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Cardiac contractions are not dependent upon a nerve supply, but the central nervous system can
affect cardiac rhythm. Innervation by the parasympathetic (vagus) and sympathetic nerves
modifies cardiac rhythm. The sinoatrial (SA) node, a group of specialized cardiac muscles
fibers, acts as the pacemaker for the heart. These cells rhythmically produce action potentials
that spread through the muscle fibers of the atria. The resulting contraction pushes blood into the
ventricles. The only electrical connection between the atria and the ventricles is via the
atrioventricular (AV) node. The action potential spreads slowly through the AV node, thus
allowing atrial contraction to contribute to ventricular filling, and then rapidly through the AV
bundle and Purkinje fibers to excite both ventricles (Figure 1).
Figure 1. Components of the Human Heart
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The cardiac cycle involves a sequential contraction of the atria and ventricles. The combined
electrical activity of the different myocardial cells produces electrical currents that spread
through the body fluids. These currents are large enough to be detected by recording electrodes
placed on the skin. The regular pattern of peaks during one cardiac cycle is shown in Figure 2.
The components of the ECG can be correlated with the electrical activity of the atrial and
ventricular muscle:
• the P-wave is produced by atrial depolarization
• the QRS complex is produced by ventricular depolarization. Atrial repolarization also
occurs during this time
• the T-wave is produced by ventricular repolarization (Figure 2)
Figure 2. One Cardiac Cycle showing the P wave, QRS complex and T wave
The action potentials recorded from atrial and ventricular fibers are different from those recorded
from nerves and skeletal muscle. The cardiac action potential is composed of three phases: a
rapid depolarization, a plateau depolarization (which is very obvious in ventricular fibers), and a
repolarization back to resting membrane potential (Figure 3).
Figure 3. A typical ventricular muscle action potential.
Heart valves and heart sounds
Each side of the heart is provided with two valves, which convert rhythmic contractions into a
unidirectional pumping. The valves close automatically whenever there is a pressure difference
across the valve that would cause backflow of blood. Closure gives rise to audible vibrations
(heart sounds). Atrioventricular (AV) valves between the atrium and ventricle on each side of
the heart prevent backflow from ventricle to atrium. Semilunar valves are located between the
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ventricle and the artery on each side of the heart and prevent backflow of blood from the aorta
and pulmonary artery into each ventricle.
The closure of these valves is responsible for the characteristic sound produced by the heart,
which is usually referred to as a ‘lub-dup’ sound. The lower-pitched ‘lub’ sound occurs during
the early phase of ventricular contraction. This is produced by closing of the atrioventricular
(mitral and tricuspid) valves. When the ventricles relax, the blood pressure drops below that in
the artery, and the semilunar valves (aortic and pulmonary) close, producing the higher-pitched
‘dup’ sound. Malfunctions of these valves often produce an audible murmur, which can be
detected with a stethoscope.
The cardiac cycle
The sequence of events in the heart during one cardiac cycle is summarized in Figure 4. During
ventricular diastole, blood returns to the heart. Deoxygenated blood from the periphery enters
the right atrium and flows into the right ventricle through its open atrioventricular (AV) valve.
Oxygenated blood from the lungs enters the left atrium and flows into the left ventricle through
its open AV valve. Filling of the ventricles is completed when the atria contract (atrial systole).
In the resting state, atrial systole accounts for some 20% of atrial filling. Atrial contraction is
followed by contraction of the ventricles (ventricular systole). Initially, as the ventricles begin to
contract, the pressure in them rises and exceeds that in the atria; this closes the AV valves. But,
until the pressure in the left ventricle exceeds that in the aorta (and in the right ventricle exceeds
that in the pulmonary artery), the volume of the ventricles cannot change. This is the so-called
isovolumic phase of ventricular contraction. Finally, when the pressure in the left ventricle
exceeds that in the aorta (and the pressure in the right ventricle exceeds that in the pulmonary
artery), the aortic and pulmonary valves open and blood is ejected into the aorta and pulmonary
arteries. As the ventricular muscle relaxes, pressures in the ventricles fall below those in the
aorta and pulmonary artery, and the aortic and pulmonary valves close. Ventricular pressure
continues to fall and once it has fallen below that in the atria, the AV valves open and ventricular
filling begins again.
Figure 4. Cardiac Cycle
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Changes in a variety of parameters during one cardiac cycle are summarized in a figure
introduced by Wiggers. A modified form of this is shown in Figure 5. The importance of this
representation is that it allows you to see the temporal relationships between the different
parameters.
Figure 5. A Wiggers' diagram.
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Equipment Setup and Electrode Attachment
1.
Make sure the PowerLab is turned off and the USB cable is connected to the computer.
2.
Connect the 5 Lead Shielded Bio Amp Cable to the Bio Amp Connector on the front panel (Figure 6). The
hardware needs to be connected before you open the settings file.
3.
Attach the Shielded Lead Wires to the Bio Amp Cable. Channel 1 positive will lead to the left wrist, Channel 1
negative will lead to the right wrist, and the Earth will lead to the right leg. Attach the Disposable Electrodes to
the end of the Channel 1 and Earth wires. Refer to Figure 6 for proper placement. Follow the color scheme on
the Bio Amp Cable. Remove any jewelry from the volunteer’s hands, arms, and right leg and stick the
Disposable Electrodes to the skin (Figure 6).
Note: Do not place the electrodes over the major muscles because muscle activity interferes with the signal recorded
from the heart.
4.
Check that all three electrodes are properly connected to the volunteer and the Bio Amp Cable before
proceeding. Turn on the PowerLab, then open the Heart Sounds Settings file
(Experiments/HumanPhysiology/ECG_and_Heart_Sounds/Settings_files/Heart_Sounds_settings)
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Resting ECG
Figure 6. Electrode Placement
1.
Have the volunteer assume a relaxed position.
2.
Start recording. Add a comment with “Resting ECG.” Record the ECG for one minute.
3.
Save your data when you are finished recording. Do not close the file.
ECG and Phonocardiography
In this exercise, you will use a Cardio Microphone placed over the chest wall to record the heart sounds, which
allows the heart sounds to be displayed graphically in real time. [Note: We have stethoscope if you want to listen
directly as well].
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1.
Connect the Cardio Microphone to Input 1 on the front panel of the PowerLab (Figure 7). The hardware needs
to be connected before you open the settings file.
2.
Open the settings file “Phonocardiography Settings” from the Experiments tab in the Welcome Center. It will
be located in the folder for this experiment.
Figure 7. Equipment Setup for PowerLab 26T
3.
Have the volunteer place the Cardio Microphone on the left side of the chest against the skin.
Start recording. Move the Cardio Microphone to get the best signal possible. When ready, Stop recording, and
tape down the Cardio Microphone.
Amplitude
Duration
(mV)
(s)
Note: It is important to tape down the Cardio Microphone instead of
P Wave
1
holding it as the hand introduces considerable noise into the
recording.
2
5. Have the volunteer sit in a relaxed position, facing away from
the monitor. Remind the volunteer not to move during the
3
recording.
4.
6.
When ready, Start recording. Record for 15 seconds, and Stop.
Save your data.
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ECG in a Resting Volunteer
1.
2.
Mean
QRS
Complex
1
Examine the data in the Chart View. Use the View Buttons to
set the horizontal compression to 5:1. Scroll through the data to
observe the regularly occurring ECG cycles.
2
Use the Marker and Waveform Cursor to measure the
amplitudes and durations of four P waves, QRS complexes, and
T waves from the ECG trace.
4
•
•
3.
4
3
Mean
To measure the amplitudes place the Marker on the
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baseline immediately before the P wave. Then move the Waveform Cursor to the peak of the wave.
To measure the durations place the Marker at the start of the wave or complex and position the
Waveform Cursor at the end of the wave or complex.
Record these values in the table at right, and find the mean amplitude and duration for each phase of the cycle.
What does the P wave amplitude represent? How does it vary? Is this similar to the variation one would expect
in skeletal muscle EMG recordings? Why or why not?
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4.
Use the View Buttons to set the horizontal compression to 10:1.
Measure the time interval (in seconds) between three pairs of adjacent
R waves using the Marker and Waveform Cursor (R-R time
interval). For each interval, calculate the heart rate using the equation
below:
Pair
2
Record these values in the table at right.
Is the heart rate regular? What determines the regularity of the heart beat?
What can influence this?
Pair
3
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ECG and Phonocardiography
1.
Examine the data in the Chart View, and Autoscale, if necessary.
Note the relationship between the R wave and the first heart sound.
2.
Place the Marker on the R wave, and place the Waveform
Cursor on the beginning of the first heart sound. Note the time
between these two events as shown at right.
3.
Repeat steps 1 and 2 for the T wave and its relationship with the
second heart sound.
4.
Record these values in the table at right
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R Wave to
First Sound
(s)
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Heart Rate
(BPM)
Pair
1
60
Heart rate (beats / minute) =
time interval (seconds)
5.
R-R
Time
Interval
(s)
T Wave to
Second Sound
(s)
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Background
The pressure in the arteries varies during the cardiac cycle. The ventricles contract to push blood into the arterial
system and then relax to fill with blood before pumping once more. This intermittent ejection of blood into the
arteries is balanced by a constant loss of blood from the arterial system through the capillaries. When the heart
pushes blood into the arteries there is a sudden increase in pressure, which slowly declines until the heart contracts
again. Blood pressure is at its highest immediately after the ventricle contracts (systolic pressure) and at its lowest
immediately prior to the pumping of blood into the arteries (diastolic pressure). Systolic and diastolic pressures can
be measured by inserting a small catheter into an artery and attaching the catheter to a pressure gauge. Such a direct
measurement might be accurate, but is invasive and often inconvenient and impractical. Simpler estimates of blood
pressure can be made with acceptable accuracy using noninvasive, indirect methods.
The modern era of blood pressure measurement started with the introduction of the mercury sphygmomanometer by
Scipione Riva-Rocci (1863-1937) in 1896. Blood pressure is estimated using a stethoscope and a blood pressure cuff
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connected to a mercury sphygmomanometer. The cuff is placed on the upper arm and inflated to stop arterial blood
flow to the arm from the brachial artery. The high pressure in the cuff collapses the artery. The pressure in the cuff
is then released slowly. When the cuff pressure begins to fall below the systolic pressure in the artery, blood begins
to flow to the arm through the partially collapsed artery. This flow is turbulent rather than streamlined and generates
sounds called Korotkoff sounds, which can be heard through the stethoscope. When blood flow is first heard, the
cuff pressure approximates systolic pressure. As the cuff pressure continues to decrease and the artery regains its
normal diameter, flow becomes streamlined and the sounds become muffled and then disappear. The cuff pressure
at the point of the sound muffling approximates diastolic pressure, but as the disappearance of sound if easier to
detect than muffling, and since the two occur within a few millimeters of mercury pressure, the disappearance of
sound is commonly used the determine diastolic pressure.
An alternative method makes use of a simple finger pulse transducer connected to the computer. The cuff is inflated
to a pressure that obliterates the finger pulse. As the cuff pressure is released, the finger pulse returns and the
pressure at which it reappears is a measure of the arterial systolic pressure.
It is conventional to reference all arterial blood pressure measurements to the position of the heart; there are
differences in pressure with the arm held in different positions. In a column of blood, a difference in height of one
meter corresponds to a pressure difference of 10.3 kPa, or 77 mmHg. Due to hydrostatic pressure of a column of
blood, any measurements made below the heart will be increased in pressure and any made above the heart will be
decreased in pressure relative to heart level.
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Equipment Setup
1.
Make sure the PowerLab is turned off and the
USB cable is connected to the computer.
2.
Connect the Pressure Transducer to Input 1 on
the front panel of the PowerLab (Figure 8).
3.
Connect the Finger Pulse Transducer to Input 2
on the front panel of the PowerLab (Figure 8).
4.
Connect the Cardio Microphone to Input 3 on the
front panel of the PowerLab (Figure 8).
Figure 8. Equipment Setup for PowerLab 26T ->
5.
Place the pressure pad of the Finger Pulse
Transducer on the tip of the middle finger of the hand with the Blood Pressure Cuff attached. Use the Velcro
strap to attach it firmly but without cutting off circulation.
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6.
If the strap is too loose, the signal will be weak, intermittent, or noisy. If the strap is too tight,
blood flow to the finger will be reduced causing a weak signal and discomfort.
Have the volunteer face away from the monitor, sit comfortably, and relax. Make sure the Finger Pulse
Transducer is not resting on any surface. The volunteer’s hands should be in his/her lap and have the leg
support the wrist, allowing the fingers to hang freely. Tell the volunteer not to move during the recording.
Blood Pressure and Arterial Sounds
In this exercise, you will record arterial sounds while measuring blood pressure. You will use a cardio microphone
instead of the stethoscope typically used in medical settings. Close the current file, and open the Blood Pressure
Settings file ((Experiments/HumanPhysiology/Blood_Pressure/Settings_files/Blood_Pressure_settings)
Note. If you opened the settings file “Blood Pressure Settings” before the Cardio Microphone was connected
to input 3 of the PowerLab the range for Channel 3 will be need to be set to 100 mV.
7.
Make sure the volunteer is relaxed with the Finger Pulse Transducer free from any surface. If the volunteer
feels any discomfort, re-adjust the transducer before continuing with the experiment.
8.
Place the Cardio Microphone over the brachial artery. Secure the microphone in place. [Note: you can do this
with a stethoscope if you’d like, but the Cardio Microphone makes recording easier]
9.
Make sure you can inflate the Blood Pressure Cuff without disturbing the volunteer’s hand or the microphone.
Adjust any of the equipment now, if necessary.
10. Start recording. Enter a comment with “CardioPressureRecord” and continue recording for 10 seconds.
11. Inflate the Blood Pressure Cuff until the pressure is approximately 180 mmHg.
12. Slowly reduce the pressure in the cuff.
Once the pressure has gone below 50
mmHg deflate the cuff completely. Stop
recording.
13. Examine your cardio microphone
recording. It should look similar to Figure
9.
14. Save your data and close the file. The
cardio microphone can be removed from
the volunteer’s arm. Leave the Finger
Pulse Transducer on the volunteer’s
finger.
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Figure 9. Cardio Microphone Recording of Arterial Sound
During Blood Pressure Measurement
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Analysis
Blood Pressure and Pulse
1.
Examine the data in the Chart View and Autoscale, if necessary. Use the View Buttons to change the
compression of the data trace so the entire exercise can be viewed at once.
2.
Use the Marker and Waveform Cursor to measure the systolic
and diastolic pressures, based on the comments. Place the
Marker on the baseline pressure (i.e., before you inflated the
cuff) and move the Waveform cursor to the systolic and diastolic
comments. Enter the pressure values in the table at right.
Explain the changes in the pulse trace as you inflate the Blood
Pressure Cuff.
Systolic
Pressure
(mmHg)
Diastolic
Pressure
(mmHg)
Pressure
Value
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Explain the changes you see in the phonocardiogram as you release the pressure in the Blood Pressure Cuff.
How are the phonocardiogram and pulse measurements related?
Mammalian Dive Reflex
Background
Many organisms have specific physiological changes when submerged in water; this set of responses is often
referred to as the dive reflex or dive response. These vary from organism to organism but usually consist of a
decrease in heart rate and a decrease in blood flow to the extremities. It appears this reflex is protective for aquatic
mammals, helping the animal to conserve oxygen while submerged beneath the surface of the water. Terrestrial
animals, however, do not gain an oxygen-conserving benefit from this reflex. In fact, in cold water, oxygen
consumption will actually rise as we attempt to produce more heat by shivering. It appears the primary benefit for
terrestrial organisms is the reduction of blood flow to the limbs to conserve heat. By constricting blood vessels in
the limbs, the body is able to maintain a greater percent of its blood in the torso.
Do humans exhibit a dive response? In many mammals, the dive response is triggered by sudden submergence of
the face in cold water, which stimulates the trigeminal nerve receptors around the nose. As water temperature
decreases, stimulation of the receptors is enhanced and the extent of the response increases. Enhanced stimulation
of these receptors results in an inhibition of the cardiovascular center, as well as a subsequent reduction in heart rate
through both enhanced parasympathetic output and reduced sympathetic output to the heart.
Using some or all of the tools you’ve used today, design an experiment or set of experiments to determine if humans
exhibit a dive response and how this shows up physiologically. What other possible causes might there be for any
changes you see. E.g., can you design an experiment to distinguish between the effects of a classic dive response
and simple breath holding? Is simple cold enough to elicit a response, or must apnea (interruption of breathing) also
occur (more analogous to actual submersion)? Design an experiment that gives you quantitative data. We will not
analyze these data statistically, but look for trends/changes, and base you conclusions on these.
State your hypothesis:
State your prediction:
Describe your experimental design. What will you measure? How? How will you control for potentially
confounding variables?
Describe your results (as quantitatively as is relevant and possible):
Interpret your results. What do they mean, and how doe they relation to your hypothesis and prediction?
Based on your results, answer the following question: Do humans exhibit a mammalian diving reflex?
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ECG in a Resting Volunteer
What does the P wave amplitude represent?
Amplitude
(mV)
P Wave
How does it vary?
Duration
(s)
1
2
3
Is this similar to the variation one would expect in skeletal
muscle EMG recordings? Why or why not?
4
Mean
QRS Complex
1
2
Is the heart rate regular? What determines the regularity of the
heart beat? What can influence this?
3
4
Mean
(
R-R
Time
Interval
(s)
Heart Rate
(BPM)
Pair
1
Pair
2
Pair
3
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ECG and Phonocardiography
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Analysis
Blood Pressure and Pulse
R Wave to
First Sound
(s)
T Wave to
Second Sound
(s)
(
Explain the changes in the pulse trace as you inflate the Blood Pressure
Cuff.
Systolic
Pressure
(mmHg)
Diastolic
Pressure
(mmHg)
Pressure
Value
(
Explain the changes you see in the phonocardiogram as you release the pressure in the Blood Pressure Cuff.
How are the phonocardiogram and pulse measurements related?
Mammalian Dive Reflex
State your hypothesis:
State your prediction:
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Describe your experimental design. What will you measure? How? How will you control for potentially
confounding variables?
Describe your results (as quantitatively as is relevant and possible):
Interpret your results. What do they mean, and how do they relate to your hypothesis and prediction?
Based on your results, answer the following question: Do humans exhibit a mammalian diving reflex?
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