Basic Cardiac Function

Biology 347 General Physiology Lab
Basic Cardiac Functions
Blood Pressure, Pulse Determinations and EKG
Objectives
•
•
•
•
•
•
Students will learn traditional techniques associated with taking a pulse and blood pressure.
Students will learn how to interpret data from a plethysmograph using the iWorx system.
Students will learn techniques associated with electrocardiograpghy.
Students will learn how to interpret the ECG in terms of depolarization and repolarization and identify the P,
QRS and T waves using the iWorx system.
Students will calculate their heart rate, QRS interval P-Q interval and Q-T interval using data produced from the
iWorx system.
Students will compare components of the ECG in resting and exercising states.
Introduction
In healthy heart, the two atria contract simultaneously. As they begin to relax, simultaneous contraction of the ventricles
occurs. According to general usage, the terms systole an diastole refer to events of ventricular contraction and relaxation
respectively. The cardiac cycle is equivalent to one complete heartbeat- during which both atria and ventricles contract and
then relax. It is marked by a succession of changes in blood volume and pressure within the heart.
We will begin the discussion of the cardiac cycle with the heart in complete relaxation (diastole). At this point, pressure in
the heart is very low, blood is flowing passively from the pulmonary and systemic circulations into the atria and on through
to the ventricles; the semilunar valves are closed and the AV valves are open. Shortly, atrial contraction occurs and atrial
blood pressure increases, forcing residual blood into the ventricles. Then ventricular systole begins and intraventricular
pressure increases rapidly, closing the AV valves. When ventricular pressure exceeds that of large arteries leaving the heart,
the semilunar valves are forced open; and the blood in the ventricular chambers is expelled through the valves. During this
phase, the aortic pressure reaches approximately 120mm Hg in a healthy young adult. During ventricular systole, the atria
relax and their chambers fill with blood, which results in gradually increasing atrial pressure. At the end of ventricular
systole, the ventricles relax; the semilunar valves snap shut, preventing backflow, and momentarily, the ventricles are
closed chambers. When the aortic semilunar valve snaps shut, a momentary increase in the aortic pressure results from the
elastic recoil of the aorta after valve closure. This event results in a pressure fluctuation called the dicrotic notch. As the
ventricles relax, the pressure within them begins to drop. When intraventricular pressure is again less than artrial pressure,
the AV valves are forced open, and the ventricles again begin to fill with blood. Atrial and aortic pressures decrease, and
the ventricles rapidly refill, completing the cycle.
The average heart beats approximately 75 beats per minute, and so the length of the cardiac cycle is about 0.80 seconds.
Of this time period, atrial contraction occupies the first 0.1 second, which is followed by atrial relaxation and ventricular
contraction for the next 0.30 second. The remaining 0.40 second is the quiescent or ventricular relaxation period. When
the heart beats at a more rapid pace than normal, this last period decreases.
Notice that two different types of phenomena control movement of blood through the heart: the alternate contraction and
relaxation of the myocardium, and the opening and closing of valves (which is entirely dependent on the pressure changes
within the heart chambers).
Two distinct sounds can be heard during each cardiac cycle. These heart sounds are commonly described as lubb and
dupp. The sequence is lubb-dupp, pause and so on. The first heart sound, lubb, is referred to as the S1 and is associated
with closure of the AV valaves at the beginning of ventricular systole. The seconds sound, dupp, called the S2 occurs as
the semilunar valves close and corresponds to the end of systole.
The following figure represents all aspects of cardiac cycle and can be used to coordinate sounds with functions or data
with functions.
Figure 1: The Cardiac Cycle
Adapted from: http://www.mscd.edu/~biology/2320course/2320images/cardiac-physv3.gif
Heart contraction results from a series of electrical potential changes (depolarization waves) that travel through the heart
preliminary to each beat. Because cardiac muscle cells are electrically connected by gap junctions, the entire myocardium
behaves as a single unit.
The ability of cardiac muscle to beat is intrinsic- as it does not depend on impulses from the nervous system to initiate
contraction and will continue to contract rhythmically even if all nerve connections are severed. However, two types of
controlling systems exert their effects on heart activity. One of these involves nerves of the autonomic nervous system,
which accelerates and decelerates the heartbeat depending on its activation. The second system is the intrinsic conductive
system or nodal system, of the heart, consisting of specialized non-contractile myocardial tissue. The intrinsic conduction
system ensures that the heart muscle depolarizes in an orderly and sequecntial manner (from atria to ventricles) and that
the heart beats as a coordinated unit.
The components of the intrinsic conduction system include the sinoatrial (SA) node, located in the right atrium just
inferior to the entrance of the superior vena cava; the atrioventricular (AV) node in the lower artial septum at the juncture
of the aria and ventricles; the AV bundle (also called the bundle of His) and left and right bundle branches, located in the
interventricular septum and the Purkinje fibers, essentially long strands of Purkinje myocytes, which ramify within the
muscle bundles of the ventricular wall. The Purkinje fiber network is much elaborate and denser in the left ventricle due to
the large size of this chamber.
The SA node, which has the highest rate of
discharge, provides the stimulus for
contraction. Because it sets the rate of
depolarization for the heart as whole, the SA
node is often referred to as the pacemaker.
From the SA node, the impulse spreads from
the atria to the AV node. This electrical wave is
followed by arterial contraction. At the AV
node, the impulse is momentarily delayed
allowing the atria to complete their contraction.
It then passes through the AV bundle, the right
and left branches, and the Purkinje fibers,
finally resulting in ventricular contraction. Note
that the atria and ventricles are separated by a
region of electrically inert connective tissue, so
the depolarization wave can be transmitted to
the ventricles only via the tract between the AV
node and AV bundle. Thus, any damage to the
AV node-bundle pathway partially or totally
insulates the ventricles from the influence of the SA node. Although auto- rhythmic cells are found throughout the heart,
their rates of spontaneous depolarization differ. The nodal system increases the rate of heart depolarization and
synchronizes heart activity.
The conduction of impulses through the heart generates electrical currents that eventually spread throughout the body.
These impulses can be detected on the body’s surface and recorded with an instrument called an electrocardiograph. The
graphic recording of the electrical changes (depolarization followed by repolarization) is called an electrocardiogram
(ECG).
The typical ECG consists of a series of three recognizable waves
called deflection waves. The first wave, the P wave, is a small wave
that indicated depolarization of the atria immediately before the atrial
contraction. The large QRS complex, resulting from ventricular
depolarization, has a complicated shape (primarily because of the
variability in size of the two ventricles and the time differences
required for these chambers to depolarize). It precedes ventricular
contraction. The T wave results from currents propagated during
ventricular repolarization. The repolarization of the atria, which
occurs during the QRS interval, is generally obscured by the large
QRS complex.
The nature of physiology labs requires the use test subjects, most often ourselves. You are under no obligation to be a test
subject for any experiment; your grade will not impacted if you choose not to volunteer as such. None of the experiments
are designed to cause serious harm to the subject. Nor should the subject feel that any results of the experiments are a
diagnosis of a medical condition. While a physician may use an ECG to identify abnormalities in heart physiology, it is not
within the scope of this course to diagnose any particular medical problems. These exercises are designed to teach students
how to properly use equipment and interpret data. If the subject feels uneasy about the results of some experiments as
they pertain to their own well-being, then the subject should seek professional medical advice.
Procedure: Blood Pressure in the Resting Subject
Blood pressure is defined as the pressure the blood exerts against any unit of area of the blood vessel walls and it is
generally measured in arteries. Because the heart alternately contracts and relaxes, the resulting rhythmic flow of blood
into the arteries causes the blood pressure to rise and fall during each beta. Thus you must take two blood pressuring
readings: the systolic pressure, which is the pressure in the arteries at the peak of ventricular ejection, and the diastolic
pressure, which reflects the pressure during ventricular relaxation. Blood pressure are reported in millimeters of mercury
(mm Hg), with the systolic pressure appearing first; 120/80 translates to 120 over 80; or a systolic pressure of 120 mmHg
and a diastolic pressure of 80 mmHg. Normal blood pressure varies considerably from one individual to the next.
The sphygmomanometer, commonly called the blood pressure cuff, is an instrument used to obtain blood pressure
readings by the auscultatory method. It consists of an inflatable cuff with an attached pressure gauge. The cuff is placed
around the upper arm and inflated to a pressure higher then the systolic pressure to occlude circulation to the forearm. As
cuff pressure is gradually released, the examiner listens with a stethoscope for characteristic sounds called the sounds of
Korotkoff., which indicate the resumption of blood flow into the arm. The pressure at which the first soft tapping sounds
can be detected is recorded as the systolic pressure. As the pressure is reduced further, blood flow becomes more
turbulent and the sounds become louder. As the pressure is still reduced further, below the diastolic pressure, the artery is
no longer compressed; and blood flows freely without turbulence. At this point the sounds if Korotkoff can no longer be
detected. The pressure at which the sounds disappear is recorded as the diastolic pressure.
Work in pairs to take each other’s blood pressure. You will need a stethoscope, alcohol swabs and a sphygmomanometer.
Clean the earpieces of the stethoscope with the alcohol swab before and after each use. When inflating the
sphygmomanometer on you subject, do not keep it inflated for over one minute as the reduced blood flow could cause
fainting.
1. Wrap the cuff around the subject’s upper left arm. Have the subject rest his/her arm on the bench while he/she is
sitting.
2. Find the location where you palpate the brachial pulse. This is where you want the stethoscope to be.
3. Inflate the cuff to about 150mmHg (less if the pressure might cause pain to the subject).
4. Using the release valve on the sphygmomanometer, slowly release the pressure.
5. At some point you should hear the sounds of Korotkoff, this is the systolic pressure.
6. At some point the sounds of Korotkoff will stop and this is the diastolic pressure.
7. Remember these values and record.
8. Remove the cuff from the subject when finished. Have the subject move his/her fingers to resume blood flow
again.
9. You may repeat this experiment; however allow two to three minutes between trials for the subject to regain
blood flow homeostasis.
10. Compute the pulse pressure for each trail. Pulse pressure is the difference between the systolic and diastolic
pressures.
11. Compute the Mean Arterial Pressure (MAP). MAP = diastolic pressure + (pulse pressure / 3)
Table 3: Blood Pressure Measurements in the Resting Subject
Systolic Pressure
Subject 1 Trial 1
Subject 1 Trial 2
Subject 2 Trial 1
Subject 2 Trial 2
Diastolic Pressure
Pulse Pressure
MAP
Procedure: Blood Pressure in the Exercising Subject
To see how exercise affects blood pressure, have your test subject run in place or do jumping jacks for one or more
minutes. The using the method described above, take the blood pressure of your subject before exercise and after. Fill in
the following table.
Table 4: Blood Pressure Measurements in the Exercising Subject
Systolic Pressure
Diastolic Pressure
Pulse Pressure
MAP
Subject 1 Resting
Subject 1 Post Exercise
Subject 2 Resting
Subject 2 Post Exercise
Procedure: Connecting the iWorx Software and Test Subject
1. Click on the Settings menu again. This time a list of labs will be in a drop down menu. Please select ECGCirculation- LS2 lab.
2. Once selected, the configurations for this lab will load onto the screen. You will also notice that an adobe file for
the lab will pop-up. You are welcome to use this file to help you through the exercises.
3. Have the test subject remove all jewelry from their wrists and ankles.
4. Gently wipe the areas on both wrists and both ankles with an alcohol swab. Allow the areas to air dry.
5. Place the sticky electrodes on these four areas. On the right wrist place another electrode adjacent to the other.
6. Attach the large end of the patient cable to the isolated Channel 1 and 2 input on the iWorx unit.
7. Now attach the leads as follows:
• The red +1 lead attached to the right wrist
• The black -1 lead attached to the left wrist
• The green “C” lead (the ground) attached to the right ankle
8. Connect your plethysmograph into channel 3 on the iWorx unit. Wrap the plethysmograph around your volar
surface (where the fingerprints are) of your middle finger. Wrap the Velcro strap around your finger to secure the
plethysmograph in place.
9. The test subject should sit quietly with their hands on their lap.
Procedure: The ECG of a Resting Subject
1. When the subject is ready, click the Record button on the top right-hand side of the screen.
2. Then click Autoscale on each of the channels.
3. When you have a suitable trace, type “(Subject’s Name) Resting ECG in the field next to the right of the Mark
button and then press enter to apply the comment.
4. To stop recording click on the Stop button.
5. Select Save As in the File menu. Type your subject’s name and RestingECG. Save the chart to the desktop.
Procedure: Analysis of the Resting ECG
1. Click the 2-Cursor icon on the icon tray (second from the right). Two blue vertical lines appear over the
recording window.
2. Drag the lines to the left and right until you have four complete heart beat cycles within the blue line.
3. Click on the Analysis icon on the icon tray. This will open the analysis window.
4. We will only be looking at Lead I for this data. However, be aware that there are many different analyses that can
occur by looking at the different leads. To view Lead I make sure that it is the only item selected in the Display
Channel Box.
5. Use the I-beam cursors (2 blue cursors) to identify regions that you are analyzing. These cursors can be moved by
dragging the lines to the location that you want. Use the following table to identify the various regions. Then, fill
in your data into Table 2: Elapsed Time for ECG Components (Resting). Remember that the computer can tell
you the time in seconds of the various phases (read the T2-T1 data box).
Table 5: Boundaries of Each ECG Component
Feature
P Wave
P-R Interval
P-R Segment
QRS Complex
S-T Segment
Q-T Interval
T Wave
End T Wave to Next R
Boundaries
Start of the P deflection to return to the isoelectric line
Start of the P deflection to start of Q deflection
End of P wave to start of Q deflection
Start of Q deflection to S return to isoelectric line
End of S deflection to start of T wave
Start of Q deflection to end of T wave
Start of the T deflection to return to the isoelectric line
End of T wave to next R spike
Table 6: Elapsed Time (time in seconds) for ECG Components (Resting)
Component
Cycle 1
Cycle 2
Cycle 3
Mean
P Wave
P-R Interval
P-R Segment
QRS Complex
S-T Segment
Q-T Interval
T Wave
End T Wave to Next R
Procedure: The ECG of an Exercising Subject
1. Have your test subject exercise for one minute. This might include jogging in place of jumping jacks. At the end
of the exercise have the subject sit down and start recording as you did earlier.
2. At the end of recording the data, save the file as your subject’s name and ExerECG.
3. Perform the same data analysis as before. Record your data in Table 3: Elapsed Time for ECG Components
(Exercising).
4. Fill-in Table 4 with the means from each study.
Table 7: Elapsed Time for ECG Components (Exercising)
Component
P Wave
P-R Interval
P-R Segment
QRS Complex
S-T Segment
Q-T Interval
T Wave
End T Wave to Next R
Cycle 1
Cycle 2
Cycle 3
Mean
Table 8: Average Duration of ECG Components (in seconds)
Component
Normal Duration
P Wave
0.06-0.11
P-R Interval
0.12-0.20
P-R Segment
0.08
QRS Complex
< 0.12
S-T Segment
0.12
Q-T Interval
0.31-0.41
T Wave
End T Wave to Next R
Resting
Post Exercise
0.16
Varies
Questions and Conclusions
As a general observation, what happens to the components of an ECG after exercise?
Are there any deviations in your data compared to the normal for both EKGs and blood pressure? Explain.
What sources of error are present with lab? How could you change the experiment to minimize error or produce better
results?
What affect does exercise have on blood pressure? Did the systolic pressure change? The diastolic pressure?
What is pulse pressure? What is a normal value? What does a lower value indicate? A higher value?
What is MAP? What is a normal value? What does a lower value indicate? A higher value?
What errors occurred during your experimentation? What might you have done differently?