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## Health and medicine

### Course: Health and medicine > Unit 2

Lesson 9: Pressure volume loops- Pressure in the left heart - part 1
- Pressure in the left heart - part 2
- Pressure in the left heart - part 3
- Left ventricular pressure vs. time
- Left ventricular volume vs. time
- Drawing a pressure-volume loop
- Understanding the pressure-volume loop
- End diastolic pressure-volume relationship (EDPVR)
- End systolic pressure-volume relationship (ESPVR)
- Reimagine the pressure volume relationship
- What is preload?
- Why doesn't the heart rip?

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# Left ventricular volume vs. time

Left ventricular volume changes over time in a way that makes a curious looking graph. Learn more in this video! Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

## Want to join the conversation?

- I understand the shape of the V vs T graph here, but the intuition is still fuzzy to me. The way you determined delta P was changing from big to small and vice versa was based on how you drew the graph with the slight differences in the aortic pressure vs LV pressure and left atrial pressure vs LV pressure, but in previous videos we just said they are close. How did you know that delta P changes in such a consistent fashion?(3 votes)
- At8:25, when the diastolic part of the curve is drawn there is no increase in flow at the near end-diastole. But this is the time of the atrial systole. I have expected increased flow as we can see in the previous videos with increased pressure at the time of atrial contraction. Has this been accounted for? Is there really no flow increase?(1 vote)

## Video transcript

So this is our left
ventricular pressure curve, and I've divided it
into four sections. And actually, I chunked
in this extra bit here. This is actually kind of going
into the second heartbeat. And the reason I
did that, remember, is because I wanted
to make sure that you can see the entire last chunk of
the heart cycle, this blue bit, instead of having to try and
remember adding this part in. So just to keep it
easy on the eyes, I just went into
the next heartbeat to make you see very
easily that this is all one part of
the cycle in a sense. Now, instead of looking at
pressure, what I want to do is actually look at volume. And so what I'm going
to do is I'm actually going to draw out the same axis. So we're going to have time
using seconds over here. And I'm going to follow
the heart, specifically the left ventricle, in the same
way I was following it before, in four sets. This is, let's say, 1.2 seconds. This could be 1 second. This could be 0.75. This could be 0.5,
and this could be 0.25, something like that. There we go. And on this side, I'm
going to do volume, so volume in milliliters. And we're going to
start with 0 down here. And we'll do 50 right there. Let's do 100 and 125. And these numbers, I'm
actually just choosing because I think they're
reasonable numbers. But, of course, we know that
these numbers can change person to person. Now, to figure out what
is happening over time, I'm actually going to label the
four points as before, where the colors change on
our graph-- A, B, C, D. And we know that
going from A to B, this we called the isovolumetric
contraction, meaning the volume in the heart
stays the same while there's a contraction going on there. And on this side in green, we
have isovolumetric relaxation, so the same idea
that volume is not changing during these times. And because of that, I
think that's an easy place to start our graph. So we're going to actually
use A, B, C, and D for isovolumetric
contraction and relaxation. Now, when we're talking
about contraction, you know that at this
point, the left ventricle is really full of blood. When it's about
to contract, it's as loaded up as
it's going to be. And so these are the two points
A and B. And then for C and D, you basically have blood,
what's residual in the heart, and that, let's
say, is around 50. So these numbers,
125 and 50, I've chosen them because, again,
they're reasonable numbers. But they could definitely
change person to person. And I really just want
you to get a sense for the overall pattern,
what it looks like. So these two parts
of our graph, I'm just sketching out like that. And in fact, I can
even draw that last A, because it gets into
the next heartbeat to be somewhere like that. That's resetting and
restarting the next heartbeat. So the question is, what is
the slope of the line here? Is it just a linear
thing like that? Is it just a line where blood
fills in, something like this? Or is it something a little
bit different from that? And to answer this
question, I think it really is helpful to
revisit that old equation delta P equals Q times R,
something like that, right? And in fact, I'm
actually going to blow up two sections of the curve. I'm going to blow up
one section like this, and I think you'll recognize
which sections they are, just based on the color and the
shape I'm drawing them in, and another section like this. So we've got two sections. And actually, maybe the
slope is not quite like that. And it looks maybe like this. So there are two sections here. The way to apply this
idea delta P equals Q times R, what I want you
to do is think about this. Think about section one,
which is right here, and section two separately. So this is section one
and this is section two. And what's happening in
terms of the aortic pressure during section one? Well, the aorta is, of
course, coming in like this. And immediately the aortic
valve opens, and then quickly the pressure rises,
and it rises, and then finally catches up with the
left ventricular pressure. And then at some point,
let's say right here, it's going to do this and
have that dicrotic notch we talked about. And on the bottom-- this is,
of course, the aortic pressure, right? And on the bottom, we've
got left atrial pressure to think about. So left atrial pressure
is coming in like this, and it's rising quickly,
and it's maxing out. Remember, it's
very full, we said. And then it begins
to fall, right? It begins to fall, and
then it picks up again. So these are the kind
of curves that you get for the left
atrium and the aorta. Why did I draw this out? Why did I make it
all big like that? Well, because if we're
talking about delta P, all that means is the difference
between the yellow line and the white line. That's all it is, delta P,
difference in two pressures. So if I look at
this, it's actually a nice difference, a
good size difference. If I look up here, it's a
medium-sized difference. And if I look over here,
it's almost no difference. So delta P is starting
big and getting small. So delta P is getting smaller
and smaller with time. And over here, it's
a similar situation. It starts out big, and
then it gets medium, and then it goes
eventually small. So it starts out big and
then eventually get smaller. So, whenever you see delta
P, change in pressure, and you see two pressure lines,
that's kind of an immediate-- that's an easy one
in a sense, right? You just have to look at the
difference between the two lines. So how does that help us? Well, you can see that
delta P equals Q times R. And what is Q exactly? Well, Q, remember, this
is flow, blood flow, and the units on Q-- maybe
you'll have an aha moment; I did when I first thought about
this-- the blood flow is simply volume over time. And if you have
volume over time, then that's nothing more
than the slope of the line. The slope down here
on this line is going to be volume over time. So if I can show differences
in delta P-- and I guess here I have to state my assumption. My assumption is that
resistance isn't changing much, so this is basically
steady within the heart. There's not a huge changes
in resistance from heartbeat to heartbeat within the heart. We don't assume that. So really, if you can
see a change in delta P, then you'll see
a change in flow. And that's exactly what happens. So initially, we said
that there's a big delta P over here, a big delta
P with aortic pressure. So that's looking at
this point right here. And that means
that there's going to be a big flow, that there's
a lot of blood flowing, in this case, out of the heart. And if there's a lot of blood
flowing out of the heart, then that means that this
line, this slope of the line is going to be really
steep and negative. And then eventually,
you get a small flow. Eventually, the delta
P becomes tinier, and the flow becomes
less significant, so then you get a smaller flow. So you get basically
something like that. So instead of that
white line, the truth is that you get
something that looks more like that yellow line. Now, what about the other
side going from D to A? Well, going from D to A-- let
me just switch colors here-- you have a similar situation. You've got a big
delta P initially, a lot of blood flowing, this
time into the left ventricle, so it kind of rises quickly. And then eventually, it
gets smaller over time. So eventually, it's
going to do this. And that's why on
the other side, we have to change that
white line as well. So this white line over here
is going to change as well, and it's going to
look more like that. So this is exactly what the
volume time graph looks like. It has this interesting shape. And instead of just having
you memorize the shape, I want you to understand
where it comes from. So there you go.