blood pressure
dynamics are well preserved in orthostatic tolerant cosmonauts, rendering a
fundamental neural deficit due to microgravity deconditioning unlikely. Postflight
88
-CHAPTER II-
autonomic adjustments made by orthostatic tolerant cosmonauts appear appropriate
for their altered hemodynamic loading conditions.
Related publications:
Verheyden B, Beckers F, Couckuyt K, Liu J, Aubert AE. Respiratory modulation of
cardiovascular rhythms before and after short-duration human spaceflight. Acta
Physiol. 2007; In press
Beckers F, Verheyden B, Couckuyt K, Jiexin L, Aubert AE. Long-term changes in
autonomic cardiovascular control after short-duration spaceflights. Eur J Appl
Physiol. 2007; In press
Verheyden B, Beckers F, Couckuyt K, Liu J, Aubert AE. Heart rate reflexes and
arterial pressure control after short-duration spaceflight. J Gravit Physiology. 2006;
3(1): 59-60
Aubert AE, Beckers F, Verheyden B. Cardiovascular function and basics of
physiology in microgravity. Acta Cardiol. 2005; 60: 129-151
Beckers F, Verheyden B, De Winne F, Duque P, Didier C, Aubert AE. HICOPS:
human interface computer program in space. J Clin Monit Comput. 2004; 18(2):1316
Beckers F, Verheyden B, Aubert AE. Evolution of heart rate variability before, during
and after spaceflight. J Gravit Physiology. 2003; 10: 107-108
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89
3 Tilt table testing in patients with neurally mediated
syncope
3.1
Introduction
Syncope is a symptom that affects up to 35% of the general population (230). It is
defined as a transient, self-limited loss of consciousness, usually leading to falling.
The onset of syncope is relatively rapid, and the subsequent recovery is spontaneous,
complete, and prompt. The underlying mechanism is a transient global cerebral
hypoperfusion, which is brought on by an underlying condition that can vary from a
temporary and mild functional disorder to severe cardiovascular disease. The
consequences of syncope may vary independent of the etiology from a single spell to
recurrent attacks with sustained injuries and/or poor quality of life. Syncope carries a
poor prognosis in the presence of underlying organic heart disease, whereas it has a
benign prognosis if no structural disease can be found (231).
Determining the most probably cause of syncope in individual patients is often a
challenging task that is facilitated by considering the possible etiologies in an
organized manner. To this end, the ESC Syncope Task Force (231) recommended
using a classification that begins with the most frequently encountered conditions, the
neurally mediated reflex faints. There are several types of neurally mediated reflex
faints, but the best known is the “common” or “vasovagal” faint. Patients
experiencing vasovagal syncope are very likely to feel nauseated and sweaty before
fainting, and often appear pale and feel clammy. It is the result of a paradoxical
autonomic reflex that, when triggered, gives rise to vasodilation due to sympathetic
withdrawal and bradycardia owing to an increase in vagal-cardiac tone.
Although a vasovagal response can be triggered by multiple stimuli, the most
important trigger includes conditions that affect orthostatic blood pressure regulation
(230;232). When the muscle pump is inadequate and the autonomic nervous system
does not regulate arterial pressure and venous return sufficiently to fulfill the demands
of the brain, this can lead to a vasovagal episode. The sequence of changes in failing
adaptive systems that leads to loss of consciousness has been studied extensively. Yet,
the triggering mechanism(s) remain unsolved. It appears that vasovagal faints can be
provoked by different stimuli affecting centrally or peripherally, but with the same
sequence of hemodynamic changes as result (232;233). This implies a multitude of
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90
triggers that can initiate a common efferent reaction. The present chapter focuses on
those aspects of orthostatic circulatory control that may contribute to a vasovagal
response.
3.1.1
Tilt table test in the evaluation of vasovagal syncope
Establishing a diagnosis of vasovagal syncope is usually possible by careful review of
the patients’ medical history, with particular attention being directed to the history of
events immediately surrounding the loss of consciousness. However, in many cases,
historical features alone are inadequate to establish a confident diagnosis. In such
cases, even the experienced clinician may desire additional diagnostic evidence in
order to feel comfortable. Moreover, the fainter may develop a greater sense of
confidence in the physician's diagnosis if additional evaluation is obtained,
particularly if that evaluation permits reproduction of the patient's symptoms at a time
when these symptoms can be witnessed by the medical practitioner.
Head-up tilt testing is widely accepted as a valuable diagnostic tool for evaluation of
patients with suspected vasovagal syncope (234). Clinical use of a tilt table is to
confirm the diagnosis in those with a typical history of vasovagal reactions, and to
teach patients counter-maneuvers such as leg crossing and muscle tensing. Repeated
tilt testing can also be considered a therapeutic option to reduce or prevent syncope
recurrence (235).
In 1991, Fitzpatrick et al. (236) showed that passive tilting at an angle of 60° for 45
min was an adequate diagnostic test, since this incorporates the mean duration to
syncope (24 ± 10 min) plus two standard deviations. This method is widely known as
the Westminster protocol. In order to increase the diagnostic yield of tilt testing, many
different protocols utilizing pharmacological agents have been proposed. Of the
various proposed provocative agents, sublingual nitroglycerine (NTG), which
enhances venous pooling of blood, is the most popular (237).
3.1.2
Repeated tilt table testing
Despite the fact that vasovagal syncope is the most frequent of all causes of fainting,
treatment strategies are as yet still based on an incomplete understanding of the
pathophysiology of the faint. The management of vasovagal syncope has largely been
empiric and many drugs have been tried (231). Although pharmacological treatment
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91
has been satisfactory in uncontrolled trials or short-term controlled trials, several
long-term prospective trials have been unable to show consistent benefit of the drug
over placebo (238). Pacemaker therapy has also been used to prevent severe
bradycardia and asystolic pauses associated with vasovagal syncope; however, this
treatment option does not prevent syncope recurrence in 4.3 % up to 22 % of the
treated patients (239-241). The efficacy of pacemaker therapy was further questioned
by recent controlled blind trials that failed to prove superiority of cardiac pacing over
placebo (242;243).
There is increasing evidence that a tilt training program is useful in the prevention of
recurrent vasovagal syncope (244). The patients perform serial tilt testing, one session
per day (235), until 2 consecutive negative tests (45 min duration) are achieved. This
therapy is usually initiated in the hospital. The patients are further followed in the
outpatient clinic for syncope. The treatment of vasovagal syncope by repeated tilt
testing is a new and fascinating therapy with promising results. In a group of 222
patients who underwent tilt training therapy for vasovagal syncope, a negative tilt test
could be obtained in every patient after 2.9 sessions on average (245).
3.1.3
Study objectives
This chapter deals with the effects of tilt testing on circulatory control in patients with
a typical history of vasovagal syncope.
In a first part we set out to determine the effects of sublingual nitroglycerine
on circulatory control in patients with distinct age-related blood pressure patterns at
the time of collapse. The purpose was to unravel whether underlying
pathophysiologies of an NTG-induced vasovagal response are age-related.
In a second part we investigate the effect of tilt training on orthostatic blood
pressure control in patients with recurrent vasovagal syncope. Up till now, the exact
mechanism by which tilt training improves symptoms associated with vasovagal
syncope remains obscure.
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92
3.2
Impact of age on the vasovagal response provoked by sublingual
nitroglycerine in routine tilt testing
3.2.1
Introduction
Vasovagal syncope (VVS) is a common clinical problem that afflicts patients of all
ages (246). Although the sequence of hemodynamic events associated with VVS
should be investigated during spontaneous episodes, this request makes adequate
hemodynamic study practically impossible, for obvious reasons. Therefore, head-up
tilting has become a widespread diagnostic test to document a tendency toward VVS
(247). In order to increase the diagnostic yield of head-up tilting, sublingual
nitroglycerine (NTG) is commonly administered (248-250), because as potent
venodilators (251), nitrates might facilitate VVS by enhancing venous pooling in the
upright posture (14). The yield of positive tilt testing has been reported to increase
substantially more after NTG provocation in older compared to younger patients
(252-254). A marked reduction in the capacity of the arterial baroreflex to buffer
blood pressure responses to vasoactive drugs with physiological aging has been
reported (255). With this information as a background, we hypothesized that
presyncope induced by NTG in older patients is brought on by baroreflex failure to
maintain arterial blood pressure, which, rather than presenting as a sudden vasovagal
reflex, is accompanied by a more gradual blood pressure decline compared to younger
patients. The purpose of this study was to test the hypothetical baroreflex involvement
by investigating the hemodynamic mechanisms underlying the direct effect of NTG,
in routine tilt tests. For this we studied patients with suspected VVS, applying pulse
wave analysis to the continuously measured Finapres blood pressure signal.
3.2.2
Methods
3.2.2.1 Study population
This is a retrospective study. The study group consists of patients with suspected VVS
who had shown a positive response to head-up tilt testing with NTG provocation at
the Syncope Unit of the Academic Medical Center. A positive response was defined
as a progressive fall in blood pressure and impending vasovagal syncope. The
patient’s medical history included at least one syncopal episode of unknown origin in
the last year, in the presence of (prolonged) orthostatic stress, pain, or an unusually
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93
strong emotional trigger. Excluded were patients with a history of cardiovascular
disease, carotid sinus syndrome, or any disease that might affect the autonomic
nervous system, as well as patients using medication that might affect the circulation
or circulatory control. In addition, we excluded patients who had experienced a
vasovagal episode before administration of NTG (n = 4). A total of 29 otherwise
healthy patients with a positive tilt test response (12 men and 17 women) and history
of VVS were included in this study. The mean age was 41 ± 18 years (range 16 – 71,
median 39 years).
3.2.2.2 Tilt test protocol and measurements
Tests were performed between 9:00 AM and 1:00 PM in a temperature-controlled
room (23°C). A manually operated tilt table with a footboard was used. The tilt test
started with 5 min of supine rest, followed by 20 min head-up tilt (60°). If no
vasovagal reaction developed, NTG was administered sublingually (0.4 mg) for an
additional 15-min tilt duration. An impending vasovagal response was aborted by
means of tilt-back, or by counter-maneuvers such as legs crossing to prevent loss of
consciousness (256). The term tilt-back is further used to indicate the moment an
impending vasovagal syncope was aborted. Blood pressure was measured
continuously and non-invasively using the Finapres Model 5 (TNO Biomedical
Instrumentation, Amsterdam, the Netherlands). The hand was held at right atrium
level and kept in place using an arm sling. The study was carried out in accordance
with the Declaration of Helsinki (2000) and has been approved by the Medical Ethics
Committee of the Academic Medical Center, University of Amsterdam, the
Netherlands.
3.2.2.3 Hemodynamic analysis
The arterial pressure signal was analog to digital converted at 100 Hz and stored on a
hard-disk. Beat-to-beat systolic and diastolic pressures were derived from the arterial
pressure waveform. Mean arterial pressure (MAP) was the true integral of the arterial
pressure wave over one beat divided by the corresponding beat interval. Heart rate
(HR) was computed as the inverse of the inter-beat interval (IBI), and expressed as
beats per minute. Beat-to-beat changes in stroke volume (SV) were estimated by
modeling flow from the finger arterial pressure waveform (Modelflow, TNO
Biomedical Instrumentation) (9;99;201). The patient’s gender, age, height and weight
were inserted in the model to determine Modelflow parameters using previously
established equations (100). Cardiac output (CO) was computed by the product of SV
94
-CHAPTER III-
and HR. Systemic vascular resistance (SVR) was obtained through a self-adapting
process in which an initial value is assumed for the first beat. The ratio of MAP to CO
for this first beat defines a new resistance value which is used in the model for the
next beat. Within 5 beats from the start, the model resistance stabilizes to actual SVR
and further allows close tracking of changes in real SVR (105). Due to this selfadapting process to estimate SVR, the tracking precision of the model is unaffected by
administration of vasoactive drugs like NTG (105). Stroke volume, CO and SVR were
set at 100% (baseline) in the upright posture, 3 min before NTG, and variations were
expressed as percentages (%) of this baseline. Beat-to-beat hemodynamic values were
averaged per 30-s periods; slopes were calculated over the initial 3-min time frame
starting at NTG administration to assess hemodynamic trends after NTG in all
patients.
3.2.2.4 Rate of arterial pressure fall in the presyncopal episode
The rate of arterial pressure fall (mmHg/s) in the presyncopal episode was computed
in each individual patient by calculating the first derivative of a 6th order Givens
polynomial approximation of MAP in the hypotensive 30-s episode prior to tilt-back
(Figure 3.1). The same procedure was adopted to calculate the rate of IBI-increase
(bradycardic response) during the presyncopal episode (ms/s).
3.2.2.5 Baroreflex sensitivity (BRS)
Time domain analysis of spontaneous BRS was performed using the cross-correlation
method (79). The SAP and IBI time series were resampled at 1 Hz. In a 10-s window,
the correlation and regression slope between SAP and IBI were computed. Delays of
0- to 5-s increments in IBI were computed, and the delay with the highest positive
coefficient of correlation was selected. The slope between SAP and IBI was recorded
as a BRS estimate if the correlation was significant at p = 0.01.
3.2.2.6 Statistical analysis
Statistical analysis was performed with SPSS version 11.5 for windows (Scientific
Packages for Social Sciences, Inc., Chicago, IL, USA). Variables were tested for
normality using the Kolmogorov-Smirnov test and expressed as the mean value ± SD,
unless stated otherwise. The BRS was transformed by calculating its natural logarithm
to achieve a normal distribution. Pearson’s correlation coefficient (r) was computed to
evaluate the relationship between age and the rate of arterial pressure fall at
presyncope. Differences between patients with distinct blood pressure patterns at
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95
presyncope were analyzed using the 2-tailed Student t-test or by one-way analysis of
variance with statistical correction for age as a continuous covariable.
3.2.3
Results
3.2.3.1 Study population
In all 29 patients, prodromal symptoms and signs such as pallor, diaphoresis,
lightheadedness and/or impending loss of consciousness were present within 4 to 11
minutes after NTG provocation. All patients recovered by counter-maneuvers or on
return to the supine position. Figure 3.1 shows blood pressure tracings during 3-min
periods (180 s) prior to tilt-back in two representative patients, aged 31 (upper panel)
and 69 years (lower panel). In both patients there is a reduction in blood pressure
during the vasovagal episode. However, the rate of blood pressure fall was much
more rapid in the younger compared to the older patient (3.5 versus 0.7 mmHg/s).
When tested in all patients, the rate of blood pressure fall in the presyncopal episode
was shown to be inversely related to age (r = 0.51, p = 0.005) (Figure 3.2). The
median rate of arterial pressure fall was 1.44 mmHg/s. Patients with a steeper blood
pressure fall (> 1.44 mmHg/s) were regarded as Type A responders. Patients with a
more gradual blood pressure decline (≤ 1.44 mmHg/s) were regarded as Type B
responders. The time from NTG administration to tilt-back was not different in
patients with a Type A (426 ± 165 s) or Type B (379 ± 129 s) blood pressure response
(p = NS). This implies that blood pressure started to fall earlier after NTG
administration in those patients with a Type B blood pressure response (257 ± 127 vs.
361 ± 166 s, p = 0.047).
96
-CHAPTER III-
Figure 3.1: Blood pressure tracings during 3-min periods (180 s) preceding tilt-back
in two patients, aged 31 (A) and 69 (B) years old. The rate of arterial pressure fall in
the hypotensive presyncopal episode is computed by calculating the first derivative of
a 6th order Givens polynomial approximation during 30-s periods prior to tilt-back or
counter-maneuver. The blood pressure fall was more rapid in the younger (upper
panel) compared to the older (lower panel) patient (3.5 versus 0.7 mmHg/s).
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97
Figure 3.2: Relationship between age and the rate of arterial pressure fall in the
presyncopal episode (r = 0.51, p = 0.005). The median rate of arterial pressure fall
was 1.44 mmHg/s. Patients with a steeper blood pressure fall (> 1.44 mmHg/s) were
regarded as Type A responders. Patients with a more gradual blood pressure
attenuation (≤ 1.44 mmHg/s) were regarded as Type B responders.
3.2.3.2 Distribution of age and gender
Figure 3.3 shows the age-distribution in patients with a Type A or Type B vasovagal
response. Patients with a Type A response were younger compared to Type B
responders (30 ± 10 vs. 51 ± 17 years; p < 0.001). All patients with a Type A response
(n = 14) were below 50 years old. Of those patients with a Type B response (n = 15),
67 % (n = 10) were older than 50 years. Accordingly, the analysis of possible ageeffects on the baroreflex response to NTG is further presented in terms of Type A or
Type B blood pressure patterns in the presyncopal episode. The distribution of gender
was not different between patients with a Type A (5 men and 9 women) or Type B (7
men and 8 women) vasovagal response.
Figure 3.3: Distribution
of age in patients with a
rapid (Type A) and more
gradual
(Type
B)
arterial blood pressure
fall in the presyncopal
episode. Note that all
patients with a Type A
blood pressure response
were younger than 50
yrs.
98
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3.2.3.3 Circulatory response to NTG
Cardiovascular trends in patients who experienced a Type A or Type B vasovagal
response are shown in Figure 3.4. Before NTG, all patients were asymptomatic, and
the average HR was 86 beats/min (range 61 to 134 beats/min; no difference between
Type A and Type B). Mean blood pressure tended to be higher in Type B patients;
however, this difference was not significant (91 ± 7 vs. 85 ± 11 mmHg, p = 0.11). The
cardiac BRS was reduced with age (r = -0.51, p = 0.003). Accordingly, Type B
patients had a lower BRS compared to Type A (5 ± 2 vs. 8 ± 3 ms/mmHg, p = 0.001).
Stroke volume, CO and SVR were stable before NTG in both groups of patients
(Figure 3.4).
Hemodynamic trends after NTG were analyzed by calculation of the slope during the
first 3 minutes after NTG provocation (Figure 3.4). Type B patients had a steeper drop
in SV compared with Type A (-11.8 ± 2.1 vs. -7.6 ± 1.5, p = 0.023). Because the
concomitant rise in HR was similar in both groups, Type B patients had a larger drop
in CO (-6.3 ± 1.4 vs. -2.0 ± 1.4, p = 0.046). This was adequately compensated by a
greater rise in SVR compared to Type A (6.2 ± 2.7 vs. 1.0 ± 1.7, p = 0.045). In both
groups, MAP was well maintained during the first 3 min after NTG in all patients
(Figure 3.4). There was a similar reduction in BRS after the hemodynamic changes
induced by NTG. After introduction of age as a covariate, no significant differences
were observed in the hemodynamic slopes following NTG provocation.
3.2.3.4 Vasovagal episode
There was marked hypotension, both in patients with a Type A and a Type B
vasovagal response (Figure 3.4). During the last 15 s before tilt-back, MAP averaged
respectively 49 mmHg (range 25 to 87 mmHg) and 55 mmHg (range 45 to 68 mmHg)
(p = NS). Heart rate dropped below 60 beats/min in 9 Type A patients (64%)
compared to only 4 Type B patients (27%). Consequently, during the last 15 s before
tilt-back, HR was lower on average in patients with a Type A vasovagal response (62
± 21 vs. 82 ± 17 beats/min, p = 0.041) (Figure 3.5).
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99
Figure 3.4: Hemodynamic response to NTG during 60° tilt table testing. Data are
presented as 30-s averages and SEM and cover 3-min time periods before and after
NTG administration. In addition, data from the last 4 min before tilt-back are
presented. Note the discontinuity in the X-axis. Shaded areas correspond to the final
minute before tilt-back. Closed circles = Type A, open circles = Type B. MAP = mean
arterial pressure, HR = heart rate, BRS = baroreflex sensitivity, SV = stroke volume,
CO = cardiac output, SVR = systemic vascular resistance.
100
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Figure 3.5: Distribution
of heart rate (HR) during
the last 15 seconds
before
tilt-back
in
patients with a Type A
and Type B vasovagal
blood pressure response.
The slope of the bradycardic response in the last 30 seconds before tilt-back was
related to the rate of fall in blood pressure within the same time period (r = -0.75, p <
0.001) (Figure 3.6).
Figure 3.6: Relationship between the rate of blood pressure fall and the bradycardic
response (rate of IBI-enlargement) during the last 30 seconds prior to tilt-back or
counter-maneuver (r = -0.75, p < 0.001).
In all patients, the last 30 seconds before tilt-back were characterized by a marked
reduction in CO, which was not compensated for by a further increase in SVR (Figure
3.4, shaded area). The drop in CO was mediated by significant reductions in both SV
and HR, while, in 24 of the 29 patients, SVR was increased compared with baseline.
-CHAPTER III3.2.4
101
Discussion
This study describes the impact of age on hemodynamic patterns induced by
sublingual NTG during routine tilt testing in patients with suspected VVS.
1. Analysis of blood pressure patterns during the presyncopal episode shows agerelated differences. We observed a gradient in the rate of arterial pressure fall
from the younger to the older patients, with the younger patients being more
likely to have a more rapid blood pressure fall compared to the older (Figure
3.2). The main determinant of the rate blood pressure drop on approach of
presyncope appears to be the rate of fall in heart rate (Figure 3.6).
2. The first 3 min after NTG, arterial blood pressure was well maintained, both in
patients with a rapid (Type A) and more gradual (Type B) blood pressure fall
prior to tilt-back (Figure 3.4). Cardiac output decreased more in Type B
patients. This implies an (initially) adequate arterial vasoconstrictor response
in the older patients to NTG-induced venous pooling.
The present findings support our hypothesis that, in the older patients, sublingual
NTG provokes a more gradual blood pressure decline compared to the younger
patients. However, this gradual fall in arterial pressure cannot be attributed to the
hypothesized failure of the arterial baroreflex buffering capacity with increasing age.
Age-related differences in the rate of fall in arterial pressure appear to depend on the
bradycardic response in the presyncopal episode.
3.2.4.1 The initial circulatory response to NTG (first 3 min)
The circulatory response to NTG was characterized by a rapid drop in SV, and despite
a rise in HR, CO was diminished. Furthermore, BRS was decreased in the early
period after NTG, which, together with the rise in HR and SVR suggests sympathetic
activation. Interestingly, sublingual NTG provoked a steeper drop in SV in the first
three minutes after application in the older patients with a Type B vasovagal response
(Figure 3.4). This steeper drop in SV in Type B patients may be surprising because, in
the older patients, reduced venous compliance is reported to account for a diminished
peripheral fluid shift, which attenuates the reduction in SV during the hypovolemic
stress of lower-body negative pressure (257). Two factors may be involved. First,
pooling in the splanchnic area of the upright human body seems more important than
previously reported in studies using simulated orthostasis by applying lower-body
negative pressure (258). Thus, increased splanchnic pooling may be hypothesized in
patients with a Type B vasovagal response. On the other hand, in the older subjects,
102
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increased myocardial stiffness and impaired diastolic relaxation make the heart more
dependent on cardiac preload to adequately fill the left ventricle (259). This age effect
becomes particularly important if heart rate is increased and thus diastolic filling time is
shortened (260;261). Therefore, age-related impairments in diastolic left ventricular
filling may account for the greater SV reduction induced by sublingual NTG in our
patients with a Type B blood pressure pattern in the presyncopal episode (Figure 3.4).
Unlike the reduction in SV, the concomitant rise in HR after NTG provocation was
similar in Type A and Type B patients (Figure 3.4). Thus, in patients with a Type B
vasovagal response, a steeper drop in SV after NTG is not accompanied by a larger
baroreflex-mediated rise in HR. This can be ascribed to a diminished HR regulation
with increasing age (262), which is further supported in this study by a lower
spontaneous cardiac baroreflex sensitivity (BRS) in the patients with a Type B
vasovagal respopnse. The steeper fall in CO compared to Type A patients is
adequately compensated by a greater rise in SVR, supporting the concept that
orthostatic blood pressure control essentially relies on sympathetic vasomotor
responses (262-264).
The precise sequence of events leading to the hypotensive, presyncopal episode is still
matter of debate. Certain data suggest that orthostatic hypotension results from
inadequate compensatory responses of the venous system (265-267); other data point
more to an underlying arterial vascular resistance deficit (268-270). In agreement with
Gisolf et al. (14), the present data signify that the NTG-triggered vasovagal response
is CO-mediated and is not preceded by a decrease in SVR (Figure 3.4). The reduction
in CO relies on the bradycardic response within the last 30 seconds prior to tilt-back;
the rate of fall in HR is clearly associated with the rate of arterial pressure fall in the
presyncopal episode (Figure 3.6). Within the last 15 seconds before tilt-back, average
HR was lower in patients with a Type A versus Type B vasovagal response (Figure
3.5). Therefore, the main determinant of age-related differences in the laboratory
presentation of a NTG-triggered vasovagal response appears to be the rate of fall in
HR.
3.2.4.2 The final vasovagal response
The more gradual development of a vasovagal response with increasing age was
formerly labeled as a dysautonomic response (271), suggesting that vasomotor failure
might be involved. In the new Vasovagal Syncope International Study (VASIS)
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103
classification of tilt-induced VVS (231), patients with a decrease in HR of less than
10% are labeled as ‘vasodepressor’ responders, further pointing to sympathetic
vasomotor failure with a decrease in SVR as the underlying mechanism. In this study,
we found that SVR did not decrease in the presyncopal episode, but remained above
baseline (100%) in all Type B patients. This, together with the adequate steady-state
arterial vasoconstrictor response to NTG-induced venous pooling, shows that the
previous assumption of sympathetic vasomotor failure in the older patients is rather
unlikely. Instead, the NTG-triggered vasovagal response appears to be CO-mediated
and is not preceded by a decrease in SVR (14).
Previous studies associated tilt-table testing in younger patients with an increased
incidence of severe vagal-cardiac inhibition (254;272;273). A much stronger vagalcardiac neural outflow in younger subjects may be involved here (274;275). The
decrease in vagal cardiac outflow with aging is reflected in the present study by a
lower cardiac BRS in patients with a Type B vasovagal response (Figure 3.4).
Although the occurrence of a bradycardic response during tilt testing seems
predominant in younger patients, the use of an implantable loop recorder (ILR) has
learned that, during spontaneous vasovagal events, asystolic syncope is a very
frequent observation in older adult and elderly patients (276;277). At present, these
contradictory findings are hard to explain. The fact that HR usually starts to fall after
the onset of hypotension may be involved (278). Only in those patients with fully
developed vasovagal faints does a marked heart rate decrement occur, without
exception (279). Thus, the timing to abort oncoming syncope, by tilt-back, may affect
the extent of reflex bradycardia. The more rapid vasovagal symptoms develop, the
more likely this will be associated with severe bradycardia in routine tilt testing.
Alternatively, the spontaneous asystolic events observed during ILR could result from
other than vasovagal mechanisms, e.g. early stage of sick sinus syndrome and/or
Adam-Stokes syndrome in the older patients.
3.2.4.3 Limitations
Results were obtained in subjects with no cardiovascular or neurological disease and
no medication. The older patients therefore represent a very healthy part of the total of
older patients referred for unexplained syncope (selection bias). Included were only
those patients who did not have a vasovagal episode before NTG, thus excluding the
most outspoken cases. Whether the present findings also account for spontaneous
vasovagal events should be addressed in future studies.
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104
3.2.5
Conclusion
It is concluded that, in the older patients, sublingual NTG generally provokes a more
gradual blood pressure decline compared to the younger. This gradual decline cannot
be attributed to failure of the baroreflex buffering capacity with increasing age. Agerelated differences in the rate of development of vasovagal symptoms depend on the
bradycardic response in the hypotensive, presyncopal episode.
Related publications:
Verheyden B, Gisolf J, Beckers F, Karemaker J, Wesseling KH, Aubert AE, Wieling
W. Impact of age on the vasovagal response provoked by sublingual nitroglycerine in
routine tilt testing. Clinical Science 2007; In press
-CHAPTER III-
3.3
105
Tilt training therapy increases vasoconstrictor reserve in patients
with neurally mediated syncope and a positive head-up tilt test
3.3.1
Introduction
Neurally mediated syncope is a common medical problem resulting from an abnormal
autonomic response with excessive vagal tone and sympathetic withdrawal (232). It
can be triggered in the upright position as a reflex consequence of thoracic
hypovolemia and is relieved by recumbence. Accordingly, head-up tilt testing can be
used to reproduce symptoms associated with neurally mediated syncope (234;247).
Despite lengthy study, the genesis and treatment strategies remain controversial.
To date, no definitive evidence exists to show that patients with neurally mediated
syncope benefit from pharmacological therapy (280), and pacing is reserved for the
few patients in whom syncope is accompanied by marked asystole (281). The recently
introduced therapy of daily repeated head-up tilt testing can restore orthostatic
tolerance to a level that prevents syncope in the majority of patients
(235;244;245;282;283). Tilt training therapy is based on the assumption that both the
clinical encounter and the head-up tilt test are themselves interventions (284-286).
The precise mechanism by which sequential head-up tilt testing improves symptoms
associated with neurally mediated syncope remains as yet uncertain.
Asymptomatic arterial pressure reductions have been reported to occur well before the
onset of tilt-induced syncope (287;288). This is a consequence of maladaptive reflex
adjustments aimed at maintaining cardiovascular homeostasis in the upright posture
(289). Certain data suggest that orthostatic hypotension in patients with neurally
mediated syncope is caused by a gradual lessening of venous return (265-267;290);
other data comprise an underlying arterial vascular resistance deficit (268;291;292).
Recent studies have demonstrated that it is the individual degree of neural and
vasomotor reserve available for vasoconstriction that significantly impacts the
maintenance of orthostatic tolerance (211;293). With the above background, we
hypothesized that in patients with recurrent neurally mediated syncope tilt training
will restore orthostatic blood pressure control by increasing sympathetic
vasoconstrictor reserve during sustained orthostasis. The purpose of this study was to
assess hemodynamic characteristic and baroreflex control of heart rate and systemic
vascular resistance during subsequent head-up tilt testing.
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106
3.3.2
Methods
3.3.2.1 Study population
For this study 17 patients were selected (6 males and 11 females, mean age 31 ± 22
years, height 170 ± 11 cm, weight 65 ± 12 kg) with a clinical diagnosis of recurrent
neurally mediated syncope and two consecutive positive head-up tilt tests. Other
possible causes of syncope were excluded by means of a systematic clinical work-up
(234). No patient suffered from cardiovascular or neurological disease and no
medication was used affecting the circulation or circulatory control.
3.3.2.2 Baseline diagnostic head-up tilt test
In all selected patients, a first positive diagnostic head-up tilt test was performed on a
motorized tilt table with foot support according to the Westminster protocol (236).
Briefly, after a 15-minute resting period in the recumbent position the patients were
moved to the 60 degrees upright position for a maximum duration of 45 min or until
syncope developed. At the time of syncope patients were returned to the recumbent
position (tilt-back). No pharmacological provocation was used to avoid a false
positive diagnosis (294). Syncope was defined as an abrupt, transient loss of
consciousness, and loss of postural tone. The tilt test was considered positive if
syncope developed in association with hypotension, bradycardia, or both. Positive
responses were classified according to the recent VASIS-recommendations (234).
3.3.2.3 Study design
A second positive tilt test was reproduced at the start of tilt training therapy around 1
month after the first positive diagnostic test. Tilt training was initiated in hospital to
motivate individual patients to continue standing training at home (282). A similar
procedure for in-hospital tilt training was adopted as for the diagnostic tilt test. The
target was to obtain two consecutive negative tilt tests without prodromal signs of
syncope.
After hospital discharge, the patients were instructed to continue a program of daily
standing training at home (282). We recommended 1 or 2 sessions per day, 30
minutes each. Patients had to stand with their feet 15 cm away from the wall and lean
with the upper back against the wall without moving. This tilt training was organized
on a safe place, without risk of injury, and with the attendance of a family member.
Each session was ended at the occurrence of the first symptoms. The patients were
asked to return to the hospital for a control tilt test after a period of at least 6 weeks.
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107
The study was approved by the local Medical Ethical Committee and informed
consent was obtained from all participants.
3.3.2.4 Data acquisition
Data recording was started at the start of in-hospital tilt training therapy. During
sequential head-up tilt testing, the electrocardiogram was recorded continuously and
beat-to-beat arterial blood pressure was measured non-invasively by means of a
servo-controlled
photoplethysmograph
(Portapres,
TNO,
Amsterdam,
The
Netherlands) placed on the midphalanx of the right middle finger (97). The hand was
positioned at heart level and held in place using an arm sling to prevent hydrostatic
pressure differences in the upright posture. Finger cuff pressures were calibrated to
intermittent arm-cuff pressures (Colin BP-88S, Komaki, Japan) and used to track
arterial blood pressure changes. The electrocardiogram and finger arterial pressure
were digitized at 1000 Hz using an external A/D converter (DATAQ Instruments Inc.,
Akron, OH, USA) and stored on a laptop computer. Respiratory rate was derived from
changes in thoracic impedance.
3.3.2.5 Hemodynamic analysis
A file consisting of consecutive RR intervals (RRI) was created from the
electrocardiogram (169). Accordingly, beat-to-beat mean arterial pressure (MAP) was
calculated as the true integral of the arterial pressure wave divided by the
corresponding beat interval. Beat-to-beat changes in stroke volume (SV) were
estimated by modeling flow from finger arterial pressure (Modelflow, TNO
Biomedical Instrumentation) (9;99;100;201). Cardiac output (CO) was the product of
SV and heart rate (HR), and systemic vascular resistance (SVR) was obtained from
MAP at heart level divided by CO (105). Stroke volume, CO and SVR were set at 100
% (baseline) immediately after assumption of the upright posture and variations were
expressed as percentages (%) of this baseline. Beat-to-beat data were averaged per
minute during the initial 4 min-period in the head-up tilt position. Syncope occurred at
different time points of the tilt protocol and so minute averages were further
calculated from recordings referenced to syncope time. Baseline HR, MAP and PP in
the supine position were inferred from the last 4-min period preceding the start of tilt.
The analysis of changes in SV, CO, and SVR from the supine to the upright tilt
position was omitted to avoid unreliable estimations during posture change (107).
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108
3.3.2.6 Baroreflex sensitivity (BRS)
Beat-to-beat RRI and systolic arterial pressure (SAP) time series were interpolated,
resampled at 2 Hz, and divided at 90 % overlapping segments of 128 s. Each segment
was detrended, Hanning windowed, and fast Fourier transformed (165). The obtained
spectral resolution was 0.0078 Hz. Power spectral density (ms2/Hz for tachograms
and mmHg2/Hz for systograms) and the SAP-RRI transfer function gain (BRS in
ms/mmHg) was then computed in the low-frequency band (0.04 to 0.15 Hz) for each
window. The reliability of each BRS estimation was evaluated by the squared
coherence function (66).
3.3.2.7 Statistical analysis
Statistical analysis was performed with SPSS version 11.5 for Windows (Scientific
Packages for Social Sciences, Inc., Chicago, IL, USA). Variables were tested for
normality with the Kolmogorov-Smirnov goodness of fit test. Spectral powers were
transformed by calculating the natural logarithm to achieve a normal distribution. The
effect of tilt training on orthostatic blood pressure control was investigated by oneway ANOVA for repeated measures. This was conducted at fixed 4-min time frames
of each tilt protocol: 1) 4-0 minutes supine before head-up tilt; 2) 0-4 minutes head-up
tilt; 3) 5-1 minutes referenced to syncope time. The first and last tilt sessions of inhospital tilt training therapy were selected and compared with a follow-up tilt test
after 6 weeks continued standing training at home. During a positive tilt test, the last
minute before tilt-back was used to investigate the presyncopal episode (presyncope).
3.3.3
Results
3.3.3.1 Clinical characteristics
The patients referred to this study had a mean number of 4.7 ± 3.5 syncopal episodes
during the last 3 months before diagnostic tilt testing (range 2 – 12; median 4). The
response to the diagnostic tilt test was: type 1 (mixed) in 9 patients; type 2A
(cardioinhibitory without asystole) in 2; type 2B (cardioinhibitory with asystole) in 1;
and type 3 (vasodepressor response) in 5 patients. Asystolic pauses were due to sinus
arrest in all cardioinhibitory cases. The mean duration of the diagnostic tilt test was 21
± 13 min (range 5 – 44 min; median 24 min) and did not differ significantly between
the different types of tilt responses. On resuming the recumbent position, all patients
recovered and returned to a stable sinus rhythm within about 15 s.
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109
3.3.3.2 Tilt training and follow-up
Head-up tilt testing was repeated day after day: one session per day. Clinical results of
in-hospital tilt training are summarized in Figure 3.7.
Figure 3.7: Flow-chart of the experimental protocol. A plus sign indicates a positive
tilt table test. A minus sign indicates a negative tilt table test. ‘n’ is the number of
subjects undergoing each procedure or the number excluded after it.
110
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The number of in-hospital training sessions ranged from 3 to 6, median 4. For all
patients, the mean number of sessions to achieve a first negative tilt test was reached
after 2.9 ± 0.7 sessions, median 3. Accordingly, a second consecutive negative test
was achieved after a mean of 4.1 ± 0.9 sessions, median 4. During subsequent tilt tests
there was no change in the type of syncope observed.
Fourteen patients returned to the hospital for a follow-up tilt test after a period of
approximately 6 weeks continued standing training at home. All these patients had
continued standing training on a regular basis and had become completely
asymptomatic. Accordingly, the follow-up tilt test was negative in all these patients.
3.3.3.3 Acute circulatory response with posture change (0-4 min head-up tilt)
Hemodynamic patterns during subsequent tilt testing are shown in Figure 3.8. Results
are obtained from 3 different tilt sessions: 1) positive tilt test at the start of in-hospital
tilt training, 2) negative tilt test at the end of in-hospital tilt training and, 3) negative
tilt test following 6 weeks continued standing training at home. Power spectral
estimates at corresponding time intervals are summarized in Table 3.1.
We did not find significant differences between subsequent tilt sessions during 4-min
periods preceding and following postural change. On average in all sessions, HR in
the supine position was 73 ± 11 beats.min-1 and increased to 93 ± 13 beats.min-1 in the
upright posture (p < 0.001). Accordingly, average MAP increased from 83 ± 8 to 89 ±
10 mmHg, while BRS decreased from 10 ± 5 to 6 ± 3 ms/mmHg due to the
hemodynamic changes induced by head-up tilt (both p < 0.001). The SAP lowfrequency power increased from supine to upright tilt (p < 0.001), and also RRI lowfrequency power tended to become higher in the upright posture (p = 0.108). Stroke
volume, CO and SVR were remarkable stable (~100 %) during the first 4 min of tilt
(Figure 3.8). During these periods, none of the patients had prodromal symptoms of
syncope.
Figure 3.8: Hemodynamic response to head-up tilting in patients with neurally mediated syncope. Values are presented
as minute averages and SEM. 0 corresponds to the supine value before the start of tilt followed by the initial 4 minutes
in the head-up tilt position and the last 5 minutes referenced to syncope time. Shaded areas indicate the presyncopal
episode in case of a positive tilt test. Solid circles = positive tilt test at the start of in-hospital tilt training. Open circles
= negative tilt test after in-hospital tilt training. Triangles = negative tilt test after 6 weeks continued standing training
at home. MAP = mean arterial pressure. HR = heart rate. BRS = baroreflex sensitivity. SV = stroke volume. CO =
cardiac output. SVR = systemic vascular resistance.
-CHAPTER III111
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112
Table 3.1: Spectral and cross-spectral data at fixed 4-min time frames throughout the
tilt test protocol before and after in-hospital tilt training therapy and after 6 weeks
continued standing training at home (follow-up).
Positive tilt
test before
tilt training
Negative tilt
test after
tilt training
Negative tilt test
at
follow-up
n = 17
n = 17
n = 14
756 ± 601
727 ± 640
739 ± 772
NS
SAP LF power (mmHg )
10 ± 5
11 ± 5
11 ± 7
NS
BRS LF gain (ms/mmHg)
11 ± 5
9 ±7
8 ±3
NS
840 ± 690
844 ± 968
933 ± 893
NS
SAP LF power (mmHg )
27 ± 13
27 ± 10
27 ± 13
NS
BRS LF gain (ms/mmHg)
6 ±3
5 ±3
6 ±2
NS
384 ± 368
942 ± 926*
985 ± 733*
0.019
SAP LF power (mmHg )
16 ± 8
37 ± 24*
37 ± 17*
0.001
BRS LF gain (ms/mmHg)
4 ±2
4 ±2
5 ±2
NS
pvalue
4-0 min before head-up tilt
RRI LF power (ms2)
2
0-4 min head-up tilt
RRI LF power (ms2)
2
5-1 min referenced to syncope time
RRI LF power (ms2)
2
Data are presented as the mean value ± SD. BRS = baroreflex sensitivity; LF = lowfrequency; RRI = R-R interval; SAP = systolic arterial pressure. * p < 0.001
compared to a positive tilt test before tilt training.
3.3.3.4 Sustained circulatory response in the upright posture
The positive tilt test at the start of in-hospital tilt training was characterized by a ~25
% reduction in SV from the initial phase after tilt (0-4 min head-up tilt) to the last
phase before presyncope (5-1 min before tilt-back). No significant differences were
observed at corresponding time intervals between subsequent tilt sessions (Figure
3.8). In each tilt session, there was a continued rise in HR in the sustained upright
posture; however, maximum HR decreased with subsequent tilt testing: 109 ± 20
beats.min-1 at the start of in-hospital tilt training; 105 ± 14 beats.min-1 after in-hospital
tilt training; 101 ± 17 beats.min-1 after 6 weeks continued standing training at home (p
= 0.038). Despite the continued rise in HR there was a ~7 % reduction in CO relative
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113
to the start of tilt and no significant differences were observed between subsequent tilt
sessions at corresponding time intervals referenced to syncope time (Figure 3.8). To
compensate for a reduction in blood pressure there was a substantial SVR increase of
~11% after in-hospital tilt-training (p = 0.034), and of ~14% after 6 weeks continued
standing training at home (p = 0.004). Otherwise, the compensatory SVR response
was insufficient during a positive tilt test at the start of in-hospital tilt training, leading
to a gradual MAP-fall between 5 to 1 minutes before presyncope (Figure 3.8). The
low-frequency powers of SAP and RRI oscillations were also depressed during
periods between 5 to 1 min before syncope; significant higher values were observed
following tilt training therapy (Table 3.1). No significant changes were observed in
the BRS low-frequency gain between subsequent tilt-sessions.
3.3.3.5 The presyncopal episode
To investigate the hypotensive, presyncopal episode of the positive tilt test at the start
of tilt training, the last minute before tilt-back was selected (shaded areas in Figure
3.8). During this period, all patients showed marked hypotension with MAP of 57 ±
17 mmHg. Heart rate ranged from 42 to 133 beats.min-1 during this period (mean 89 ±
25 beats.min-1). Five of the 17 patients had a HR above 100 beats.min-1, whereas four
had a HR below 60 beats.min-1. During the presyncopal episode in all patients HR
decreased compared to its peak value that was reached about three minutes before
syncope occurred. The fall in HR during presyncope was accompanied by a sudden
rise in the cardiac BRS together with an abrupt drop in CO (Figure 3.8). Stroke
volume in the presyncopal episode was 76 ± 18 % of baseline (p < 0.001), CO was 73
± 20 % (p < 0.001), and SVR was 98 ± 26 % (p = NS).
3.3.3.6 Respiratory frequency
In all subjects, the respiratory frequency averaged 14 ± 5 breaths per minute in the
supine position before tilt. Upon assuming the upright posture the respiratory
frequency remained stable around 15 ± 5 breaths per minute in all tilt sessions. During
the positive tilt test at the start of in-hospital tilt training, respiratory rate further
increased towards 23 ± 10 breaths per minute within periods between 5 to 1 min
before syncope (p = 0.007).
3.3.4
Discussion
The purpose of this study was to clarify underlying mechanisms by which tilt training
may improve symptoms in patients with a clinical diagnosis of neurally mediated
114
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syncope. The principle findings are that at the start of therapy there is an inadequate
increase in systemic vascular resistance that cannot compensate for a postural
reduction in stroke volume. The subsequent fall in cardiac output contributes to
hypotension in the presyncopal episode. Our findings confirm the hypothesis that
daily repeated tilt testing restores orthostatic tolerance by increasing the degree of
neural and vasomotor reserve available for vasoconstriction in the sustained upright
posture. The increased vasoconstrictor reserve is preserved after 6 weeks continued
standing training at home.
3.3.4.1 Vasoconstrictor reserve in neurally mediated syncope
As expected, in comparison to the start of tilt, we found a relative reduction in stroke
volume during time intervals referenced to syncope time (Figure 3.8). This reduction
in stroke volume is the result of orthostatic pooling of venous blood, which, together
with a rise in transcapillary fluid filtration rate provokes a reduction in venous return
in the sustained upright posture (295-297). The compensatory rise in heart rate cannot
account for the postural reduction in stroke volume and so leads to a drop in cardiac
output (Figure 3.8). In healthy individuals, progressive reductions in stroke volume
and cardiac output are followed by a reflex increase in systemic vascular resistance
(152;153). This baroreflex mechanism of systemic vascular resistance plays an
important role not only in acute but also in sustained orthostatic blood pressure
control (297).
One important mechanism underlying the individual variability in orthostatic
tolerance is the degree of vasomotor reserve that is available for vasoconstriction in
the sustained upright posture (211). Our findings further extent this knowledge by
showing that in tilt-positive patients the additional vasomotor reserve that can
ultimately be made available during orthostatic stress is lacking (Figure 3.8).
Furthermore, the lack of increase in SAP low-frequency power (Table 3.1) points to
an underlying neural deficit affecting sympathetic vasomotor control (65). Likely, this
is detected by arterial baroreceptors and further translates into a diminished RRI lowfrequency power compared to the start of tilt (37).
3.3.4.2 Tilt-induced syncope is cardiac output-mediated
Although our data indicate that inadequate sympathetic baroreflex control of the
peripheral vasculature causes an asymptomatic reduction in blood pressure several
minutes before syncope; the main determinant of reflex hypotension in the last minute
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115
before syncope is a fall in cardiac output (Figure 3.8). According to Jardine et al.
(296) failure to maintain cardiac output appears to involve a combined reduction in
heart rate and stroke volume. The sudden rise in spontaneous cardiac baroreflex
sensitivity illustrates that presyncopal bradycardia involves a vagal-cardiac reflex. At
the same time, due to sympathetic withdrawal prior to syncope (298), rapid changes in
splanchnic blood flow (266) and/or ventricular contractility (299) may contribute to
the additional drop in stroke volume. The increased respiratory rate before syncope
suggests a period of hyperpnoea and/or hyperventilation (300). The consequent
reduction in cerebral blood flow could further predispose to the development of reflex
syncope (301).
3.3.4.3 Tilt training therapy restores vasoconstrictor reserve
Some early reports have implied a therapeutic impact of repeated head-up tilt testing.
Sheldon et al. (284) observed an apparent reduction in the risk of syncope recurrence
after a first positive tilt test. Morillo et al. (285) were the first to report a striking
reduction of positive tilt test responses during acute serial tilt testing. This has been
confirmed more recently by Sagrista-Sauleda et al. (286) in a study specifically
designed to assess the reproducibility of sequential head-up tilting in patients with
neurally mediated syncope. Our study is the first to provide a mechanistic description
on how tilt training may improve symptoms associated with neurally mediated
syncope. Rather than suppressing the final trigger, our data show that tilt training
prevents the hypotensive mechanism preceding reflex syncope. During sustained
passive orthostasis, restoration of the sympathetic vasoconstrictor reserve adequately
compensates for a postural reduction in cardiac output. This training effect is
preserved after 6 weeks continued standing training at home.
3.3.4.4 Impact of tilt training on autonomic control mechanisms
Di Girolamo et al. (244) proposed that daily orthostatic training may have a
desensitizing effect on the cardiopulmonary receptors that are believed to trigger
neurally mediated syncope. Studies in humans with totally denervated hearts,
however, have challenged the relevance of ventricular afferents in the genesis of
reflex syncope (302). Three studies have investigated the role of heart rate variability
as an adaptive pathophysiological mechanism in tilt training (303-305). Piccirillo et
al. (305) reported that patients who have abnormal autonomic nervous function with
increased vagal-cardiac tone may benefit from prolonged tilt training by increasing
sympathetic neural outflow to the sinus node at baseline supine rest. In our patients,
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116
however, tilt training did not affect the spectral data in the supine position, suggestive
of a normal autonomic nervous function at the start of therapy. Two other studies
point to a substantial influence of tilt training on the sympathovagal balance of heart
rate control in the upright posture, showing a shift toward less sympathetic dominance
and lower heart rate after training (303;304). This seems at odds with our findings on
spontaneous baroreflex sensitivity; no differences were found in the upright posture
before and after tilt training. On the other hand, we did observe a reduction in the rise
in maximum heart rate with subsequent tilt testing. Stewart et al. (306) showed that
the origin of presyncopal tachycardia depends on the degree of thoracic hypovolemia,
related to splanchnic hypervolemia, and so is the result of baroreceptor deactivation.
In our baroreflex computations, we did not explicitly distinguish between
baroreceptor stimulation and baroreceptor deactivation (307). Lack of significant
effects of tilt training on baroreflex control of heart rate in the upright posture may
therefore be ascribed in part to methodological issues.
3.3.4.5 Limitations
Head-up tilt testing has some diagnostic limitations related to the reproducibility
(308;309). According to the guidelines of the European Society of Cardiology (310),
the overall reproducibility of an initial positive tilt test (31–92 %) is lower than for an
initial negative test (85–94 %). In the present study, effects of tilt training therapy
were only assessed in patients with a reproducible positive tilt test. This strategy was
held to exclude false-positive responders from further analysis. Another point of
critique on daily orthostatic training implies that it may only be effective in highlymotivated patients (311). This drawback explains why many patients abandon tilt
training and are reluctant to undergo prolonged therapy (311). Maximum compliance
to therapy can be achieved by initiating a program of in-hospital tilt training because
it restores orthostatic tolerance in only a few days (312). In addition, the patients
should be instructed to return to the hospital for a long term follow-up.
3.3.5
Conclusion
The major findings of this study are that patients with neurally mediated syncope fail
to increase systemic vascular resistance in response to a postural reduction in stroke
volume. This is followed by a subsequent fall in cardiac output contributing to
hypotension in the presyncopal episode. Daily repeated tilt testing restores orthostatic
-CHAPTER III-
117
tolerance by increasing the amount of sympathetic vasoconstriction that can
ultimately be made available during sustained orthostatic stress. The reduction in peak
heart rate with subsequent tilt testing is suggestive of a decreased baroreceptor
deactivation in the upright posture. Effects of tilt training on circulatory control are
preserved after 6 weeks continued standing training at home. We recognize that
patients with recurrent neurally mediated syncope often suffer severe psychological
burden too. Information about the benign nature of the disorder, reassurance,
counseling and coaching on appropriate postural maneuvers to prevent presyncope
from progressing to syncope, may also produce a powerful impact on syncope
recurrence. Finally, tilt training may be effective also because it strengthens muscle
tone in the legs thus improving the efficiency of the venous pump (313). More than
one mechanism may operate in concert in the same patient.
Related publications:
Verheyden B, Beckers F, Ector H, Aubert AE, Reybrouck T. Tilt training therapy
increases the vasoconstrictor reserve in patients with neurally mediated syncope and
a positive head-up tilt test. Submitted
-CHAPTER IV-
119
4 Concluding remarks and perspectives
4.1 Cardiovascular variability and circulatory control mechanisms
Blood pressure and heart rate continuously fluctuate over time under the direct
influence of autonomic control mechanisms. This implies a condition that
dynamically aims at achieving stability, without entirely reaching it. As a result,
cardiovascular variability reflects the activity of cardiovascular control mechanisms.
Greatest physiological significance has been ascribed to the two primary short-term
oscillations in humans: those occurring at respiratory frequencies and those occurring
at a slower, approximate 10-s cycle. Progress offered by multivariate models further
allows appreciating the complex hemodynamic interaction underlying cardiovascular
regulation (218).
Although
previous
experiments
succeeded
to
eliminate
frequency-specific
cardiovascular oscillations by means of (selective) autonomic blockade, no
quantitative relationship has been established between the oscillatory components of
cardiovascular variability and underlying autonomic nervous activities (219). This
lack of correlation indicates that, instead of providing an index of neural outflow,
cardiovascular variability rather reflects the dynamic feature of autonomic modulation
(215). However, perfect correlations do not exist in biology; therefore, it may be too
sceptic not to recognize cyclic components of cardiovascular variability as autonomic
measures.
Owing to the complex and largely undiscovered physiological basis of cardiovascular
oscillations, distinct analysis methods should be applied with wisdom and caution.
One important consideration comprises the normalization of variability data. On the
one hand, normalized units of variability may be useful to assess alterations in
sympatho-vagal balance of heart rate control during orthostatic stress (173). On the
other hand, normalizing frequency-specific oscillations to each other can uncouple
their amplitudes from the underlying physiology, e.g. when the amount of total
variability becomes negligible (219). Rather than exploiting the observation that
cardiovascular oscillations have autonomic components to justify them as measures,
future research should focus on a better understanding of integrated cardiovascular
control mechanisms.
120
-CHAPTER IV-
The question remains whether parameters of cardiovascular variability may be used in
daily clinical practice to measure autonomic activities? Strangely enough, no one has
ever questioned the outcome of classical autonomic function tests, like the
phenylephrine test (314) to ‘measure’ autonomic reflex responsiveness. The highly
variable outcome between successive runs of these tests clearly indicate that
quantification of autonomic outflow by direct or indirect measures may give only
rough estimates of the patients’ condition. The true power of cardiovascular
variability, therefore, lies in the follow up of longer periods of illness, rather than in
the one-point measurement, unless the condition is extremely clear cut (315).
4.2 Orthostatic blood pressure control: the challenge of gravity
On standing up, venous blood is redistributed toward the splanchnic, pelvic and leg
vasculature. The result is a reduction in venous return to the heart leading to
tachycardia and systemic vasoconstriction. It is due to gravity that postural changes in
humans result in fluid shifts. Maintaining arterial pressure in the erect human body
occurs through stimulation of baroreceptor reflexes and is of vital importance to
preserve brain perfusion. Considering the circulatory demands of the brain, fast and
efficient responses to gravity-induced fluid shifts are crucial.
The dynamic behavior of autonomic reflex responses can be illustrated during
transient (20 – 25 s) gravity periods of parabolic flight. The gravity phases of
parabolic flight are in a range between 0 and 1.8 Gz (1 Gz: 9.81 m/s2). In subjects
aboard the aircraft, body fluids are homogenously distributed in the supine position;
however, significant fluid shifts occur in the longitudinal axis of the erected body. In
the upright position, a decrease in Gz is followed by proportional sympathetic
withdrawal in association with enhanced parasympathetic influences on heart rate
control, suggestive of a central push-pull pattern of neural organization (173).
Accordingly, postural differences in heart rate are amplified during transient
hypergravity, while they are abolished during microgravity induced by parabolic
flight.
Thermoneutral (34.5°C) head-out of water immersion (WI) is one means of
simulating microgravity under standardized conditions in humans on Earth. This is
not only because buoyancy does reduce the body weight, but a thoracic fluid shift also
increases central venous blood volume, due to increased hydrostatic pressures on the
-CHAPTER IV-
121
lower limbs. The bradycardia and systemic vasodilatation during WI correspond to
the circulatory responses seen at the very onset of weightlessness and until at least a
week into spaceflight (123). Underlying autonomic mechanisms likely include an
increased vagal-cardiac outflow together with diminished sympathetic influences on
the heart and peripheral vasculature. These findings agree with most studies showing
increased vagal-cardiac baroreflex control of heart rate up to 10 days into spaceflight
(138;139;141), but are in contrast with higher levels of muscle sympathetic outflow
previously observed in space (138). It is possible that the vascular sensitivity to
sympathetic nervous activity is reduced by weightlessness; however, more studies are
needed to unravel this issue.
On Earth, increased hydrostatic pressures in the lower compared to the upper parts of
the upright human body predisposes to vasoconstriction and thickening of vessel
walls (316). Therefore, during every day life, chronic effects of gravity can contribute
to the development of hypertension and cardiovascular disease. Because in space, the
human vasculature is chronically dilated, and lower-limb hydrostatic pressures are
cancelled out, it may be rather healthy for the human circulation to fly into space
(123). Similar responses to WI add further support to the beneficial circulatory effects
of hydrotherapy, which has been used as a therapeutic model for more than 20
centuries.
Whether it is healthy for the human heart to fly in space is not clear cut. Leftventricular muscle mass appears to be decreased after 10 days spaceflight, suggestive
of cardiac muscle atrophy (130) and/or reduced cardiac muscle contractility (317).
This does not seem to be healthy for the heart; however, the decrease in leftventricular muscle mass can also be ascribed to the well-known loss in extracellular
fluid during spaceflight (318). Besides, enhanced vagal-cardiac outflow, due to the
chronically microgravity-induced thoracic fluid shift, could have significant antiarrhythmic effects, leading to a decreased hazard of fatal arrhythmias (319-321).
4.3 Baroreflex function and orthostatic intolerance
Baroreflex sensitivity (BRS) of heart rate is a popular parameter in both clinical trials
and experimental studies. However, the ability of BRS to determine orthostatic
tolerance in human subjects is rather limited. Although the majority of investigators
who addressed this issue did not find clear evidence of alterations in the arterial
122
-CHAPTER IV-
baroreflex control of heart rate in subjects with tilt-induced syncope (292;322;323), a
few have reported an abnormality which, in most cases, consisted in a reduction
(324), rather than an increase (307). The reasons for these discrepancies are likely to
be multifold; however, a major contribution may come from the fact that BRS
estimates represent an epiphenomenon, rather than a primary cause of orthostatic
intolerance. As such, it appears that orthostatic tachycardia alone is not sufficient in
preventing a vasovagal response. Subjects with a high baseline BRS do not
necessarily have a good orthostatic tolerance. The popularity of measuring BRS is
mainly due to its accessibility, and the information it provides on the sympathetic and
parasympathetic influences on heart rate.
A common feature of orthostatic fainting implies diminished stroke volume and
cardiac output in the lead up to syncope. Diminished cardiac output predisposes to
orthostatic hypotension if not appropriately compensated for by an increased vascular
resistance. Whether orthostatic hypotension results primarily from inadequate control
of the venous system (265-267), rather than from an underlying arterial resistance
deficit (268-270), remains unsolved at present. Our finding of an adequate arterial
resistance response to compensate for a reduction in cardiac output following
sublingual nitroglycerine in patients prone to syncope suggests a cardiac outputmediated vasovagal response (14). On the other hand, repeated tilt testing restores
orthostatic blood pressure control by restoring the relative vascular resistance
response that accounts for a progressive reduction in cardiac output during passive
orthostasis. This finding implies an arterial vascular resistance deficit in patients with
recurrent vasovagal syncope.
At this moment, the most likely explanation of orthostatic intolerance in postflight
astronauts is a limited compensatory elevation of systemic vascular resistance upon
standing (145-147). However, our observations in 5 cosmonauts returning from shortduration (10 days) spaceflight did not yield any symptoms of orthostatic intolerance:
postflight blood pressure was well maintained during a 10 min stand test at one day
after landing. Instead of a fundamental neural deficit, postflight orthostatic
tachycardia in orthostatic tolerant cosmonauts is suggested to result from a change in
the regulatory operating set point. This post-spaceflight autonomic adaptation does
not persist until 25 days upon return to Earth.
123
SUMMARY
So far, we have dealt with the impact of gravity on multiple aspects of circulatory
control in standing humans. Five studies have been included in this work. The first
two studies described cardiovascular response changes to (simulated) microgravity in
healthy subjects. In a third study, the circulatory response to standing was examined
in 5 cosmonauts before and after short duration spaceflight. The last two studies
investigated failing adaptive mechanisms of orthostatic blood pressure control in
patients who are prone to head-up tilt induced syncope. The primarily objectives and
main findings of this work are summarized in the following paragraphs.
Spectral characteristics of heart rate fluctuations during parabolic flight
Parabolic flight is used to create short, successive periods of microgravity and
hypergravity. Changing gravity levels elicit altered hydrostatic pressure gradients in
standing subjects aboard the aircraft. Stabilization of blood pressure occurs through
altered autonomic nervous system responses. We set out to assess whether the
respiratory component of heart rate variability may accurately reflect underlying
parasympathetic outflow during ultra-short gravity periods (20 – 25 s) of parabolic
flight. Therefore, during the 29th and 32nd ESA parabolic flight campaign, ECG and
respiration were recorded in 13 healthy male volunteers (22-44 yrs) in both standing
and supine postures. We developed and validated a spectral algorithm especially
adapted to study respiratory frequency components of heart rate variability in ultrashort time series. With our method, we were able to reproduce normal findings in the
upright posture, i.e., proportionally less high frequency power compared to supine (p
< 0.001). Postural differences in the distribution of spectral powers were eliminated
during microgravity, while they were amplified during hypergravity conditions
induced by parabolic flight. In the supine position no differences were shown among
different gravity periods. Our findings support the hypothesis that spectral analysis of
heart rate variability allows inferring parasympathetic influences on heart rate control
during the transient gravity periods of parabolic flight. During microgravity periods,
postural differences in heart rate are abolished due to enhanced parasympathetic
influences on standing heart rate control. The reverse sequence of events occurs
during hypergravity induced by parabolic flight.
-Summary-
124
Dynamic cardiovascular control during 60 min of thermoneutral water immersion
Thermoneutral (34.5°C) head-out of water immersion is one means of simulating
microgravity under standardized conditions in humans on Earth. Up till now, only
little attention has been directed towards the initial time course of cardiovascular
control during water immersion. The purpose of this study was to determine
hemodynamic characteristics and baroreflex control of heart rate and systemic
vascular resistance during 1 hour of thermoneutral water immersion in healthy young
subjects. We further sought to assess pre- and post-immersion baroreflex responses to
standing. After a 10-min baseline recording in the seated position, 20 subjects (19-23
yrs, 10 females) were immersed to the xiphoid during 60 min. Seated control
experiments were carried out in 10 age and gender matched controls (22-25 yrs, 5
females). Mean finger arterial pressure was monitored continuously (Finapres). Left
ventricular stroke volume, cardiac output and systemic vascular resistance were
computed from the pressure pulsations (Modelflow). Spontaneous baroreflex control
of heart rate (BRS) was estimated in the time domain. There was a rapid rise in stroke
volume during water immersion, whereas heart rate decreased significantly compared
to baseline (p < 0.05). The BRS increased immediately after the hemodynamic
changes induced by water immersion (p < 0.05). This was followed by progressive
reductions in vascular resistance and mean blood pressure within the first 20 min of
water immersion, resulting in lower values compared to baseline seated control (p <
0.05). At the same time, heart rate partially recovered, but remained significantly
suppressed over the 60 min of water immersion (p < 0.05). Appropriate time control
experiments showed no measurable changes in the baroreflex control of heart rate and
systemic vascular resistance. It is concluded that thermoneutral water immersion leads
to
baroreflex-mediated
systemic
vasodilatation
and
sustained
vagal-cardiac
bradycardia, mimicking the circulatory response to real microgravity. Integrated
circulatory control during water immersion is a dynamic process that stabilizes after a
period of 20 minutes. Whether this could be related to a new equilibrium between
transcapillary intravascular fluid shifts and the renal diuretic response should be
further investigated. The circulatory response to standing was not changed after 60
min of thermoneutral water immersion, suggestive of a preserved plasma volume and
intact control mechanism.
-Summary-
125
Respiratory modulation of cardiovascular rhythms before and after short-duration
human spaceflight
Astronauts commonly return from space with altered short-term cardiovascular
dynamics and blunted baroreflex sensitivity. Although many studies have addressed
this issue, postflight effects on the dynamic circulatory control remain incompletely
understood. It is not clear how long the cardiovascular system needs to recover from
spaceflight as most postflight investigations only extended between a few days and
two weeks. We aimed to assess the effect of short-duration spaceflight (1-2 wks) on
respiratory mediated cardiovascular rhythms in 5 cosmonauts. Two paced-breathing
protocols at 6 and 12 breaths per minute were performed in the standing and supine
positions before spaceflight, and after 1 and 25 days upon return. Dynamic baroreflex
function was evaluated by transfer function analysis between systolic pressure and the
R-R intervals. Postflight orthostatic blood pressure control was preserved in all
cosmonauts. In the standing position after spaceflight there was an increase in heart
rate of ~20 bpm or more (p < 0.05). Averaged for all 5 cosmonauts, respiratory sinus
arrhythmia and transfer gain were decreased by ~40% the day after landing (p < 0.05),
and had returned to preflight levels after 25 days. Low-frequency gain decreased
from 6.6 ± 3.4 preflight to 3.9 ± 1.6 postflight and returned to 5.7 ± 1.3 ms/mmHg
after 25 days upon return to Earth. Unlike alterations in the modulation of heart rate,
blood pressure dynamics were not significantly changed postflight. These
observations show that short-duration spaceflight reduces the respiratory modulation
of heart rate and decreases cardiac baroreflex gain without affecting postflight arterial
blood pressure dynamics. There appears to be a change in the regulatory operating
‘set point’, rather than a fundamental neural deficit upon return from spaceflight. The
altered respiratory modulation of human autonomic rhythms does not persist until 25
days upon return to Earth.
Impact of age on the vasovagal response provoked by sublingual nitroglycerine
Vasovagal syncope is a common medical problem hat afflicts patients of all ages. It is
the result of an abnormal autonomic reflex with excessive vagal tone and sympathetic
withdrawal. Nitroglycerine is used in routine tilt testing to elicit a vasovagal response.
We hypothesized that with increasing age nitroglycerine triggers a more gradual blood
pressure decline due to a diminished baroreflex buffering capacity. The purpose was
to examine the effect of nitroglycerine on baroreflex control of blood pressure in
126
-Summary-
patients with distinct age-related vasovagal collapse patterns. The study groups
consisted of 29 patients (16-71 yrs, 17 females) with clinically suspected vasovagal
syncope and a positive tilt test. Mean finger arterial pressure was monitored
continuously (Finapres). Left ventricular stroke volume, cardiac output and systemic
vascular resistance were computed from the pressure pulsations (Modelflow).
Spontaneous baroreflex control of heart rate (BRS) was estimated in the time domain.
In the first 3 minutes after nitroglycerine, blood pressure was well maintained in all
patients. This implies an adequate arterial resistance response to compensate for
steeper reductions in stroke volume and cardiac output with increasing age. Heart rate
increased and the BRS decreased after nitroglycerine. The rate of blood pressure-fall
leading to presyncope was inversely related to age (r = 0.51, p = 0.005). Accordingly,
patients with a blood pressure-fall > 1.44 mmHg/s (median) were generally younger
compared to patients with a slower blood pressure-fall (30 ± 10 yrs vs. 51 ± 17 yrs; p
= 0.001). The main determinant of the rate blood pressure drop on approach of
presyncope was the rate of fall in heart rate (r = 0.75, p < 0.001). It is concluded that,
in the older patients, sublingual nitroglycerine provokes a more gradual blood
pressure decline compared to the younger. This gradual decline cannot be ascribed to
failure of the baroreflex buffering capacity with increasing age. Age-related
differences in the laboratory presentation of a vasovagal episode depend on the
magnitude of the underlying bradycardic response.
Tilt training therapy increases vasoconstrictor reserve in patients with neurally
mediated syncope and a positive head-up tilt test
Tilt training is a useful therapeutic option in patients with recurrent neurally mediated
syncope. In this study, we tested the hypothesis that tilt training restores blood
pressure control by increasing the degree of vasomotor reserve during orthostatic
stress. Seventeen patients (age 31 ± 22 years, 11 females) with a clinical diagnosis of
neurally mediated syncope and two consecutive positive tilt tests were included in a
follow-up study. The head-up tilt test was repeated day after day: one session per day.
After two negative sessions the patients were instructed to continue daily standing
training at home. A control tilt test was repeated after six weeks follow-up. ECG and
finger arterial blood pressure (Portapres) were recorded during subsequent tilt testing.
Left ventricular stroke volume, cardiac output and systemic vascular resistance were
-Summary-
127
computed from the pressure pulsations (Modelflow). Spontaneous cardiac baroreflex
sensitivity (BRS) was estimated by cross-spectral analysis of heart rate and blood
pressure. At the start of tilt training there was an inadequate rise in vascular resistance
that failed to compensate for a postural reduction in stroke volume. This vascular
resistance-deficit resulted in a gradual blood pressure-fall between 5 to 1 minutes
preceding reflex syncope (p < 0.001). A second negative tilt test was obtained after
4.1 ± 0.9 tilt sessions and follow-up tilt testing was negative in every patient. A
negative tilt test was associated with a substantial increase in vascular resistance at
time intervals referenced to syncope time (p < 0.001). Spontaneous cardiac BRS did
not differ between subsequent tilt sessions. It is concluded that in patients with
neurally mediated syncope daily repeated tilt testing restores orthostatic tolerance in
only a few days. Tilt training increases the amount of sympathetic vasoconstrictor
reserve in the sustained upright posture. This training effect is preserved after 6 weeks
continued standing training at home.
129
SAMENVATTING
Meer nog dan bij andere levende wezens op aarde heeft de zwaartekracht een
belangrijke invloed op de bloedcirculatie bij de mens. Door de werking van de
zwaartekracht is er bij (langdurig) rechtstaan een verschuiving van ongeveer 800 ml
bloed naar de buikstreek en de onderste ledematen. Als er veel bloed in de onderste
lichaamshelft achterblijft kan het hart minder bloed rondpompen. Dit verklaart de
daling in hartdebiet met zo’n 20% in rechtopstaande positie. Bij gezonde personen
zorgt het lichaam voor allerlei aanpassingen, zoals het krachtiger en sneller pompen
van het hart en samenknijpen van de bloedvaten. Deze aanpassingen zijn noodzakelijk
om de bloeddruk te handhaven om zo de weefsels, en in de eerste plaats de hersenen,
van voldoende bloed te voorzien.
Het is omwille van de zwaartekracht dat houdingsveranderingen de bloedcirculatie
beïnvloeden: bij het wegvallen van de zwaartekracht (gewichtloosheid) is er geen
verschil meer tussen liggen en staan. In gewichtloze toestand is er een chronische
verplaatsing van bloed naar de thorax en het hoofd. Bij cosmonauten in de ruimte
heeft dit de zogenaamde ‘chicken legs’ en ‘puffy faces’ tot gevolg. Het is bekend dat
cosmonauten na hun terugkeer uit de ruimte te kampen krijgen met problemen om
rechtop te blijven staan. Men spreekt van orthostatische intolerantie. Zelfs 10 minuten
staan kan bij 1 op 3 cosmonauten leiden tot een bloeddrukdaling, al dan niet gevolgd
door bewustzijnsverlies. Deze problemen zullen zich na verloop van tijd spontaan
herstellen.
Situaties waarbij de vulling van het hart in het gedrang komt kunnen leiden tot een
autonome reflex met plotse daling in hartritme en bloeddruk. Deze vorm van
flauwvallen wordt vasovagale syncope genoemd (VVS). Hoewel VVS kan worden
uitgelokt door tal van factoren treedt het sneller op tijdens kanteltafelonderzoek bij
personen die er het meest vatbaar voor zijn. Dit laatste is nuttig om de diagnose van
VVS te bevestigen, alsook om eventuele voortekenen van syncope te leren herkennen.
Patiënten leren tijdig maatregelen te nemen om syncope af te wenden (benen kruisen
en spieren aanspannen, of voorovergebogen gaan zitten of hurken). Lang niet altijd
lukt het om met kanteltafelonderzoek syncope uit te lokken. Daarom wordt er vaak
nytroglycerine (een vaatverwijder) onder de tong toegediend om het onderzoek in te
korten.
130
-Samenvatting-
Dit proefschrift handelt over de gevolgen van zwaartekracht voor de regeling van de
bloedsomloop bij de mens, in het bijzonder bij houdingsveranderingen. Verschillende
hoofdstukken behandelen specifieke aspecten van de circulatie, en omstandigheden
waarin de bloedsomloop tekort kan schieten. De eerste twee studies onderzoeken de
onmiddellijke effecten van gewichtloosheid op de regeling van het cardiovasculair
systeem. Een derde studie beschrijft de veranderingen in cardiovasculaire controle bij
5 cosmonauten vóór en na ruimtevaart. Tenslotte worden in een tweetal studies
patiënten onderzocht tijdens kanteltafelonderzoek omdat ze vaak flauwvallen. De
doelstellingen en belangrijkste bevindingen van deze studies worden hieronder kort
samengevat.
In de eerste studie werden parabolische vluchten gebruikt om de effecten te
onderzoeken van uiterst korte periodes van afwisselend hyper- en micrograviteit op de
para-sympathische beïnvloeding van het hartritme. Tijdens twee ESA campagnes voor
parabolische vluchten werden het ECG en de ademhaling continu geregistreerd bij 13
mannelijke proefpersonen tussen 22 en 44 jaar. De proefpersonen werden zowel in
rechtopstaande als liggende positie gemeten. Er werd gezocht naar een optimale
techniek om tijdens zeer korte periodes (20 – 25 s) de variaties in hart ritme afkomstig
van parasympathische (baroreflex) invloeden te bestuderen in het frequentie domein.
Er werd gebruik gemaakt van ‘zero padding’ waarbij nullen worden toegevoegd aan
het origineel signaal om de frequentieresolutie artificieel op te drijven. De resultaten
tonen een toename in parasympathische modulatie van hart ritme tijdens
gewichtloosheid, en een afname tijdens hypergraviteit in rechtopstaande positie. Als
gevolg hiervan verdwijnen de verschillen in hart ritme tussen staan en liggen tijdens
gewichtloosheid maar zijn ze meer uitgeproken tijdens hypergraviteit.
In een tweede studie werden cardiovasculaire reflexen onderzocht tijdens
onderdompeling in thermoneutraal (34.5 °C) water. Head-out of water immersion
(WI) is een erkend model om de fysiologische invloeden van gewichtloosheid te
bestuderen onder gecontroleerde omstandigheden op aarde. Als gevolg van de
hydrostatische druk op de onderste ledematen is er een opwaartse verplaatsing van
veneus bloed richting thorax. Bijgevolg zijn cardiovasculaire autonome reflexen
tijdens WI vergelijkbaar met deze tijdens gewichtloosheid. Een gedetailleerde studie
-Samenvatting-
131
van het initiële tijdsverloop tijdens WI werd tot op heden nog niet uitgevoerd. Daarom
werd bij 20 proefpersonen (19 - 23 jaar; 10 vrouwen) de baroreflex controle van hart
ritme en perifere vasculaire weerstand onderzocht tijdens 60 min WI. De resultaten
van deze studie wijzen op een toename in slagvolume en een daling in hart ritme en
vasculaire weerstand. Dit is een dynamisch proces dat een geleidelijke
bloeddrukdaling induceert gedurende de eerste 20 min WI. Vervolgens wordt een
dynamisch evenwicht bereikt dat mogelijk het gevolg is van plasma infiltratie
enerzijds, en diuretische effecten van WI anderzijds.
In een derde studie werden 5 cosmonauten (40 - 45 jaar) opgevolgd vóór en na een
verblijf van 10 dagen in de ruimte. Het is gebleken dat cosmonauten na ruimtevaart
vaak een hogere hartfrequentie vertonen, hetgeen gepaard gaat met een daling van de
baroreflex gevoeligheid (BRS). Onderliggende mechanismen zijn onvoldoende
gekend en het is bovendien onduidelijk hoe lang het cardiovasculair systeem nodig
heeft om zich te herstellen na ruimtevaart. In deze studie werd onderzocht of een
daling in BRS bij terugkeer op aarde gevolgen heeft voor de bufferwerking van de
bloeddruk.
Onder
normale
omstandigheden
worden
natuurlijke
bloeddrukschommelingen gebuffered via feed-forward en feed-back effecten van hart
ritme. De doelstelling was om frequentie-specifieke fluctuaties in de bloeddruk te
vergelijken vóór, en tot 25 dagen na ruimtevlucht. Daartoe werd geademd op
gecontroleerde vaste frequenties van 0.1 en 0.2 Hz. Eigenschappen van de baroreflex
werden bestudeerd door middel van transfertfunctie analyse van hart ritme en
bloeddruk. De resultaten tonen aan dat een lagere BRS na een ruimtevlucht geen
enkel effect heeft op de dynamische eigenschappen van de bloeddrukregeling. Een
lagere BRS na ruimtevlucht is dus geen neuraal probleem, maar is eerder een
fenomeen dat gepaard gaat met een veranderde ‘set point’. Deze effecten zijn volledig
verdwenen na 25 dagen terug op aarde.
In een vierde studie werd het gebruik van nitroglycerine tijdens kanteltafelonderzoek
bestudeerd bij patienten met syncope. De vaatverwijdende effecten van nitroglycerine
verhogen de vatbaarheid voor syncope tijdens kanteltafelonderzoek, en dit vooral bij
oudere patiënten. De vraagstelling was of de effecten van nitroglycerine groter
worden bij oudere patiënten omwille van onvoldoende baroreflex buffering van de
bloeddruk. Er werden 29 patiënten (16 – 71 jaar; 17 vrouwen) opgenomen met een
132
-Samenvatting-
positieve kanteltafeltest. De resultaten tonen aan dat na toediening van nitroglycerine
de hartfrequentie en de vasculaire weerstand stijgen, terwijl de bloeddruk initiëel
constant blijft. We vonden dat het hartdebiet onmiddelijk na toediening van
nitroglycerine daalde. Deze daling was groter bij oudere patiënten en werd
opgevangen door voldoende toename in vasculaire weerstand. De bloeddrukdaling bij
syncope zelf verliep langzamer bij oudere patiënten. We concluderen uit deze studie
dat het klinisch beeld van syncope voornamelijk afhangt van de snelheid waarmee
symptomen zich ontwikkelen. Dit gebeurt langzamer bij oudere patiënten maar staat
los van de baroreflex controle van de bloeddruk onmiddelijk na nitroglycerine
toediening.
In een laatste studie werden 17 patiënten (11 - 67 jaar, 11 vrouwen) met een klinische
diagnose van vasovagale syncope opgevolgd tijdens herhaald kanteltafelonderzoek
(tilt-training). Tilt-training houdt in dat patiënten dagelijks een kanteltafeltest
ondergaan totdat ze tweemaal achtereenvolgens gedurende 45 min zonder symptomen
rechtop kunnen blijven staan (negatieve test). Thuis wordt de tilt-training verdergezet
door gedurende 30 min met de rug rechtop tegen de muur te staan, met de voeten op
ongeveer 30 cm van de muur verwijderd. De klinische effecten van dit soort therapie
zijn duidelijk maar onderliggende mechanismen blijven onbekend. De resultaten
tonen aan dat patiënten gemiddeld 2.9 sessies nodig hebben om een eerste negatieve
test te bereiken. De positieve effecten van tilt-training hebben waarschijnlijk te maken
met een verbetering van de sympathische vasoconstrictor reserve capaciteit. De
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149
Curriculum vitae and publication list
PERSONAL DATA
Name:
Date of birth:
e-mail:
Bart Verheyden
April 17, 1979
[email protected]
EDUCATION
•
PhD in Medical Sciences
Thesis: Cardiovascular Control in Space and on Earth: the Challenge of
Gravity
Laboratory of Experimental Cardiology
K.U.Leuven – Belgium
2003 – 2007
•
Master of Biomedical and Clinical engineering
Thesis: Ligamentous anatomy of the wrist and impact of the scapholunar
interosseous ligament on radiocarpal wrist pressure
Faculty of Engineering
K.U.Leuven – Belgium
2001 – 2002: graduated cum laude
•
Master of Physical Education and Kinesiology
Thesis: Analysis of the service risk in volleyball as function of the set score
and set course
Faculty of Kinesiology and Rehabilitation Sciences
K.U.Leuven – Belgium
1997 – 2001: graduated cum laude
•
Geaggregeerde voor het secundair onderwijs – groep 2: Lichamelijke
Opvoeding
K.U.Leuven – Belgium
2000 – 2001
LIST OF PUBLICATIONS
Articles in reviewed journals
Verheyden B, Gisolf J, Beckers F, Karemaker J, Wesseling KH, Aubert AE,
Wieling W.
Impact of age on the vasovagal response provoked by sublingual
nitroglycerine in routine tilt testing.
Clinical Science. 2007; In press
150
Verheyden B, Beckers F, Couckuyt K, Liu J, Aubert AE.
Respiratory modulation of cardiovascular rhythms before and after shortduration human spaceflight.
Acta Physiologica, 2007; In press
Beckers F, Verheyden B, Couckuyt K, Jiexin L, Aubert AE.
Long-term changes in autonomic cardiovascular control after short-duration
spaceflights.
Eur J Appl Physiol, 2007; In press
Verheyden B, Eijnde BO, Beckers F, Vanhees L, Aubert AE.
Low dose exercise training does not influence cardiac autonomic control in
sedentary healthy men 55-75 yrs old.
Journal of Sports Sciences. 2006; 24(11): 1137-1147
Verheyden B, Beckers F, Couckuyt K, Liu J, Aubert AE.
Heart rate reflexes and arterial pressure control after short-duration
spaceflight.
J Gravit Physiology. 2006; 3(1): 59-60
Beckers F, Verheyden B, Aubert AE.
Aging and nonlinear heart rate control in a healthy population.
Am J Physiol Heart Circ Physiol. 2006; 290: 2560-2570
Beckers F, Verheyden B, Ramaekers D, Swynghedauw B, Aubert AE.
Effects of autonomic blockade on non-linear cardiovascular variability indices
in rats.
Clin Exp Pharmacol Physiol. 2006; 33: 431-9.
Beckers F, Verheyden B, Couckuyt K, Aubert AE.
Fractal dimension in health and heart failure
Biomed Tech. 2006; 51(4):194-197
Verheyden B, Beckers F, Aubert AE.
Spectral characteristics of heart rate fluctuations during parabolic flight.
Eur J Appl Physiol. 2005; 1-12
Aubert AE, Beckers F, Verheyden B.
Cardiovascular function and basics of physiology in microgravity.
Acta Cardiol. 2005; 60: 129-151
Aubert AE, Beckers F, Verheyden B, Pletser V.
What happens to the human heart in space? Parabolic flights provide some
answers.
ESA-Bulletin. 2004; 119: 30-38
151
Aubert AE, Beckers F, Ramaekers D, Verheyden B, Leribaux C, Aerts J,
Berckmans D.
Heart rate and heart rate variability in chicken embryos at the end of
incubation.
Exp Physiol. 2004; 89(2):199-208
Beckers F, Verheyden B, De Winne F, Duque P, Didier C, Aubert AE.
HICOPS: human interface computer program in space.
J Clin Monit Comput. 2004; 18(2):131-6
Beckers F, Ramaekers D, Speijer G, Ector H, Vanhaecke J, Verheyden B, Van
Cleemput J, Droogne W, Van de Werf F, Aubert AE.
Different evolutions in heart rate variability after heart transplantation: 10-year
follow-up.
Transplantation. 2004; 78(10):1523-31
Verheyden B, Beckers F, Aubert AE.
Heart rate variability during water immersion.
J
Gravit
Physiology.
2003;
10:
81-82
Verheyden B, Beckers F, Aubert AE.
Frequency analysis of cardiovascular variability during parabolic flight.
J Gravit Physiology. 2003; 10: 85-86
Beckers F, Seps B, Ramaekers D, Verheyden B, Aubert AE.
Parasympathetic heart rate modulation during parabolic flights.
Eur J Appl Physiology. 2003; 90(1-2): 83-91
Beckers F, Verheyden B, Aubert AE.
Evolution of heart rate variability before, during and after spaceflight.
J Gravit Physiology. 2003; 10: 107-108
Articles in books
Verheyden B, Beckers F, Aubert AE.
Exercise Physiology: Bioenergetics and Systemic Responses to Exercise. 2006
In: Wiley Encyclopedia of Biomedical Engineering (Metin Akay, ed.)
Hoboken: John Wiley & Sons, Inc. Doi.org/ 10.1002/9780471740360.ebs1132
Beckers F, Verheyden B, Aubert AE.
Space Physiology. 2006 In: Wiley Encyclopedia of Biomedical Engineering
(Metin Akay, ed.) Hoboken: John Wiley & Sons, Inc. Doi/
10.1002/9780471740360.ebs1107
Aubert AE, Beckers F, Verheyden B, Van de Voorde P.
Electrodes. 2006 In: Wiley Encyclopedia of Biomedical Engineering (Metin
Akay,
ed.)
Hoboken:
John
Wiley
&
Sons,
Inc.
Doi/
10.1002/9780471740360.ebs1107
152
Couckuyt K, Verheyden B, Beckers F, Liu J, Aubert AE
Complex demodulation of the baroreflex during parabolic flight. In: IEEE
Computers in Cardiology 33: 869-872, 2006
Verheyden B, Couckuyt K, Beckers F, Aubert AE.
Complex Demodulation of Heart Rate Variability during Parabolic Flight. In:
IEEE Computers in Cardiology 2005; 32:271-274
Beckers F, Anné W, Verheyden B, van der Dussen de Kestergat C, Van Herk E,
Janssens L, Willems R, Heidbüchel H, Aubert AE.
Determination of Atrial Fibrillation Frequency Using QRST-Cancellation with
QRS-Scaling in Standard Electrocardiogram Leads. In: IEEE Computers in
Cardiology 2005; 32:339-342.
Aubert AE, Beckers F, Verheyden B.
Studies in Space related Life Science: Cardiology. IN: Space research in
Belgium. Report to the 35th COSPAR assembly, 2004; 84-88
Beckers F, Verheyden B, Aubert AE.
Human interface program (HiCop) guidance for the cardiovascular experiment
during Odissea mission. In: Adaptation to extreme conditions. Proceedings of
the meeting for the 40th Anniversary of IBMP, Moscow 2003; 394 – 396.
Beckers F, Verheyden B, Aubert AE.
Influence of spaceflight on heart rate variability. In: Adaptation to extreme
conditions. Proceedings of the Meeting for the 40th Anniversary of IBMP,
Moscow 2003; 396-398