BIOEN 3102 Bioengineering Lab II

Laboratory IX: 2 weeks

Human Cardiovascular Activity

Goal

· To introduce techniques for measuring

· Heart sounds

· Blood pressure

· Respiratory-rate and volume

· Human ECG responses

· 3-lead measurements

· 12-lead measurements

· Computation of axis

· To examine body-surface potentials and contours

· To examine cardiovascular modulation

· Dive

· Aerobic Exercise

· Mental activity

Assignment

Submit a lab report by answering the specific questions for each experiment

Additional reading

Experiments

I. Heart sound measurements

Auscultation of heart sounds means to listen to and study the sounds arising from the heart as it pumps blood. These are the result of vibrations caused by the opening and closing of the valves in the heart, and by the blood rebounding against the ventricular and blood vessel walls. Sounds may be heard by using a stethoscope, or monitored more accurately using a phonocardiogram.

First heart sound Start of systole when the AV valve closes and the semilunar valves open (those to the pulmonary artery and the aorta). Has a low pitched "lub"sound.
Second heart sound. End of systole when the SL valves close and the AV valves open. Has a higher pitched "dub" sound.
Third heart sound. As the ventricles fill up- probably produced by vibrations of the ventricular walls.
Fourth heart sound. At the time of atrial contraction- probably due to accelerated rush of blood in the ventricles.

Using a stethoscope, listen to the heart of a group member, paying special attention to the four major auscultatory areas on the chest (shown below). Also record the heart sounds using the piezoelectric transducer.

II. Blood pressure measurement

The cardiovascular system consists of a system of blood vessels, and the heart, which pumps the blood throughout the system. Blood pressure is a common physiological measurement performed on the cardiovascular system, and gives a measurement of the pressure in the blood vessels during the cardiac cycle. Systolic pressure is the maximum pressure in the arteries, and occurs when the blood is ejected from the ventricles into the arteries. Diastolic pressure is the minimum pressure in the arteries, and occurs when the heart is filling with blood, and results from the recoil of the elasticity of the blood vessels. The normal range for systolic pressure for a resting adult is 100-139mmHg, and the normal range for diastolic pressure for a resting adult is 60-89mmHg. In this experiment, you will be measuring blood pressure by the auscultatory method which involves the use of a stethoscope or microphone and a sphygmomanometer. You will observe the sounds detected during blood pressure measurement, referred to as Kortokoff Sounds. The pressure cuff is inflated so that the sound of blood flowing through the arteries is no longer heard, which means that it is at a higher pressure than the pressure in the artery. It is slowly allowed to deflate. The first sound occurs when the systolic pressure is reached, and blood starts to flow through the artery again. When the sounds become muffled, the diastolic pressure has been reached.

Two other commonly measured blood pressure parameters are: 1) Pulse pressure - the difference between the systolic and diastolic pressures. The normal value is 40 mm Hg and 2) Mean blood pressure – the diastolic pressure plus one third of the pulse pressure. This is the average effective pressure forcing blood through the circulatory system. The normal value is 96 to 100 mm Hg. The mean blood pressure is a function of two factors - cardiac output (CO) and total peripheral resistance (TPR). Peripheral resistance depends on the calibre (diameter) of the blood vessels and the viscosity of the blood.

Mean BP = Cardiac output (ml/sec) x TPR

Cardiac output (ml/min) = Heart rate/min x Stroke volume (ml)

Thus, the measurement of blood pressure provides us with information on the heart's pumping efficiency and the condition of the systemic blood vessels. In general, we say that the systolic blood pressure indicates the force of contraction of the heart, whereas the diastolic blood pressure indicates the condition of the systemic blood vessels (for instance, an increase in the diastolic blood pressure indicates a decrease in vessel elasticity).

Experimental procedure

Sit in a comfortable position, with your legs and ankles uncrossed and your back supported.
Place your arm, raised to the level of your heart, on a table or a desk, and sit still.
Wrap the correctly sized cuff smoothly and snugly around the upper part of your bare arm. The cuff should fit snugly, but there should be enough room for you to slip one fingertip under the cuff.
Be certain that the bottom edge of the cuff is 1 inch above the crease of your elbow.
Put the stethoscope ear pieces into your ears, with the ear pieces facing forward.
Place the stethoscope disk on the inner side of the crease of your elbow.
Rapidly inflate the cuff by squeezing the rubber bulb to 30 to 40 points higher than your last systolic reading. Inflate the cuff rapidly, not just a little at a time. Inflating the cuff too slowly will cause a false reading.
Slightly loosen the valve and slowly let some air out of the cuff. Deflate the cuff by 2 to 3 millimeters per second. If you loosen the valve too much, you won't be able to determine your blood pressure.
As you let the air out of the cuff, you will begin to hear your heartbeat. Listen carefully for the first sound. Check the blood pressure reading by looking at the pointer on the dial. This number will be your systolic pressure.
Continue to deflate the cuff. Listen to your heartbeat. You will hear your heartbeat stop at some point. Check the reading on the dial. This number is your diastolic pressure.
Write down your blood pressure, putting the systolic pressure before the diastolic pressure (for example, 120/80).
If you want to repeat the measurement, wait 2 to 3 minutes before reinflating the cuff.

After making measurements by hearing the Kortokoff sounds, also use the piezoelectric pressure transducer to record the sounds as you deflate the cuff. Can you record the sounds in the oscilloscope. Finally, lift your arm by a specific distance of say 1.5 ft (450 mm) and measure blood pressure. What would have predicted the systolic pressure to be? How does this compare to the actual results? For your predictions, assume that the specific gravity of blood is 1 and that of mercury is 13.5. Knowing the millimeter distance your head is above your heart, can you calculate what the systolic pressure is in the arteries of your brain? Can you explain why the increased gravitational force or “gs” during flight might cause a pilot to pass out? Optional question: A 16’ giraffe typically has its heart some eight and one-half feet above ground level. If the brain of a giraffe requires a systolic pressure of 120 mm. Hg, what systolic pressure must be produced by the heart to satisfy this demand?

III. Arterial pulse wave

The blood pressure within an artery varies during each cardiac cycle. The highest pressure (systolic) occurs when the heart is in its relaxation phase and no blood is flowing through the semilunar valves. The difference between the systolic and diastolic pressures is called the pulse pressure. A recording of these changes is called an arterial pulse wave. A normal pulse wave over the aorta is shown in Figure 3. The dicrotic notch results when the aortic semilunar valves close, causing the blood in the aorta to rebound against the arterial walls to produce a slight elevation in pressure.

The magnitude and contour of the arterial pulse wave are directly related to the stroke volume and inversely related to the compliance (elasticity) of the arterial vessels. As the vessels lose their compliance (as with age or in arteriosclerosis), the stroke volume increases and the height of the pulse wave increases (pulse pressure increases). An examination of the pulse wave can give valuable clues to the functioning of the arteries and heart, as is seen in the abnormal waves pictured in Figure 4.

The velocity of the pulse wave as it travels down the artery is also an important clinical measurement. The arterial pulse wave moves over the large arteries at a rate of 3 to 5 m/sec and over the small arteries at 14 to 15 m/sec. The difference in velocity is related to the compliance of the vessels - the less compliance a vessel has, the faster the pulse wave will move over it (as in the small arteries). Thus, a measurement of the velocity of the pulse wave can also provide useful information about changes in the vessel's elasticity (compliance). The velocity will vary with the age of the individual (table1).

Recording the Peripheral Pulse

In this experiment, you will record the pulse wave from the tip of the finger; a peripheral pulse. It is recorded using a photoelectric pulse transducer, which measures changes in blood volume (plethysmography). A light source in the transducer transilluminates the finger tip, and the photoconductor detects changes in light intensity within the finger caused by pulsatile variations in blood volume.

Experimental Procedures

1. With the subject seated, attach the transducer snugly to the palmar surface of the middle finger. Record the pulse for 20 seconds with the subject's arm resting on the table. Now have the subject raise the transducer above his head (arm extended) for 30 seconds and record the pulse during the last 10 seconds.

Then have the subject lower the transducer (arm hanging at his side) for 30 seconds. Note the characteristics of the pulse wave profile.

IV. Electrocardiogram

Every living cardiac cell undergoes a regular sequence of electrical changes that initiate the contractile activity (systole) and the relaxation (diastole) of the cell. Thus, the contraction of the heart is associated with a compound action potential that is initiated at the sinus node and sweeps over the conduction path of the heart, preceding the mechanical contraction of the cardiac fibers. During this depolarization and repolarization of the myocardium, a potential difference is created between different regions on the surface of the heart. A separation of charge or potential difference is called a dipole. The electrical potential of the dipole is conducted through an electrolyte solution, such as the interstitial fluid and blood plasma, and eventually reaches the surface of the skin. By placing electrodes on the skin surface, we are able to detect and record the electrical activity over the heart surface prior to its contraction. By measuring the potential changes in various directions across the heart, it is possible to detect abnormalities.

Here is a diagram of normal heart conduction.

(see Shockwave movie)

The electrocardiogram (EKG) is a graphic record of the action potentials of the heart. It is recorded with an electrocardiograph, and the study of this cardiac electrical activity is called electrocardiography.

Standard Limb Leads. Various types of electrode positions can be used to record an ECG. The particular arrangement of the two recording electrodes is called a lead. The relative position of the two electrodes influences the direction and amplitude of the potentials recorded. To standardize the procedure so that results can be compared and evaluated from different laboratories, cardiologists have agreed on certain conventional requirements for the recording of the ECG. The most common recording procedure is to use the standard limb leads. Electrodes are placed on the left arm, right arm, left leg, and right leg. The electrode on the right leg is a ground connection that prevents unwanted external potential fields from distorting the record. The other three electrodes are used in pairs to detect the cardiac potential.
Einthoven's Law. Einthoven, the father of electrocardiography, originated many of the conventions used in the recording of electrocardiograms. He visualized the three standard limb leads as enclosing the heart in a triangle, often referred to as Einthoven's triangle (Figure 6). Einthoven also found a relationship between the amplitude of the QRS complexes in each lead, such that lead I + lead III = lead II (Einthoven's law).

Lead I. Right arm to left arm.

The right arm is connected to the negative terminal of the electrocardiograph, and the left arm to the positive terminal. When the right arm is negative to the left arm the record shows an upward deflection. Thus, lead I measures the potential difference between the electrodes on the left and right arms, or across the base of the heart. We can use a lead switching box to change recording modes.

Lead II. Right arm to left leg.

The right arm is connected to the negative terminal, and the left leg to the positive terminal. Thus, lead II measures the potential difference between the left leg and the right arm, or along the long axis of the heart from base to apex.

Lead III. Left arm to left leg.

The left arm is connected to the negative terminal, and the left leg to the positive terminal. This combination allows lead III to measure the potential difference between the left leg and left arm, or along the left side of the heart.

The sinoatrial (SA) node initiates the cardiac impulse (epicardium in this area becomes negative first), and this wave of negativity sweeps over the heart. Because the SA node is nearer the right arm, this arm becomes negative while the left arm and left leg are still positive, and the deflection of the record is upward in those leads (I and II). The left arm is closer to the SA node, so in lead III the first deflection is also upward as the left arm becomes negative in reference to the left leg.

Other lead configurations that can be derived from the standard leads are the Augmented unipolar limb leads (frontal plane):

Lead aVR: RA (+) to [LA & LF] (-) (Rightward)
Lead aVL: LA (+) to [RA & LF] (-) (Leftward)
Lead aVF: LF (+) to [RA & LA] (-) (Inferior)

In some experiments, unipolar (+) chest leads (horizontal plane) are also used as shown in the figure below. These leads are : V1: right 4^th intercostal space, V2: left 4^th intercostal space, V3: halfway between V2 and V4, V4: left 5^th intercostal space, mid-clavicular line, V5: horizontal to V4, anterior axillary line and V6: horizontal to V5, mid-axillary line. In addition to these intercoastal leads, we can also measure the potentials on various points on the surface of the chest to obtain a “body surface potential map”.

In all these experiments, we will use the “Wilson’s central terminal” as a reference electrde. To obtain a Wilson’s central terminal, connect a 5K resistor in series with the two arm leads. Connect the two ends of the resistor together and use this as your reference electrode.

Lead placement diagrams

An example of a body surface map.

Components of Normal ECG Complex

The normal ECG comprises of :

· P wave. Represents the spread of electrical activity (wave of negativity) over the atria after the initial depolarization of the SA node.

QRS Complex. Represents the spread of the negativity wave (depolarization) through the ventricular musculature (Figure 7). A small amount of atrial repolarization also occurs at the same time.
PR Interval. Time from the beginning of the P wave to the beginning of the QRS complex; interval between activation of the SA node and beginning of ventricular depolarization. Any abnormal lengthening of this interval suggests some interference with conduction of the impulse through the atria, atrioventricular (AV) node, bundle of His, and Purkinje fibres.
T Wave. Represents the repolarization of the ventricular musculature. It is of longer duration and lower amplitude than the depolarization wave (QRS complex), which indicates that the ventricular repolarization process is less synchronized and slower than the depolarization process.
QT Interval. Represents the time from the beginning of the QRS complex to the end of the T wave; that is, from the beginning of ventricular depolarization to the end of ventricular repolarization. The QT interval varies with the heart rate, becoming shorter as the heart rate increases.
PR Segment. From the end of the P wave to the beginning of the QRS complex. During this time the impulse is travelling through the AV node, AV bundle, and Purkinje fibers. These structures are within the heart myocardium; therefore, during this time there is no change in the negativity of the surface of the heart, and we say that the record is isoelectric (no change in potential is occurring).
ST Segment. From the end of the S wave to the beginning of the T wave. During this time the heart is completely depolarized and therefore the record is again isoelectric. The position and shape of the ST segment are important in diagnosis.

Experimental procedures for ECG recordings

1. Make baseline measurements of the:

PR interval
P wave amplitude and duration
QRS interval
QT interval
T wave amplitude and duration
PR segment
ST segment

Does the cycle length ever vary (arrhythmia)? Is there a change in the cycle length (heart rate) with inspiration or expiration? Are any of the waves abnormal? A PR interval (adult) greater the 0.2 sec is abnormal and indicates first degree heart block. In second degree heart block, there are P waves that are not followed by QRS waves; this may occur regularly or irregularly. Third degree heart block is a complete AV dissociation in which P waves occur quite regularly but have no relation to R waves. The normal duration of QRS complex is 0.08 to 0.12 sec. A duration of more than this indicates bundle branch block, or that the beat has arisen in one of the ventricles- a so called ventricular beat or extra systole. Variations in the T wave are quite numerous and require an expert diagnosis. Inversions of the T wave are not abnormal, especially in lead III. Elevation of the ST segment by more than 2mm is associated with acute injury or anoxia

2. Measure the QRS axis (How to )

3. Obtain a body-surface contour map of the peak deflection of the QRS complex. For these measurements, use two-channel acquisition where one channel is always fixed in position. Use the peak of the QRS of that channel to time-align the QRS peak of the second channel. Now change the position of the second channel to different positions along the front of the body and for each position, determine the magnitude of the second channel, time aligned to the QRS peak of the first channel. From this data, make a contour plot of the body-surface potential.

Hints for minimizing ECG measurement errors:

Apply the electrodes at least 5 minutes before recording. Sweating tends to affect electrode adhesion to the skin.
Nothing should rub against the electrodes and Subject’s clothing should not interfere with electrodes.
No pressure should be placed on the electrodes or the baseline will be distorted.
Subject should remain still during all recording segments. Any extraneous movement will be passed through the electrodes. Subject should try to minimize EMG artifact generated in the arms and chest, which will interfere with the ECG signal. Do this by relaxing and not moving.
Subject should maintain the same position for each recording segment, including angle of arms at rest.
Subject should be at rest and should not have exercised within the last hour.
The optimum setup is Subject relaxed and in a supine position.
Wait at least 30 seconds after moving the precordial lead before recording each segment.
Someone other than the Subject should unclip/clip the precordial (chest) lead to rotate it from position V1-V6 so Subject does not have to move between or during recording segments — it is imperative that the Subject remain relaxed so the ECG is not corrupted with EMG artifact.
Make sure the electrodes adhere securely to the skin. If they are being pulled up, you will not get a good ECG signal.

V. Respiration and Airflow measurements

To measure respiratory-rate and respiratory volume, Connect the differential pressure transducer MPX10DP using the pin-out diagram below. Hook the transducer up to a mouth-piece and breathe into it. The output of this transducer measures air-flow. Integrate the air-flow measurement to obtain lung volume. Use a calibration of 3.5 mv/Kpa assuming a 3 V power-supply. Measure tidal volume and inspiratory and expiratory reserve volumes (see figure below).

V. Integrated cardiovascular measurements

Hook up transducers to simultaneously measure heart-rate (using arterial pulse), ECG (using any one-lead) and respiratory-rate.

VI. Modulation of Cardiovascular activity

1. Dive reflex

Dip your head in ice-cold water and observe changes in your cardiovascular response. Record and discuss your observations.

2. Aerobic exercise

Examine the effects of exercise on circulatory and respiratory physiology. Obtain a baseline Measure blood pressure, ECG, heart rate, Breathing rate for 30 seconds. Start exercise: 200-300 jumping jacks. Measure ECG, heart rate, Blood pressure, Breathing rate, Measure for 30 seconds immediately following exercise. Exercise should be only at a moderately strenuous level unless you are an athelete who is comfortable with strenuous exercise in which case you can exercise more. Measure after the start of exercise. Exercise should continue for about 3 to 5 minutes or until you feel that you are tired. Then measure the same quantities every minute. How long does it take for the various measurements to return to baseline?

3. Mental activity

Pick a mental activity that you believe will modulate cardiovascular activity. Formulate a hypothesis about cardiovascular modulation and demonstrate evidence in favor of your hypothesis.