Cardiovascular Physiology Laboratory Clive M. Baumgarten, Ph.D.

Transcription

1 Cardiovascular Physiology Laboratory Clive M. Baumgarten, Ph.D. To pump blood, the heart has a rhythmical sequence of both electrical and mechanical events, the cardiac cycle. The electrical activity, recorded as an electrocardiogram (ECG), initiates the mechanical activity of the heart (contraction and relaxation of atria and ventricles). Ejection of blood from the left ventricle into the aorta increases arterial pressure from diastolic to systolic pressure and produces a pressure wave that passes down the arterial tree. This lab comprises four exercises that are designed to: 1. Measure the propagation velocity of the pressure wave 2. Examine the effect of postural changes on heart rate 3. Visualize the Korotkoff sounds used to measure systolic and diastolic blood pressure 4. Examine the effect of exercise and recovery from exercise on heart rate, pulse pressure, and components of the ECG. To accomplish this, you will: 1. Record the Lead II ECG 2. Estimate pressure pulse by plethysmography 3. Record systolic and diastolic blood pressure and Korotkoff sounds with a sphygmomanometer and stethoscope microphone. A. EXPERIMENT 1: MEASURE THE PROPAGATION VELOCITY OF THE PRESSURE WAVE 1. Overview The pressure pulse is initiated by ejection of blood from the LV into the aorta. The pulse propagates down the arterial tree as a sound wave moving through a liquid. The velocity of propagation of the pressure pulse is much faster than the velocity of the flow of blood. The arterial walls are elastic and are distended by the passing of the pressure pulse. The compliance of the vessel walls acts to slow the velocity of propagation of the pressure pulse as compared to that in a rigid tube. Nevertheless, the elastic recoil of the vessel serves to maintain diastolic pressure and flow. When does ejection of blood begin? The events in the ECG reflect the timing of electrical activity of the heart rather than the mechanical events. The QRS complex corresponds to depolarization of the ventricle. The ejection of blood occurs about 50 ms after the end of the QRS complex because (1) contraction follows electrical activity and (2) LV pressure must increase from end-diastolic pressure to aortic pressure (isovolumetric contraction) before the aortic valve opens and ejection occurs.

2 B. Set up of computer and physiological sensors 1. Make sure the BIOPAC MP30 sensor interface is turned OFF (switch on the back left side) 2. Turn the Computer ON. 3. Plug in the sensors as follows: CH 1: Electrode lead (SS2L) CH 2: Pulse transducer (SS2LA or SS4L) 4. Turn the BIOPAC ON. C. Attach electrodes to record Lead II ECG 1. Peel off an electrode holding the tab. Try not to touch the adhesive. The sponge at the center of the electrode contains a conductive gel. 2. Place electrodes on right arm, left leg, and right leg as shown in the diagram. 3. Connect the cables. The pinch connectors should be oriented with the metal contacts facing the electrode, and the cables should be connected according to the following color code: RED Left Leg (+ for Lead II) WHITE Right Arm ( for Lead II) BLACK Right Leg (ground) Lead II (LA-RA) is recorded; Right leg is ground. D. Attach the plethysmography pulse transducer 1. Wrap the pulse transducer around the tip of your index finger with the window facing the finger pad and secure the transducer with the velcro strap, as shown in the diagram. Contact between the transducer and finger should be firm without compressing the finger.

3 E. Start data acquisition program 1. Double click on BCL2 Lessons. 2. Select Lesson 7 (L7-ECG&P-1), and click OK. 3. Type in a filename (e.g., student s name), and click OK. F. Calibration of System 1. The subject should relax so that signals from skeletal muscle electrical activity do not corrupt the ECG. 2. Click on Calibrate in the upper left corner of the Setup window. The calibration recording will stop automatically after 8 s. 3. Check the calibration data (see example in figure). There should be an ECG with reduced amplitude and a relatively flat baseline in the upper band (ECG), and a recording that looks like the arterial pressure waveform in the lower band (Pulse). 4. The scale for each trace is automatically adjusted by the computer so that recordings will occupy nearly the full height of the trace. If the record is different, redo the calibration. If it is similar, proceed to Data Recording. G. Data Recording WARNING: Experimental directions on the screen should be ignored. Follow directions in the handout. 1. Click on Record (upper left) and record for about 15 s. Then click on Suspend and review the data. 2. If the recording doesn t look right (see figure), click on Resume and obtain a second tracing. (Warning: Clicking on Redo will erase the data). 3. After a satisfactory record is obtained, remove the electrode cable pinch connect- ors, peel off the electrode patches, and throw them away. Wash the electrode gel residue from the skin. The electrodes may leave a slight ring on the skin for a few hours.

4 H. Data Analysis 1. Measure the time for pressure pulse to move from the heart to the finger tip. 2. Measure the distance between the heart and finger tip. 3. Calculate the propagation velocity of pressure pulse by dividing the distance by the time. 4. Report the result in m/s. I. Making the Measurements 1. Using the tape measure, measure the distance from the spine at heart level to the shoulder and out to the finger tip. The distance should be about 1 meter. 2. The pressure pulse is initiated by ejection of blood from the LV into the aorta. Ejection of blood occurs about 50 ms after the end of the QRS complex because (1) contraction follows electrical activity and (2) LV pressure must increase from end-diastolic pressure to aortic pressure (isovolumetric contraction) before the aortic valve opens and ejection occurs. Therefore, measure the time from the end of the QRS to the beginning of the pressure pulse at the finger tip and subtract 50 ms to account for the delay between the QRS and ejection of blood. 3. Select a section of the record to measure using the horizontal scroll bar. 4. Expand the time scale by clicking on Horizontal (Time) Scroll Bar or by using the Zoom Tool. 5. Click on I-beam cursor (lower right). Using the cursor, highlight a section of record from the end of the QRS to the beginning of the pressure pulse.

5 6. Measurement Boxes are located near the top of the screen. Each measurement has three sections: a channel number box, a measurement pull-down menu box that will display when you click on it, and a results box, that reports the designated measurement. 7. Select ΔT and channel 1 for the first box. ΔT is the difference in time (in s) between the end and beginning of the selected area. 8. Repeat measurement on 3 successive beats. Average and subtract 50 ms. 9. The Zoom Previous and Autoscale Horizontal tools can be used to restore the time scale to its original value. The horizontal scroll bar can be used to select a different section of the record. 10. The propagation velocity of the pressure pulse was: m/s II. EXPERIMENT 2: CHANGES IN HEART RATE ON ALTERING POSTURE. A. Overview We will observe the effect of postural changes on resting heart rate. In the supine position, arterial pressure is nearly independent of location. Immediately on moving to the standing position, blood pressure in the arteries above the level of the heart decreases and blood pressure below the level of the heart increases (see figure). The reason for this is the hydrostatic pressure due to the force of gravity on the column of fluid (blood) depends on body location when standing. In contrast, there is no difference in hydrostatic pressure in the supine position. Note that the density of Hg is 13.6 g/cm 3. Reflexive control of HR. The large change in arterial pressure upon standing is immediately sensed by the carotid baroreceptors initiating the baroreceptor reflex. Within seconds, there are autonomic actions on both the vasculature and heart. Vasoconstriction occurs on both the arterial and venous sides of the circulatory network due to increased sympathetic stimulation to the walls of the vessels. Heart rate is increased due to withdrawal of parasympathetic and enhanced sympathetic traffic to the SAN. Sympathetic stimulation of ventricular muscle and the increased heart rate lead to enhanced contractility. Both the enhanced contractility and increased right atrial pressure (due to vasoconstriction) cause augmentation of stroke volume. In turn, the elevated stroke volume results in increased pulse pressure.

6 B. Set up computer and physiological sensors: No changes from Experiment 1. C. Experimental Protocol The subject lies on the table for 3-5 min and then rapidly moves from the supine to standing position with a minimum of exertion. With assistance of a second student, the subject should rotate their body horizontally, so that their legs are off the table, and then rapidly stand with assistance. A second assistant should make sure the wires do not become entangled during the change of posture. Heart rate will be recorded: (1) at steady state while supine (e.g., after 3-5 min), (2) immediately upon standing, and (3) at 30 s intervals for the next 3 min. (Practice the supine to standing maneuver with leads attached to insure a smooth transition without excessive exertion.) D. Data Acquisition 1. After 3-5 min in the supine position, begin recording by clicking on Record. 2. Obtain at least 30 s of baseline data. 3. Continue the recording without interruption during the posture change and for 3-5 min in the standing position. 4. Then click on Suspend to stop the recording and review the data. E. Data Analysis Measure heart rate in bpm: 1. In the steady state (e.g., after 3-5 min) in the supine position 2. Immediately upon standing 3. At 30 s intervals for the next 3 min F. Making the Measurement 1. Select the section of the record corresponding to the last 10 s in the supine position using the horizontal scroll bar. 2. Expand the time scale by clicking on the Horizontal (Time) Scroll Bar or by using the Zoom Tool (lower right). 3. Click on I-beam cursor. 4. Using I-beam cursor, highlight a section of record from one R wave to next. 5. Select BPM and channel 1 for 2nd measurement box. BPM first calculates the difference in time between the end and beginning of the area selected by the I-Beam tool (same as ΔT) and converts to bpm. 6. Repeat the measurement for 3 consecutive beats and average results. 7. Repeat the determination of heart rate (1) immediately upon standing and (2) at 30 s intervals for 3 min. 8. Plot your results as heart rate as a function of time.

7 III. EXPERIMENT 3: MEASUREMENT OF BP AND VISUALIZATION OF KOROTKOFF SOUNDS A. Overview You will record systolic and diastolic blood pressure and visualize the Korotkoff sounds. The following figure represents a recording of systemic arterial blood pressure measured directly by inserting a catheter into an artery and attaching the catheter to a pressure measuring and recording device. Four pressures can be measured: Systolic Pressure: The maximum arterial pressure (during systole). Diastolic Pressure: The minimum arterial pressure (during diastole). Mean Arterial Pressure: Estimated as Diastolic Pressure + 1/3 Pulse Pressure Pulse Pressure: Systolic Pressure Diastolic Pressure By convention, systemic arterial BP is expressed as a ratio: systolic pressure/diastolic pressure. For example, if systolic and diastolic pressures are 135 and 80 mm Hg, BP would be expressed as 135/80. For these values, pulse pressure would be 55 mm Hg and mean arterial pressure would be 98 mm Hg. Pulse pressure is directly related to stroke volume of the heart. For example, with exercise the increase in stroke volume results in a large increase in systolic pressure, while diastolic pressure either increases slightly, remains constant, or falls slightly. The large increase in systolic pressure with only small changes in diastolic pressure results in increased pulse pressure during exercise. Systemic arterial blood pressure is commonly measured with indirect methods because direct methods are invasive and neither practical nor convenient for routine use. The most common method, auscultatation (listening to sounds made by internal organs), uses a stethoscope and a sphygmomanometer. The sounds detected when measuring blood pressure are referred to as Korotkoff Sounds. B. Method for Measuring Blood Pressure Arterial pressure is determined by placing an inflatable rubber cuff attached to a pressure gauge around the arm, inflating it to collapse the underlying artery, and listening with a stethoscope positioned over the vessel and below the cuff. Sound is created by the turbulent flow of blood through the partially compressed vessel. When cuff pressure exceeds systolic arterial pressure, the artery is collapsed, blood flow ceases, and no sound is produced.

8 Systolic Pressure: As cuff pressure is slowly reduced, blood flow through the artery begins when cuff pressure falls just below systolic arterial pressure. At this point, a sharp tapping sound (first sound of Korotkoff) is heard. Cuff pressure when this sound is first heard is taken as an approximation of systolic pressure. Diastolic Pressure: As cuff pressure is further reduced, sound intensity increases, then suddenly becomes muffled (second sound of Korotkoff) at the level of diastolic pressure, and finally disappear. Sounds disappear when the vessel is no longer compressed by the cuff and normal nonturbulent blood flow resumes. Because it is easier to determine when the sound disappears than when it becomes muffled, and because only a few mm Hg pressure differential exist between the two points, the disappearance of sound is commonly used as an indicator of diastolic pressure. The diagram above shows the temporal relationship between the ECG, Korotkoff sounds, cuff pressure, the blood pressure pulse waveform (at the arm), and the condition of the brachial artery under the cuff. The pulse waveform represents the brachial pressure in the artery above the cuff. The shaded area represents the blood flow that can pass below the cuff as soon as the arterial pressure exceeds the cuff pressure. Timing of Korotkoff sounds relative to the ECG is important. The first sound appears during the T-wave. This sound occurs at the time of peak pressure (systole), which, if measured at the heart, would occur earlier during the S-T segment. The delay in detecting the sound at the arm is due to the time it takes the pressure wave (sound) to reach the arm. The time between the R wave and the first sound should be consistent. Using this fact, you can distinguish actual Korotkoff sounds from extraneous noise. NOTE: If your blood pressure as determined from this exercise is high, you should not be too concerned. A mistake may have been made in the measurement, or there may be other factors that resulted in a temporarily high reading. If you are concerned, please consult your physician, but do not try to diagnose or treat yourself based on these laboratory blood pressure readings.

9 C. Set up of Computer and Physiological Sensors Note: It is necessary to restart both the program and the BIOPAC sensor interface. 1. Make sure BIOPAC interface is turned OFF (switch on the back left side) 2. Exit the program. Click on X in upper right. 3. Plug the sensors as follows: 4. CH1: BP Cuff (SS19L) CH3: Stethoscope (SS30L) CH4: Electrode lead set (SS2L) 5. Turn BIOPAC back ON 6. Start the BSL2 Lessions and choose Lesson 16 (L16-BP-1). 7. Type the filename, typically a different name than before, and click OK. D. Attaching and Preparing the Sensors 1. Place three ECG electrodes on the subject as before. 2. Release all pressure from the cuff by opening the cuff valve and rolling the cuff in on itself, then press to flatten and close the valve tight. E. Calibration 1. With cuff not on the subject but closed in loop with velcro, click on Calibrate (upper left corner) and inflate cuff to 100 mmhg according to the dial gauge. You may have to pump times to have enough pressure in the cuff for any gauge reading. After having reached 100 mmhg, click on OK. 2. Deflate the cuff pressure to 40 mmhg using the pressure release valve. Then click OK. When you click OK the calibration of the microphone will begin. 3. After the calibration recording begins, lightly tap the stethoscope diaphragm twice. The recording stops automatically after 8 s. If the record is similar to the following, proceed to Data Recording. If different, Redo the calibration. 4. The pressure should be constant at 40 mmhg during the calibration period. If it declines or has large spikes, the valve is not fully closed or the hose connections are not tight. If the ECG is excessively noisy or drifts even though the subject was relaxed, one or more of the electrodes are probably not making good electrical contact with the skin.

10 F. Using the Sphygmomanometer 1. To obtain accurate measurements, it is important to release the cuff pressure at a rate of 2-3 mmhg per second. Practice before putting the sphygmomanometer on the subject. 2. To practice, open the valve and roll the cuff in on itself, flatten the cuff to get out the air, and close the valve. Pump the bulb until the pressure dial reads 160 mmhg. Slowly turn the valve counter-clockwise to begin releasing the cuff pressure. Have someone time how long it takes pressure to decline from 160 to 100 mmhg. Open the valve slowly so that you don t have a large pressure drop, and try to maintain an even release. To keep the release rate constant, you may have to open the valve more as the cuff pressure gradually diminishes. It should take s for pressure to drop 60 mmhg. G. Setting Up Sphygmomanometer and Stethoscope microphone 1. Place the cuff on subject s LEFT arm so that the artery label is over the brachial artery (with arrow on label pointing down). 2. Position the cuff with the lower edge 1.5 to 2 inches above the antecubital fossa and high enough to avoid covering any part of the stethoscope diaphragm. 3. Wrap the cuff evenly and snugly around the subject s arm. The velcro wrap should hold the cuff in place, but you may wish to inflate the cuff slightly (10-20 mmhg) so that it will stay in place. 4. Position the sphygmomanometer pressure dial so you can read the face of the dial straight on. A strap on the cuff above the artery label that allows the dial indicator to be clipped on. 5. Position the subject s arm at heart level. Palpate the brachial artery between the antecubital fossa and the lower edge of the cuff to find the greatest pulse and align the artery label of the cuff with the pulse point. 6. Place the stethoscope over the artery and apply firm but not excessive contact pressure.

11 H. Experimental Protocol 1. One student inflates and deflates the cuff, while another operates the computer. By auscultation, mark the time at which Korotkoff sounds begin by pressing the F9 key (An arrowhead appears above the record). 2. Start the recording by clicking on Record, then inflate the cuff to 160 mmhg, and click OK. 3. After clicking OK, release cuff pressure at a 2-3 mmhg/second and call out when the Korotkoff sounds first appear (systolic). Insert a marker by pressing F9. 4. Continue to release cuff pressure and call out when sounds completely disappear (diastolic). Insert a marker by pressing F9, and click on Suspend. 5. After the sounds disappear, cuff pressure should be deflated as quickly as possible to reduce venous congestion. Never leave an inflated pressure cuff on the subject s arm. 6. Record the BP as measured by auscultation and reading pressure dial. 7. Review the data. Record should look like the figure. Pressure should decrease over time. Korotkoff sounds should be observed. ECG trace should not have excessive noise. 8. If something went wrong, click Redo to repeat the recording. I. Making the Measurement 1. Select the section of the record beginning at the first Korotkoff sound or marker for systolic pressure using the horizontal scroll bar. 2. Expand the time scale by clicking on the Horizontal (Time) Scroll Bar or by using the Zoom Tool (lower right). 3. Click on I-beam cursor (lower right). 4. Sequentially position the I-beam cursor at each time point of interest (first and second Korotkoff sounds and markers for systolic and diastolic pressure by auscultation). 5. Select Value and channel 1 for the third measurement box. Value displays the calibrated BP for that time point. 6. Repeat for the additional time points of interest.

12 J. Data Analysis 1. What was BP based on auscultation (reading dial)? / 2. What was BP based on auscultation (markers on chart)? / 3. What was BP based on visualization of Korotkoff sounds? / IV. EXPERIMENT 4: THE EFFECT OF EXERCISE A. Overview The purpose of this experiment is to examine the effect of exercise and recovery from exercise on heart rate, the pressure pulse, and components of the ECG. During exercise, metabolic demands increase, sympathetic tone is augmented, and parasympathetic tone is withdrawn. As a result, heart rate and cardiac contractility are increased, arterioles serving muscle beds vasodilate, whereas vascular beds serving certain other nonessential organs (e.g., GI system) constrict, and venoconstriction elevates right atrial pressure and enhances cardiac filling. Overall, increases in cardiac output, stroke volume, pulse pressure, systolic pressure, and mean arterial pressure are observed. Increases in heart rate are accompanied by changes in the ECG. First, the times between successive P waves, representing depolarization of the atria from the SAN, and between successive QRS complexes, representing depolarization of the ventricle, are decreased. Cycle length, the time between heart beats, is usually measured as the R-R interval, and the inverse of cycle length is heart rate. The autonomic effects responsible for increased heart rate also modulate the speed at which action potentials conduct through the AVN and decrease the effective refractory period of the AVN, so it is ready to conduct a second action potential more sooner. Conduction time through the atria and AVN is measured as the P-R interval, about half of which is attributed to conduction through the AVN. An abbreviation of the P-R interval accompanies increases in heart rate during exercise. Heart rate also alters action potential duration. Action potential duration is reflected in the Q-T interval, which is inversely proportional to heart rate. In contrast, the duration of the QRS complex, which reflects the spread of depolarization throughout the ventricle, normally is unaffected by heart rate. SUBJECT EXCLUSIONS This laboratory procedure asks the subject to perform a standard brief amount of strenuous exercise. You should not be a subject if: 1. You are not able to climb 3 flights of stairs without a marked shortness of breath or chest discomfort;

13 2. You have a medical history of angina, myocardial infarction, congestive heart failure, cardiac valve disease, arctic or mitral stenosis, mitral valve prolapse, endocarditis, pulmonary embolus, deep venous thrombosis, sick sinus syndrome, atrial fibrillation/flutter/tachycardia, conduction system disease, an abnormal ECG, supraventricular or ventricular tachycardia, a family history of sudden cardiac death, implanted pacemaker or defibrillator, heart valve replacement, coronary artery bypass or angioplasty, congenital heart disease, are on any cardiac medication, or have a history of asthma, or hypertrophic cardiomyopathy; or 3. If any of the following occur during the pre-exercise period: resting heart rate <45 bpm or >90 bpm; premature ventricular contractions; PR interval > 200 ms or <100 ms; QRS interval > 110 ms; QT interval >450 ms; blood pressure <95/50 or > 140/85. SUBJECTS FOR THE EXERCISE PORTION OF THE LABORATORY MUST READ AND UNDERSTAND THE ABOVE EXCLUSIONS. B. Set up of Computer and Physiological Sensors. No change from Experiment 3. To be able to record blood pressure, the pressure pulse, and the ECG immediately after exercise, the sensors should be left in place during exercise. An assistant should hold the cables during the exercise procedure to avoid entanglement. The sphygmomanometer cuff and stethoscope may need to be adjusted after exercise. C. Experimental Protocol 1. Target Heart Rate There are several guidelines for determining a subject s age-predicted maximum heart rate. A common method is: Target HR = (220 AGE) (% of maximum selected) In this formula, the maximum heart rate is (220 AGE). Typically, the aim is to increase HR to 75-85% of the calculated maximum. 2. An alternative, the Karvonen formula, is reported to better correlate with oxygen consumption. The Karvonen formula should be used for the laboratory exercise. The calculation is: Heart Rate Maximum = 220 AGE Heart Rate Reserve = Heart Rate Maximum Resting Heart Rate Target Heart Rate = Heart Rate Reserve (% of maximum selected) + Resting Heart Rate Use 75% of the Heart Rate Maximum for this exercise Calculation for a 24 year-old subject with a resting heart rate of 72. Heart Rate Maximum = 220 AGE = 196 Heart Rate Reserve = Heart Rate Maximum Resting Heart Rate = = 124 Target Heart Rate = Heart Rate Reserve (% of maximum selected) + Resting Heart Rate = (124 75%) + 68 = = 161

14 3. Exercise Regimen We will use a variation of the YMCA Three-Minute Step Test. This entails rapidly stepping on and off of a bench. Bench Height = 8-12 inches (2 pink blocks on each side) Step Rate = 24 steps/min Duration = 3-5 minutes Medical Warning The subject should stop exercising immediately and lie down if any of the following happen during or after exercise: dizziness shortness of breath chest discomfort abdominal discomfort HR >85% of predicted maximum sudden drop in heart rate D. Recording the Data widening of PR interval to > 200 ms widening of QRS to > 120 ms ventricular activity (PVCs or worse) supraventricular tachycardia atrial fibrillation/flutter failure to record a good pressure pulse 1. Data will be recorded in 2 parts. First, a baseline resting recording will be made. Then, after the exercise is completed, the recording will be restarted and continued until heart rate returns to normal (about 3 min). 2. To obtain the baseline resting recording, click on Resume. Determine the resting blood pressure as before. Click on Suspend after the blood pressure has been determined. 3. Exercise for 3-5 min. Do not exceed the Target Heart Rate. 4. Immediately after stopping exercise, a. Click on Resume to begin to record the ECG, sounds, and pressure pulse. b. Immediately determine blood pressure c. Repeat the blood pressure determination at 1 min intervals until heart rate has returned to its pre-exercise resting level. 5. Click on Suspend to end the recording. 6. Remove the electrode cable pinch connectors, peel off the electrodes and throw them away (BIOPAC electrodes are not reusable). Wash the electrode gel residue from the skin. The electrodes may leave a slight ring on the skin for a few hours. E. Data Analysis 1. Record HR, systolic and diastolic BP, mean BP, and the amplitude of the pulse pressure on the chart and graph heart rate. 2. Use the I-beam cursor and the ΔT measurement box to determine P-R interval, QRS interval, and Q-T interval at each time point. See diagram of ECG for the definitions of these intervals. 3. To approximate the time for ventricular filling, measure from the end of the T wave (marks beginning of isovolumetric relaxation) to end of next QRS (marks beginning of isovolumetric contraction) and subtract 90 ms (duration of isovolumetric relaxation at HR of 72 bpm). (This approximation ignores small changes in the duration of isovolumetric relaxation during exercise that result from altered aortic, LV, and LA pressures.)

15 4. Plot HR as a function of time. 10-cardiovascular physiology laboratory_2009.doc 8/6/2008