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.
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.
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.
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.
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.
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.
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.
And you can see the first, you can see the original
Einthoven sort of triangle for the electrodes.
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.
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.
We usually interpret it from the two planes that we
have. Again, frontal plane and sagittal plane.
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.
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.
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.
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.
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.
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.
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.