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# Understanding the pressure-volume loop

Video transcript

We have our
pressure-volume loop. And what I wanted to do is kind
of explore this a little bit further. And kind of a nice place to
start might be with the name, right? Pressure-volume, or PV loop. And part of it
makes perfect sense. You've got P there, and you got
V there, so there's your PV. And the loop, one loop
in this kind of sense, is really going to
represent one heartbeat. So you're going to
start at one point and kind of go all the
way around from systole and diastole and back. And so using our
PV loop, the one that we kind of
drew together, let's actually show where
systole would be. So if this is where
we start, the red represents all of systole
kind of going on and on. And this part is
really quick, right? Just a fraction of a
second, 0.05 seconds. And then finally, the
aortic valve pops open. And systole continues. It's not like it
just started there. Systole starts with
the initial contraction of the left ventricle,
continues through all of this, and also on this part where
the pressure is falling. That's all systole. In fact, let me label
that very clearly so it's clear that the
red part is systole. And then that leaves, of course,
another whole half of our loop. Although in terms of
time much more than half, because some of these
segments take longer. I'm going to do
all this in blue. And all this stuff
in blue represents diastole, the other
half of the heart cycle. So this is when the left
ventricle is now relaxing. So this is systole and diastole
you can see kind of next to each other on this graph. So this represents one
loop, or one heartbeat. So sometimes, you hear phrases. You'll hear the phrase
end-diastolic such and such, or end-systolic such and such. So what do they mean? Well, end-diastolic is
literally what it sounds like. It's the end of diastole,
kind of where I drew an arrow. And what they're really
talking about is a time point. So at that point in time,
where diastole is done, you have a certain pressure,
or sometimes they'll talk about end-diastolic volume,
so I'm just going to write or. But these two terms,
end-diastolic pressure or end-diastolic
volume, are really referring to a time point where
diastole has come to an end. I guess you could also
say start systolic. And that's kind of
the same idea, right? That's where systole
is also beginning, where diastole is ending. But you don't really
hear that term. You usually just hear
the term end-diastolic. In fact, I think
start-systolic is a word I might
have just made up. So let me actually
just erase that. But I do want to point out that
the concept would be the same. Right, it's just a
certain time point. So end-diastolic pressure
volume is that point in time. And at that point in
time, just remember a few things are happening. You've got, for example, the
mitral valve just closed. So I'm going to
write mitral closed. And if the mitral
valve is closing, also means the
tricuspid is closing. And of course, I'm going to put
it in parentheses because this is the pressure-volume loop
for the left ventricle. So we're not really thinking
about the right ventricle, but some of the events
are going to be the same. So the tricuspid is going to
close at this point, as well. And if the mitral and
tricuspid are closing you know they're
going to make a noise. They don't close silently. And that noise we call the
first heart sounder, S1. And if you think
more along the lines of what it might
sound like, we always kind of think of
the idea of lub. You know that sound of lub dub? Well, the lub part of it
comes from the closing. And now you can see on this
loop where that might happen. Now, on the other
side, you've got kind of similar set
of stuff going on. So you've got what
we call end-systolic. So if there's
end-diastolic, you better believe there's going
to be end-systolic. And this is going to be,
again, pressure or volume. So the pressure or volume
part gets kind of confusing. But just remember
what people mean when they say end-systolic--
all they're trying to get at is that point in time. And just as before, you could
pretend to make up a term, I suppose. You could say, well, isn't that
the same as start-diastolic? And I suppose you'd be right. But because no one
uses it, again, I'm writing it out just to
kind of prove the point. But I'm going to erase it
so you don't get confused. Because end-systolic
is the term everyone has kind of come to adopt. Now, certain events are
happening here, too. You've got the
aortic valve closing. So this is the
closing of a valve. And you've also got-- although
not here-- the pulmonary valve closing. So you could also say, isn't
the pulmonary valve closing? And the answer is yes. And these two together
make a noise, of course. I'm going to write
it right here, S2. And this is the dub noise. When we hear lub
dub, now you can see where the dub
part comes from. It comes from that
second point on our loop. So we've got a couple
points on our loop, and these loops are
used all the time. In fact, the main reason
we use these loops is because they convey so much
information very, very quickly. So for example, let
me just show you why we might use these loops
or how they're useful to you by showing you some
of the information you can glean from them. So you can actually,
for example, take a look at
these two numbers. And you'll say,
well, what is that? Well, this is your
pulse pressure. And you might have heard
pulse pressure before. And usually the way we
think of pulse pressure is if someone says, hey, the
ratio of your blood pressure is-- I'm going to
make this up-- 130/80. That means that my
pulse pressure is just the two numbers
subtracted by each other. So pulse pressure would
be 130 minus 80, or 50. So this is just an example. So this would be
pulse pressure, kind of the way we usually
think of pulse pressure, just your systolic pressure
minus your diastolic pressure. But looking at your
PV loop, remember this is not your
aortic pressure, which is what we measure
usually in your arm. This is actually your
left ventricular pressure. So left ventricular pressure is
going to be very, very similar. You've got kind of the
low end right here, and you've got the high end. And the low end is
really the lowest that the blood pressure in
the aorta is going to be, and the high end is the highest
that the aortic pressure is going to be. So your pulse pressure you
can kind of figure out. Using this loop, I would
say, well, on this loop, the pulse pressure equals
120, because that's the high end right
there, minus 80. And so I would say
my pulse pressure equals 40 millimeters
of mercury. That would be my answer
if someone asked me, what is the pulse
pressure on this PV loop? So the cool thing is
that you can actually use these PV loops to calculate
things like pulse pressure. Now, another interesting thing
you can quickly calculate is this. I'm going to just draw it
with green, a different color. And this is my stroke volume. All that means is
the volume of blood that leaves my heart
with every stroke. So at the one end when my
left ventricle is really full, you've got 125. And then, when it finally
kind of squeezes as much as it's going to,
you end up with 50. So my stroke volume is
literally just 125 minus 50, and that ends up being 75. So if someone said, hey,
what's your stroke volume here, I would say, well,
it's 75 milliliters. So one final thing then-- I
don't want to overwhelm you, but I want to
really point out all the kind of interesting things
you can learn from this PV loop-- is what we call
the ejection fraction. Now, this is just the
stroke volume, which we just calculated to be 75, divided
by what they call peak volume, or total volume in the left
ventricle when it's full. I'm going to call
it peak volume. So in this case, what is the
peak volume when it's 125? That was the highest
amount of blood that we got into
this left ventricle. So the ejection fraction
would be 75 divided by 125. And notice that
the volumes cancel. So all you're left with is just
two numbers over each other. And in this case, you get 60%. I'm writing it as a percent. You could also say it's 0.6. But usually, we talk
about it in percentages. So I would say the
ejection fraction using this PV loop is 60%. So you can look
at these PV loops, and you can learn certain
words like end-diastolic, end-systolic. You can calculate things
like your pulse pressure, your stroke volume,
or ejection fraction. So they basically give
you a ton of information, and that's why we
always use them.