Electrocardiogram Basics and correlation with Cardiac Cycle

Electrocardiogram Basics and correlation with Cardiac Cycle


Okay, so one thing we didn’t get to in
class today was a discussion of the electrocardiogram. You actually only have
one goal, although it has two parts, for this chapter about the electrocardiogram
and this is the goal: to match the events of the electrocardiogram — I’ll talk
about what that means in a minute– with what’s happening electrically in the
heart– so what does it mean, so how does it correspond to the depolarization and
contraction of the different parts of the heart, and functionally in the heart
as well, so how does it match up with the events of the “cardiac cycle” which just
means blood, how does blood move through the heart. So I’ll give you– there’s an
example for each one of these in the slides. The first one is “Which part of
the electrocardiogram”– this is an example, “which part of the electrocardiogram
represents depolarization of the atria?” So if you look at the electrocardiogram
itself, all you need to know here is what these three different parts are for. So
there’s this P, called the P wave, this little thing, alright? And then over here
there’s the T wave, and in between there’s this thing that has three letters,
because it kind of has three little parts, the Q R S, and it’s called a
complex instead of a wave because it has three parts. But you don’t have to know
the parts separately at all, so there’s just three things: the P wave, the QRS
complex, and the T wave. Now of course the blood is in the atria first, right? So the
first thing that has to happen is for the atria to contract. And the thing that
makes a muscle contract is for it to depolarize; so just logically, the P wave
is the depolarization of the atria. So the correct answer to this question is “a.”
It seems like there should be four things here, right? The depolarization of
the atria and then when they repolarize, and then the depolarization
and repolarization of the ventricles. And the reason that there are three instead
of four is that two of them are happening at the
same time. So the QRS complex is the depolarization of the ventricles; so
first you depolarize the atria, and that depolarization spreads as we saw in
class today to the ventricles, and they contract; that’s what this is; it makes
sense that it would be bigger than the P wave because it’s a much, there’s much
more tissue in the ventricles, it has to direct blood all over the body– at least
on the left side– whereas the atria are only squeezing it into the ventricles right
next door. So this is a much bigger event, and while it’s happening, sort of hidden
in there, is the repolarization of the atria. So you can’t really see the
repolarization of the atria. Okay so we depolarize the atria, and then they go
back to normal somewhere in here, and now we’ve depolarized the ventricles, and
this is the repolarization of the ventricles. If you’re looking at this
like you would look like, look at an action potential, it doesn’t make any
sense, because why would repolarization be
going up? You just can’t think of it that way; we don’t have time to go into it in
detail; if you want to hear more about it, you can come ask me about it in my
office; but this isn’t an action potential, it’s a whole bunch of action
potentials added together; and it’s not directional like an action potential is,
or in the same way. So it’s very, like, schematic; just think of it just as sort
of representational, that this “represents” the depolarizing atria, the depolarizing
ventricles, and then the repolarization here of the atria that you can’t see; and
the T wave is the ventricles returning to normal. Now we’re just seeing a little
sort of bit of it here, but after this T there’s gonna be a pause, and then the next P.
So all of this, right, is basically systole. The heart is contracting; it
depolarizes and contracts, and then the space in between is diastole, and we’ll
see that on the last slide here. So if we go to the second question that I would
have asked you today, the second and last question about this, this is the second
part of the goal, where you have to correspond the ECG to the cardiac cycle
which means what’s happening with the blood. And the
way you want to think about this is just that, if [muscle] tissue’s gonna contract and move
blood, like the ventricles, it has to depolarize first; so if you know– so it
says what part are you in when your ventricles are ejecting blood into the
arteries– just logically we know that they can’t eject blood except by
squeezing, and that that squeeze, that contraction, is caused by the
depolarization; so if this is the depolarization, QRS, of ventricles, then
they must be squeezing blood out somewhere in here, right? So it can’t be
the P wave, because that’s way later, that’s when the– or earlier, depending on how
you think of it; later for the next cycle and earlier in this cycle; it can’t be
during the QRS wave, because that’s when they’re depolarizing; they can’t have
contracted yet. S to T, now that looks good, because it happens right after they
depolarize, so it makes sense that they would now be[ing] contracting. T to P, which
is the end of this wave to beginning of the next one, well we know that period is
just diastole, the heart is resting during that time. So the only one that
makes sense, even without looking at the next figure that I’m going to show you,
the only one really makes sense there is the S to T, the space from S to T. Now
what you should look at to get a better idea about this, is this figure, which
shows, so up here is the electrocardiogram, you can ignore the heart sounds because
we’re not talking about it, all right? And then you see systole and diastole. Okay,
so you don’t have to worry too much about atrial, we’re really worried about
ventricular, when the ventricles are contracting, and then when the ventricles
are relaxing here; and it shows you where the blood is, right? So here are the atria
depolarizing, and then they contract right after, right? The atria depolarize
and then right after that they contract, it makes sense– the depolarization causes
the contraction. And then here the ventricles depolarize, and then they
contract, and as they finish contracting, they stop– they return to their resting
membrane potential as muscles, and then we have this rest period, and then the
whole thing repeats. So here’s the depolarization of atria, there of
ventricles; depolarization leads to contraction, and that leaves– so the blood
being squeezed out, so here’s the blood and you can see this arrow, it’s moving
now into the aorta, and from the right side of the heart into the pulmonary
arteries. And then will, and then it refills, right? See how the arrows are
pointing to blood coming into the heart while it’s resting, and then it pushes
the blood out, and so on. So have a cup of tea with this figure, if you, if you
need to, but also just think of it logically, that any tissue is going to
depolarize and then it’s going to contract; so as long as you know what
these P, QRS and T waves are, so just three little facts, you can sort of logic out
the rest of it. Okay, let me know if you have questions about that.