Cardiac Physiology Lab

Goals

(1) Gain experience in important scientific and clinical methods
(2) Integrate this technical capability with your learning in physiology

The end points

The product of your efforts will be particularly important this time. Each lab group should get nice records and annotate them in a journal that can be easily found (by yourselves and by us). Each individual will write a lab report. It will suffice, in the report, to refer to figures in that we can find on your computers in your journals. The due date will be Oct. 26, after the midterm, and the exact expectations will be finalized announced at a later time.

Background

You saw in the electrophysiology demo how you could just grab two wires, one with each hand, and record the electrocardiogram (ECG, EKG). How is that possible? It is because (1) a large number of cells are depolarizing and repolarizing fairly synchronously (making a big signal), and (2) there is virtually no resistance in between cells (through the extracellular fluid). The biggest challenge, in terms of resistance, is through the skin, and today we will overcome some of that resistance with the EKG electrodes that stick on and that have a high conductance gel.

If you go to the doctor for a clinical EKG, leads are put in many locations, and the heart's proper or impaired function can be inferred by measuring the EKG from different directions. Today, we will do the three "leads" of Einthoven's triangle, Fig. 14.19 in your text:
(1) "lead I" (left arm POS, right arm NEG, left leg EARTH)
(2) "lead II" (left arm EARTH, right arm NEG, left leg POS) [switch left arm and left leg from I]
and
(3) "lead III" (left arm NEG, right arm EARTH, left leg POS) [switch left & right arms from II]
to assess the differing EKG shapes.

A stress EKG involves being on a treadmill to challenge the cardiovascular system while recording with a diagnostic array of electrode leads.

Recorded on the computer, the EKG has PQRST components (as mentioned in your electrophysiology quiz). P is atrial depolarization, QRS is ventricular depolarization, and T is ventricular repolarization. (Note, myocardial action potentials have long duration relative to nerve action potentials, see Fig. 14-13 in your text.) Now you can appreciate the name of a well-known cardiac disorder (mentioned in your text) called "long Q-T syndrome."

Overview - We will make 3 measurements separately (and simultaneously)
(1) The EKG
(2) The peripheral pulse
and
(3) Blood pressure

Preliminaries

Start iWorkx
Go to settings, ECG circulation

Peripheral pulse transducer:

Each student in a group can try this.
(1) Start with your arm resting on the table.
(2) Raise your arm.
(3) Lower your arm toward the floor.

Here is what happened to my peripheral pulse when I held my breath. (The rate did not change much, but the size did.) Try this.

EKG - Initial Electrode Placement and Connections:

There are 3 wires plugged into the other end or this special wire, one to green "earth, common" (most people call this "ground"). The other two wires are red and black. Three BioTac ECG conductive adhesive electrodes will be attached: one to each wrist and a third to the left ankle.

Connect Lead 1. I was confused between the difference between what your book says and what iWorx says, so I tried it, and I am pretty sure you should connect red to the right wrist, black to the left, and green to the ankle, left (instead of right).

Remember high-pass and low-pass filters from the electrophysiology lab? Those are not easy to set on I worx, but I wrote the tech rep and he told me how. With full scale, the ECG is not very big. Click auto scale.Now the ECG is bigger but the noise is worse. Click stop. With a right mouse click, get a menu that gives a choice for filtering. I liked low=2 to get less movement noise and high=15 to get less fast noise. This is done with a right mouse click (as well as the keyboard arrows for finer control).

Click "start" and sit still until you get a nice recording, then you can stop. Note that you can scroll back and forth and that you can compress and re-expand the time scale with those little buttons that look like mountains.

After getting the record for lead I, do it again for leads II and III, reminder:
(1) "lead I"
(2) "lead II" [switch left arm and left leg from I]
and
(3) "lead III" [switch left & right arms from II]

Here is a PVC (premature ventricular contraction) [note the lack of a P-wave] followed by a compensatory delay.

Electrical axis

To determine the mean electrical axis (M.E.A.) begin with a QRS complex from Lead 1 and count the number of millimeters that it projects above the base line. Add to this value the number of millimeters that it extends below the base line. If the QRS complex extends 7 mm above the baseline and 1 mm below the baseline, sum these values as 7+ (-1)= +6. Repeat the process for Lead 3. Find the Lead 1 scale on the grid chart (paper copies will be provided in lab) and draw a line perpendicular to the scale at the +6 point. Repeat this process for the Lead 3 value. Draw a line from the center (0) point of the grid chart through the intersect of the two vectors out to the mean electrical axis scale.

Like the old fashioled academic that I am, I pulled a half dozen books off the shelf and found tidbits. Only then did I remember the internet and typed "electrical axis" into Sherlock (get it [?] -- Sherlock Holmes) a Mac-specific smorgasbord of search engines that is very friendly. Try this. (It's under the apple). I got a bunch of nice sites. 0 to 90 degrees (60) is normal, >90 = right axis deviation, <90 = left axis deviation.

In September, 2004, I obtained these results for a 4 s period.
A previous student has a left ventricular hypertrophy from a heart murmur shows a left axis deviation.

Next ECG trial

Pull down the ECG six electrode and work out the same determination with simultaneously recorded ECGs

Blood Pressure

Record blood pressure using a sphygmomanometer. First locate the position of the brachial artery by feeling for the pulse on the inner surface of the left forearm just below the bend of the elbow. That is where you will place the stethoscope diaphragm. Wrap the deflated cuff of the sphygmomanometer just above the stethoscope diaphragm and inflate the cuff until the pressure exceeds 180 mm Hg (no higher!). At this pressure, there are no audible sounds of blood flow due to the compression of the arterial walls. While observing the pressure gauge and with the stethoscope diaphragm placed firmly over the brachial artery, slowly release the cuff pressure using the valve located on the bulb. Listen carefully and note the pressure when you hear a loud thump sound as the blood rushes past the cuff. The pressure at the first detectable sound (tapping) is called the systolic pressure reading. The idea is that the artery is closed off at first, but when the arterial pressure is higher than the cuff pressure (as the cuff pressure is lowered), then the flow is turbulent and noisy. Continue monitoring and lowering the pressure until the rhythmic thumping sounds are no longer audible. The point at which the sound is no longer discernible is known as the diastolic pressure reading. The idea here is that, once the cuff pressure is so low that it does not obstruct the arterial flow, blood flow is no longer turbulent. If you are lowering the pressure slowly, you will also note that the needle bounces, starting at the systolic pressure and ending at the diastolic pressure. Repeat this procedure several times until you are confident that you can measure the blood pressure quickly and accurately before you begin the experiment.

The actual measurements

(1) Pick your favorite lead, in terms of nice waveform, for the EKG.
(2) Get practiced at all the above, EKG, peripheral pulse and blood pressure simultaneously.
(2a) work out a nice geometry for the subject to sit and the student who measures blood pressure
(2b) be prepared for quick release and re-hook-up, i.e. unsnap the EKG but leave on the electrodes, unvelcro the thumb, but keep wearing the blood pressure cuff
(3) Get good baseline records and measurements (resting)
(4) Unhook and walk or run the stairs across the hall.
(5) Rehook and get good after exercize records and measurements.

Note, for the diving lab, the person taking blood pressure will have to be practiced enough to be fast so that the determination can be made while the subject is holding his or her breath.

The next blood pressure measurements

Look at the blood pressure (two set-ups: vagal tone and peripheral circulation. We won't do exactly what they say. Instead, what I want is:

(1) While watching the dial go down, use the push button to mark every 10 so that you can calibrate the Y axis of the feed from the blood pressure cuff.

(2) Then with the plethysmograph, get a nice tracing of the restoration of the pulse as systolic and diastolic pressures are passed.

A few years ago, I gave a quiz, and here are the answers:

Put your name here-> Key

The cardiac glycoside digitalis is most like
(a) acetylcholine. that's a neurotransmitter
(b) tetrodotoxin. that blocks the sodium channel
(c) atropine. that blocks muscarinic receptors
(d) ouabain. YES they're both cardiac glycosides blocking sodium pump
(e) anticholinesterases. like nerve gas, blocks acetylcholine breakdown

When taking the blood pressure with a sphygmomanometer,
(a) one records from the two wrists and left ankle. that's the EKG
(b) one needs to put a catheter into the aorta. that would measure pressure but not with a sphygmomanometer
(c) one listens for turbulent arterial flow. YES
(d) one listens for the "dup." you do not listen for heart sounds
(e) one listens for the "lub." you do not listen for heart sounds

If the EKG trace is very noisy
(a) use a low pass filter. YES
(b) substitute the peripheral pulse monitor since it tells you everything the EKG tells you. no it doesn't
(c) use lead II instead of lead I they would have the same noise
(d) get the paddles. not on me you won't
(e) use an inotropic agent. they change contraction strength not recording noise

Connections to both wrists and the left leg
(a) are used to measure blood pressure. no, EKG not pressure
(b) are named after Einthoven. YES Einthoven's triangle
(c) are described by Starling's law. more ventricular filling, more blood pumped
(d) originated with Paul Harvey in the 1600's. Paul Harvey? The "news" commentator? "Page 2"? Certainly you mean Sir William Harvey and they did not have electricity back then
(e) are used to start the pacemaker after fibrillation. paddles are connected to the chest

How come you can measure the EKG so far away from the heart?
(a) There is easy conduction through the salt water in the body. YES. Bode's exact wording and what I said in the handout
(b) Because of bradycardia. low heart rate is irrelevant
(c) Because of desmosomes in intercalated disks. structural cell connections are irrelevant
(d) The action potentials are many more mV than nerve action potentials. no they aren't
(e) You need different amounts of amplification for lead I, II or III. no you don't

What do you do to measure pressure from the brachial artery?
(a) Listen for the heart sounds. no, you listen for arterial sounds
(b) First administer beta blockers. blocking sympathetic heart influence is irrelevant
(c) Crank it up to the diastolic level and look for the veins to bulge. Need to inflate higher and listen
(d) Nonsense! Blood pressure has nothing to do with the arteries. Nonsense
(e) Inflate the cuff past the systolic pressure and slowly release the pressure. YES

The time between QRS and T might decrease during mild exertion. This would be indicative of
(a) dangerous pathophysiology. happens all the time
(b) the AV node controlling the heart beat instead of the SA node. not because of exertion
(c) a myocardial action potential of shorter duration. YES
(d) parasympathetic control. sympathetic influence
(e) Nonsense! The time would only increase. no it would decrease

If all parasympathetic and sympathetic inputs to the heart were instantly abolished while you were relaxed
(a) nothing would happen. The autonomic nervous system does not influence the heart. yes it does
(b) there would be an A-V conduction block. not from this intervention
(c) the heart would stop. heart is automatic
(d) the heart would speed up. YES. I ran that by you last week
(e) the heart would slow down. opposite

For the mean electrical axis, one measures
(a) the time between P and T at rest and after exertion. of interest but not for axis
(b) QRS above and below baseline for lead I and lead III. YES
(c) the average of the diastolic and systolic pressures. pressure is irrelevant to electrical
(d) the pacemaker potential from the bundle branches. they may have slow pacemaker activity but it never shows up
(e) stroke volume and heart rate. not relevant to electric axis

The arterial diastolic pressure is
(a) higher than the systolic pressure. systolic is higher
(b) the force that opens the A-V (bicuspid and tricuspid) valves. they are open because of low ventricular pressure in diastolic ventricular filling
(c) equal to the atrial pressure during diastole. very low, only from venous return
(d) usually higher than 180 mm Hg. hope not
(e) way higher than ventricular pressure during diastole. yes because ventricles are virtually zero in diastole


This page was last updated 9/29/06

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