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Electrocardiogram – The Patient's History (Diagnostics)

by Joseph Alpert, MD
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    00:00 Well, so now here we are at the electrocardiogram. We've taken the patients history including their past history, their family history, their lifestyle history. Now we have gone on and we've done the physical exam. We have some little clues from that. Not only from listening to the heart, examining the chest and so forth but also looking at the rest of the body. Now we come to the start of some objective diagnostic test. The simplest and least expensive test is the electrocardiogram.

    00:30 I'm going to spend a few slides talking about the electrocardiogram. Let me warn you at the start, you will not be able to read electrocardiograms following what I tell you. Again, entire textbooks of four, five hundred pages are written about how to interpret the electrocardiogram. And like anything else, like playing the piano, like playing tennis, it takes a lot of practice in order to be able to read the electrocardiogram and pull some information out of it. You can get more information about it if you know something about the patient in addition to looking at the electrocardiogram. Now, what you see here in front of you is the typical 12 lead electrocardiogram. It looks at the heart, it is an electrical biopsies of the heart done in two planes. A frontal plane like this and a sagittal plane like this. And I'm going to show you examples of that. On the left hand side of the slide, you see six leads that are biopsed as if in the frontal lead we were looking from here, we're looking from here, we're looking from here, we're looking from here. In the right hand six leads, we are looking as if we took electrical biopsies around the chest like this. Right through the chest. I will show you a little more examples in the next slides. Remember we talked before about how the electrocardiogram starts with atrial depolarization, the P wave. Then goes into the QRS which is the ventricular depolarization.

    02:06 And then the T wave which is the resetting of the electrical mechanisms. I'm going to show you enlarged example of that in a moment. But it's important to know that when we are using the electrocardiogram, we have to use it in the setting of what we know already about the patient.

    02:25 We are looking for example for signs that the heart muscle has increased in thickness. We are looking for signs that the heart may have been damaged at some point in the past with a heart attack.

    02:35 We're looking possibly even for an acute heart attack. If the patient comes to the emergency room complaining of chest pain. These days, most electrocardiograms are read by the computer.

    02:46 However, the computer is not faultless. It's about 80% accurate and needs an experienced cardiologist or internist who knows a lot about reading electrocardiograms to read that cardiogram, to interpret it so that we correct mistakes that the computer reading makes. Here, for fun is one of the first electrocardiograms taken in the Netherlands by Dr. Einthoven. And you can see the patient has their legs in buckets and arms in buckets. That was salt water. And they were hooked up to a very large and complex apparatus. And it gave a very rudimentary electrocardiogram. So here is a modern electrocardiogram. You see it's all computerized with a screen right there. The electrodes, we don't have to put the patients legs and arms in salt water anymore. We use little paper electrodes.

    03:38 I'm sure many of you have seen this done before and probably had it done to yourself. But in any case, it's quite clear that what we get these days is a much nicer and cleaner tracing compared to what Dr. Einthoven got more than a hundred years ago. So here is what I talked about before, and that is we get the electrocardiogram in two planes. A frontal plane that is like this and a sagittal plane like this.

    04:06 And you can see the first, you can see the original Einthoven sort of triangle for the electrodes.

    04:14 And then the later one, the Goldberger, which is one of the modern ones. And then on the right hand side you can see the sagitally it's the ones that go around the chest on the left hand side. What they are doing is they are taking a look at the electrical activity of the heart from different angles.

    04:31 And in fact, you can put that all together and get a three-dimensional image of what the electrical activity is in the heart. But in fact we usually don't bother with the three-dimensional electrocardiogram.

    04:43 We usually interpret it from the two planes that we have. Again, frontal plane and sagittal plane.

    04:51 So, here we see the frontal plane. You can see the six leads that I talked about. And you'll see that they are coming in from different angles. So that lead I comes in like this from the left side, lead aVF comes up from below, lead aVR comes from down from the right shoulder. So we have basically a electrical picture of the heart from six different points around the compass in the frontal view.

    05:21 When you look at the transverse you get six views too but mostly of the left side because that's what you are most interested in is what's happening to the left ventricle. And you can see in this diagram again it implies that really what we're seing is we're seing as if we were doing electrocardiogram in a sphere. We're looking at the heart all the way around as if it were the center of a globe.

    05:44 But again we can do that electrically. We can actually show the three-dimensional image of the electrical activity in the heart. But we usually don't bother because we get as much information just by looking at the frontal and the sagittal planes. Now here we see, two sets of electrocardiograms.

    06:04 On the left hand side is a normal cardiogram, on the right hand side is a patient with ischemic heart disease. I am not going to try and take you through all the details of recognizing what is happening. But I will show you an enlarged electrocardiogram, one complex what we are looking for when we're worried that the patient may be having an acute heart attack. Again just a quick review. We talked about this in the first lecture. What we're seing on the electrocardiogram is the electrical wave of depolarization that sets off the mechanical activity as it passes through the heart. You can see in this little diagram, it starts with the sinus node up in the upper right atrium. It passes down through a little delay factor, the AV node with the Bundle of His. And that delay there is to allow the atria to finish its contraction.

    07:01 And then it passes down into the Purkinje fibers that go out throughout the ventricular muscle and result in the depolarization wave that causes both right and left ventricular contraction. And again, remember the P wave is atrial depolarization. The large deviation is the QRS. That's the ventricular depolarization.

    07:21 And then you see the T wave which is the electrical resetting. Here we see it enlarged. And you can see a number of measurements that are taken. Normally, again the computer does this these days. Although in the past we used to do it with a little pair of calipers. But the computer is usually quite accurate in doing this intervals. You'll notice the P wave and the interval between the P wave and start of the QRS, that's called the PR interval. And then there's the width of the QRS. And then there is from the the end of the QRS to the end of the T wave is the QT interval. And all of these intervals have some implication for how well the conduction is going through the heart. And they also can have changes related to drugs that we give. And sometimes dangerous changes can be picked up on the electrocardiogram.

    08:11 So again, the important thing to remember is that each heartbeat has, each normal heartbeat that is, has a P wave with atrial depolarization, a QRS with ventricular depolarization and a T wave with resetting of the ventricular muscle for the next heartbeat.


    About the Lecture

    The lecture Electrocardiogram – The Patient's History (Diagnostics) by Joseph Alpert, MD is from the course Introduction to the Cardiac System.


    Included Quiz Questions

    1. All the following
    2. Ischemia
    3. Acute myocardial infarction
    4. Ventricular hypertrophy
    5. Arrythmia
    1. V2
    2. aVL
    3. aVF
    4. I
    5. II
    1. PR interval
    2. QRS complex
    3. QT interval
    4. PR segment
    5. ST segment
    1. SA node Inter-nodal and inter-atrial pathways A-V node Bundle of His Purkinje fibers
    2. Purkinje fibers SA node Inter-nodal and inter-atrial pathways A-V node Bundle of His
    3. SA node A-V node Bundle of His Inter-nodal and inter-atrial pathways Purkinje fibers
    4. SA node A-V node Inter-nodal and inter-atrial pathways Purkinje fibers Bundle of His
    5. SA node Bundle of His Purkinje fibers Inter-nodal and inter-atrial pathways A-V node
    1. PU interval
    2. QT interval
    3. PR interval
    4. PR segment
    5. ST segment

    Author of lecture Electrocardiogram – The Patient's History (Diagnostics)

     Joseph Alpert, MD

    Joseph Alpert, MD


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