ECG, Blood Pressure, and Exercise Lab

ECG, Blood Pressure, and Exercise Lab
ECG, Blood Pressure, and Exercise Lab
Rob MacLeod
March 31, 2006
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Purpose and Background
Purpose:
The purpose of the lab is to learn about measuring the ECG and blood pressures and observing
the effects of exercise on blood pressure, heart rate, and electrocardiogram (ECG).
Background
This lab will build on the class material we have covered on blood flow and pressure, cardiac
contraction, regulation of heart rate and function, and the ECG.
To prepare, please review the notes and text on the ECG, the cardiac cycle, and blood pressure
measurements and also read the section in your text (or any other good physiology book) on
exercise.
1.1
Blood Pressure measurement
See the web site www.ktl.fi/publications/ehrm/product2/part iii3.htm for a description of this
measurement.
Arterial blood pressure is measured by a sphygmomanometer. This consists of:
1. A rubber bag surrounded by a cuff.
2. A manometer (usually a mechanical gauge, sometime electronic, rarely a mercury column).
3. An inflating bulb to elevate the pressure.
4. A deflating valve.
Figure 1 below shows how blood pressure is measured. After the cuff is placed snugly over the
arm, the radial artery is palpated while the pressure is increased until the pulse can no longer be
felt, then 30 mm Hg. more. As the pressure is released the artery is palpated until the pulse is felt
again. This palpatory method will detect systolic pressure only.
The auscultatory method detects diastolic as well as systolic pressure. The sound heard when a
stethoscope bell (or diaphragm) is applied to the region below the cuff were described by Korotkow
in 1905 and are called Korotkow’s sounds.
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Figure 1: Schematic diagram of arterial blood pressure measurement by Korotkow sounds.
The artery is compressed by pressure and as the pressure is released the first sound heard is a
sharp thud which becomes first softer and then louder again. It suddenly becomes muffled and later
disappears. Most people register the first sound as Systolic, the muffled sound as the first diastolic
and the place where it disappears as the second diastolic. It requires practice to distinguish the first
diastolic, so, for our laboratory, we will record only the first sound (systolic) and the disappearance
of the sound (second diastolic). These will not be difficult to elicit, and a little practice will enable
you to get the same reading on a fellow student three times in succession.
1.2
1.2.1
Sources or error in blood pressure measurement
Faulty Technique
1. Improper positioning of the extremity. Whether the subject is sitting, standing, or supine,
the position of the artery in which the blood pressure is measured must be at the level of the
heart. However, it is not necessary that the sphygmomanometer be at the level of the heart.
2. Improper deflation of the compression cuff. The pressure in the cuff should be lowered at
about 2 mm Hg per heartbeat. At rates slower than this venous congestion will develop and
the diastolic reading will be erroneously high. If the cuff is deflated too quickly the manometer
may fall 5 or 10 mm Hg between successive Korotkow sounds, resulting in erroneously low
readings.
3. Recording the first Blood Pressure. Spasm of the artery upon initial compression and the
anxiety and apprehension of the subject can cause the first blood pressure reading to be
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erroneously high. After the cuff has been applied, wait a few minutes before recording the
blood pressure. Make several measurements. Generally the third value recorded is the most
basal.
4. Improper application of the cuff. If the rubber bladder bulges beyond its covering, the pressure
will have to be excessively high to compress the arm effectively. If the cuff is applied too
loosely, central ballooning of the rubber bladder will reduce the effective width, thus creating
a narrow cuff. Both bulging and ballooning result in excessively high readings.
1.2.2
Defective Apparatus
A defective air release valve or porous rubber tubing connections make it difficult to control the
inflation and deflation of the cuff. The aneroid manometer gauge tube should be clean.
If an aneroid manometer is used, its accuracy must be checked regularly against a standard
manometer. The needle should indicate zero when the cuff is fully deflated.
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Procedure
The lab is divided into 4 sections, the first 3 of which you should do in parallel in the usual paired
groups. Half the class should start with the blood pressure measurements while the other half
starts with the ECG measurements. For the final section, please form groups of 4–7 as this exercise
needs more people to carry out.
2.1
Arterial blood pressure measurement
The subject lies down with both arms resting comfortably at his sides or sits quietly with arm
hanging down, elbow slightly bent–the goal is to have the arm at the same level as the heart. Wrap
the sphygmomanometer cuff about the arm so that it is at heart level. The air bag inside the
cuff should overlay the anterior portion of the arm about an inch above the antecubital fossa—the
depression on the inside of the elbow joint. Note the “Artery” label and the arrow that should sit
over the center of the inside of the elbow. The cuff should be wrapped snugly about the arm.
Palpatory method: Palpate the radial pulse with the index and middle fingers near the base of
the thumb on the anterior surface of the wrist. While palpating the radial pulse, rapidly inflate the
cuff until the blood pressure manometer reads 200 mm Hg pressure. Set the valve on the rubber
bulb so that the pressure leaks out slowly (about 5 mm per second). Continue palpating the radial
pulse, and watch the manometer while air leaks out of the cuff. Note the pressure at which the
pulse reappears.
mm Hg.
Record the pressure:
This is Systolic pressure as detected by palpation. Allow the pressure to continue to decrease,
noticing the changes in the strength of the radial pulse.
Auscultatory method: Elevate the pressure in the cuff 20 mm Hg higher than the pressure
at which the radial pulse reappeared in A. Apply the stethoscope bell lightly against the skin in
the antecubital fossa over the brachial artery. There will be no sounds heard if the cuff pressure
is higher than the systolic blood pressure because no blood will flow through the artery beyond
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the cuff. As the cuff is slowly deflated, blood flow is turbulent beneath the stethoscope. It is this
turbulent flow that produces Korotkow’s sounds. Laminar flow is silent. Thus when the cuff is
deflated completely, no sounds are heard at the antecubital fossa. Deflate the cuff completely and
allow the subject to rest for a few minutes. DO NOT REMOVE THE CUFF
Palpatory and Auscultatory methods simultaneously: Palpate the radial artery, and elevate the pressure in the cuff to 20 mm Hg. Higher than that at which the radial pulse reappeared.
Apply the stethoscope to the skin over the brachial artery, and allow pressure to leak slowly from
the cuff. Note the pressures:
mm Hg.
(1) At which the radial pulse is first felt:
(2) At which the sound is first heard with the stethoscope:
mm Hg.
The pressure at which the sound was first heard is recorded as systolic blood pressure. Allow
the pressure to continue to fall. The Korotkow’s sounds grow more and more intense as the pressure
is reduced. Then they suddenly acquire a muffled tone and finally disappear.
The pressure observed at the first muffled tone is the first Diastolic pressure.
The pressure observed when the sound disappears is the second diastolic pressure. Record this
pressure as the diastolic pressure for this laboratory. (Note: In practice you should record both
diastolic pressures.)
Repeat the blood pressure determination at least three times, or until sufficient proficiency is
acquired that agreement is obtained between consecutive readings. Blood pressure is recorded with
the systolic pressure reading first,
e.g., 120/80 means Systolic 120 mm Hg; Diastolic 80 mm Hg.
Pulse Pressure is the difference between Systolic and Diastolic blood pressure.
How do the measurements from these two methods compare? Which do you think
was more accurate and why? Try and explain any differences in results.
2.2
2.2.1
Venous blood pressure
Estimation of Venous Pressure
One can measure the approximate venous pressure by noting how much above the level of the heart
an extremity must be so that hydrostatic and venous pressures are equal. At that point, there is
barely enough venous pressure to lift blood against the hydrostatic pressure of the elevate limb.
With the subject sitting quietly next to a bench, with one arm lying on the bench-top, observe
the veins on the back of the relaxed hand1 . While the subject is reclining, passively raise and
lower the subject’s arm and observe for filling and collapsing of the veins of the back of the hand.
Measure the distance in millimeters from the position where the veins are just barely collapsed to
the level of the heart (in the sitting subject approximately at mid-thorax. This will give the venous
pressure in mm of blood.
=
Venous pressure in mm of blood
The specific gravity of blood is 1.056.
The specific gravity of mercury is 13.6.
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If it is difficult to see the veins on the back of the subject’s hand, have the subject hang the hand loosely down,
with muscles relaxed. If veins do not become visible, slap the back of the hand gently few times to stimulate blood
flow.
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Compute the venous pressure in mm Hg using the equation
mm of blood ∗ Sp.Gr. of blood = mm of mercury ∗ Sp.Gr. of mercury
Venous pressure in mm Hg. =
2.3
ECG Measurement
Figure 2: Electrocardiographic lead field.
The “limb leads” are part of the legacy of Wilhems Einthoven, developer of the string galvanometer and winner of the 1926 Nobel Prize for his advances in electrocardiography. The idea
was to capture the projection of the cardiac dipole in the frontal plane based on an equilateral
triangle coordinate system. The underlying formalism of the limb lead (and the Frank lead) system is the lead field, a function that projects a current dipole source to any point on the body
surface as shown in Figure 2. The lead field vector H is specific to a set of electrode locations and
when multiplied by the current dipole vector P, the result is a scalar value equal to the potential
difference between the electrodes of the lead.
The goal of this part of the lab is just to learn the basics of measuring an ECG. Figure 3
illustrates the limb lead ECG, with three electrodes forming three difference measurements or
“leads”. We will use the remaining electrode as the reference in each case and record the lead from
the other two electrodes. Note the polarity of each of the limb leads and try to mimic them in your
measurements.
The steps in setting up this basic ECG measurement are as follows:
1. Identify the following four sites on the torso of the subject and use an alcohol swab to
thoroughly clean the skin beforehand:
• Right anterior shoulder, just below the clavicle
• Left anterior shoulder, just below the clavicle
• Left lower ribs, near the mid-axillary line (equivalent to left leg) Note: the mid-axillary
line runs along the side of the thorax mid way between front and back of the chest; the
axilla are the armpits.
• Right lower ribs, near the mid-axillary line (equivalent to right leg)
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Figure 3: Limb system of the ECG.
2. At each site, apply one of the disposable, pre-gelled ECG electrodes. Find locations as free
of subcutaneous fat and muscle as possible.
3. Using the bundled ECG connector wires and short splitter cables, connect the lead wires in
set of three to into the connectors that run to the inputs of the 4-channel bioamplifier so that
you can record all three leads at once. Take careful note of the polarity of the leads
and make connections accordingly. For example, to record Lead I, this would require:
• Right anterior shoulder: -, G2 input, blue dot on the connector
• Left anterior shoulder: +, G1 input, yellow dot
• Right leg (or lower torso): reference, COM, green dot
You can check that you have proper polarity by comparing the measured signals to the sample
in Figure 5 below.
4. For the bioamplifier start with the following settings:
• Switch calibrator switch to “USE”
• Set the LO FREQ. setting to 0.1.
• Set Amplification to 5 (and turn the “ADJ. CAL” screw all the way to the left; the
resulting gain is approximately 1700.
• Set HI FREQ. to 1 kHz
• Make sure to use the same settings for all channels.
5. Put a T-connector on any two of the outputs of the bioamplifier, with two BNC cables, one
to a channel on the oscilloscope (both ends of the cable should be BNC) and the other to the
A/D input box connected to the computer. Connect the A/D ground input (“Ain Grnd”) to
the ground of the oscilloscope (near the power switch).
6. On the oscilloscope:
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Figure 4: Circuit diagram for the limb lead measurements
• On the oscilloscope, push the Menu button and for each channel, first set the tracing
position with the small knob and then select DC coupling with one of the screen buttons.
• Vertical scaling control knob so that it is the same on both channels and start with a
setting of 2 or 5 V/division.
• Adjust the horizontal control knob to .2 SEC/DIV.
• Trigger should be set to AUTO.
Now ensure that you have a good signal on the oscilloscope, make adjustments as necessary,
and then:
1. Note the sensitivity of the ECG to motion of the subject and experiment to create the best
conditions.
2. Save an image of the ECG on the oscilloscope using the memory function–you will have to
use the memory function once for each channels and save each one in a different reference
location on the oscilloscope.
3. From the saved signals, compute the period and heart rate for your subject.
4. Using the acquisition program, record a baseline ECG; perform the same measurements of
heart rate.
5. Try swapping electrode connections around so that you measure and record all three limb
leads. Also try using another lead, e.g., the right leg, as the reference and repeat the measurements. Do the signals change noticeably when the reference changes?
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Figure 5: An example of a normal 12-lead ECG
2.4
Response to exercise
The goal of this part of the lab is to record the response of a test subject to moderate exercise. For
this, each team needs 4–6 people organized as follows (See Figure 6):
1) Subject: in comfortable clothes with ECG electrodes applied and blood pressure cuff applied
loosely around an upper arm.
2) Blood pressure monitor: stationed at the side of the subject with stethoscope and blood
pressure manometer and bulb in hand. This person will carry out the BP measurements
during the breaks in the exercise.
3) Pulse monitor: stationed on the other side of the subject, this person’s job is to measure
heart rate during the breaks in the exercise.
4) ECG/Computer operator: sitting at the bench, this person’s task is to make ECG measurements and record all other measurements from the blood pressure and pulse monitors.
This person is also responsible for tracking the time and setting the pedal frequency. Use the
ECG lead combination that produced the largest amplitude signals of the three possibilities.
There are two protocols for these experiments, but before beginning, let the subject warm up
and make sure he/she is comfortable on the bike and has selected a comfortable gear and resistance
setting to be able to complete 8–10 minutes of pedaling with moderate exertion.
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Figure 6: The minimal exercise lab team.
2.4.1
ECG measurement
For this part of the protocol, use the same ECG setup as above, recording all 3 limb leads simultaneously on the computer and monitoring at least 1 of them on the oscilloscope.
2.4.2
Setting exercise workload with the metronome
To ensure a constant or controlled load, we will use a metronome to determine the pedaling cadence
(rate) of the subject. Make the own metronome from a signal generator, as follows:
1. Set the Agilent (for Hewlett-Packard) 33120A function generator to the following settings:
• Function: square wave
• Amplitude: about 1 VPP (volts peak-to-peak)
• Duty cycle: 20%
• DC offset: 0.0
2. Connect a T-connector from the output of the function generator and connect one side of it
to the Channel 2 input of the oscilloscope. use the oscilloscope to monitor the output of the
function generator, especially its frequency.
3. Connect a cable from the other end of the T-connector to the BNC/Banana converted and
then to the adapter cable to a 1/8” female plug for the headphones. Adjust volume with the
headphone controls and the amplitude control of the function generator.
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4. Adjust the frequency of the signal generator to a level that the subjects find comfortable.
Sample the signal with the oscilloscope and note the period and associated frequency.
2.5
Tips
Some additional technical aspects to note:
• Carry out measurements during the breaks at the end of each 2-minute interval as quickly
as possible! Figure 7 shows such a measurement taking place. The subject will recover
during these breaks and this will reduce the accuracy of the study; the breaks should be no
longer than 30 seconds.
• Measure pulse rate using the count during 15 s as soon as the subject stops pedaling.
• Subjects should try and be as still as possible during the breaks and the ECG operator
is responsible for measuring during an interval when the signals are as quiet and stable as
possible.
• Adjust the A/D converter range for the acquisition program so as to capture the signal with
the best possible resolution. Try and reduce baseline drift as much as possible; if the problem
persists, try turning the Bioamplifier to AC coupling. Note that turning the oscilloscope to
AC will appear to improve the baseline instability but that this effect does not pass to the
A/D converter, which measures DC amplitudes.
Figure 7:
exercise.
Simultaneous measurements of heart rate and blood pressure during a break in the
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2.5.1
Constant load protocol
1. Give the subject a 5-minute recovery period after the warmup and take resting measurements
of BP, pulse, and ECG. Set the metronome to the cadence you worked out beforehand with
the subject.
2. Exercise 2 minutes: Let the subject pedal at the set rate for 2 minutes and then stop and
as quickly as possible, measure blood pressure and pulse, and take a sample of the ECG on
the computer. Keep the breaks below 30 s.
3. Exercise 4 minutes: Let the subject pedal another 2 minutes and repeat measurements.
4. Exercise 6 minutes: Let the subject pedal another 2 minutes and repeat measurements.
5. Exercise 8 minutes: Exercise for another two minutes and then measure again. Stop the
exercise at this point but keep subject sitting on bicycle.
6. Recover 2 minutes: no pedaling, repeat measurements.
7. Recover 4 minutes: no pedaling, repeat measurements.
8. Recover 6 minutes: no pedaling, repeat measurements.
9. Let subject relax and cool down.
2.5.2
Graded load protocol
The goal for this protocol is to apply a graded stress to the subject and observe the response.
For this, have the subject select a gear that he/she can maintain over a cadence range of about
60–90 rpm. The subject will spend 2 minutes at each cadence, then stop for measurements, then
continue at an increased cadence for 2 minutes, and so on.
Work out beforehand a sequence of cadences and associated periods that will span at least
60–90 rpm in 4 steps.
1. Give the subject a 5-minute recovery period and take resting measurements of BP, pulse, and
ECG. Set the metronome to produce a cadence rate of 60 bpm.
2. Exercise 2 minutes: Let the subject pedal at the set rate for 2 minutes and then stop and
as quickly as possible, measure blood pressure and pulse, and take a sample of the ECG on
the computer. During the measurement break, set the new cadence on the metronome.
3. Exercise 4 minutes: Let the subject pedal another 2 minutes at the new cadence and
repeat measurements. Increase the cadence again.
4. Exercise 6 minutes: Let the subject pedal another 2 minutes at the new cadence and
repeat measurements. Increase the cadence again.
5. Exercise 8 minutes: Let the subject pedal another 2 minutes at the new cadence and
repeat measurements. Stop the exercise at this point but keep subject on bicycle.
6. Recover 2 minutes: no pedaling, repeat measurements.
7. Recover 4 minutes: no pedaling, repeat measurements.
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8. Recover 6 minutes: no pedaling, repeat measurements.
9. Let subject relax and cool down; feed water but hold off on the cake until the end of the next
protocol.
3
Lab Report
As with the previous report, concentrate on presenting the results and discussion of them rather
than the methods and background sections. You may choose to include the discussion with the
results or have separate results and discussion sessions. It is up to you.
Include results and discussion for the following parts of the lab:
Blood pressure: include all the pressure measurements and answer all the questions in the lab
description. Be sure to include all three results of the measurements and explain reasons for
the variation you might see.
ECG: include tracing of 1-3 beats from each of the electrode arrangements you measured; describe
any differences in signal morphology you see. For at least one tracing, add arrows to mark
each of the major features of the ECG: P, Q, R, S, and T waves, ST segment, PQ segment,
and TQ segment.
Exercise: create a table and graphs for both systolic and diastolic blood pressures, heart rate
using palpation, and heart using the measured ECGs for each of the exercise protocols; these
should all be graphs of the measured value, e.g., heart rate, as a function of time through your
the protocol. Discuss the results of these graphs—did they go as you expected? What do they
suggest about the body’s response to exercise? Did you see any changes in ECG shape with
exercise? Specifically, what changes did you see in blood pressure with exercise—provide a
model of what happens during exercise and discuss how the data you measured support that
model.
Mechanisms: Describe briefly the physiological mechanisms of as many as possible of the responses to exercise that you observed. Specifically, make sure to explain the different factors
that will alter blood pressure and decide which ones might be dominant from your data.
Experimental problems: Describe any experimental challenges you had to face in the lab and
how you dealt with them or how you would plan to deal with them were you to repeat these
experiments in the future.
Miscellaneous:
• When displaying the results of the ECG recordings, try and use the same scaling on the
axes so that it is possible to compare results between different recordings.
• You can, in principle, do all the graphs with Excel but please try and use MATLAB if
possible. This suggestions is especially important if you wish to do any signal processing
such as filtering of the ECG signals. If you have questions, simply ask the TA or me for
explanations or suggestions.
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