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# What is afterload?

Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

Video transcript

One of the really nice things
about preload and afterload is that the two have
so much in common. So if you're trying to
figure out afterload, then remembering
what preload is about is a really good idea because
the definitions are so similar. So we have volume and
pressure on this graph. I'm actually going to
start by sketching out very quickly a
pressure-volume loop. And you remember
that, to do that well, you always have to kind
of start with the two lines, the end-diastolic
pressure-volume relationship, which is here, which
basically tells you how pressure and volume
relate to each other when the heart is
completely relaxed. So this is the line
that would form if we were to fill up a
relaxed left ventricle. And then, we have another line
that goes something like this. And this one is called the
"End Systolic Pressure Volume Relationship." And this is when the heart
is completely contracted, something like that. So these are our two lines. And now we have to
just draw in our loop. I'm going to draw a loop that
starts here and goes down. This is, of course,
during diastole where the heart is filling up. The left ventricle's
filling up, anyway. And then, of course,
there's contraction. And finally, blood is
ejected out into the aorta. So that's what the
pressure-volume loop looks like, right? So that's how we start. And let me throw up the
definition of afterload, and we'll actually
start by looking at this, pressure-volume
loop and what part of it is afterload. Because I think sometimes
it's easier to just see it. The definition of
afterload, again, it's very similar to the
definition of preload. It's left ventricular
wall stress. So, so far, it's identical
to the preload definition. And this time, it's during--
so this is the key word. It's not at any specific time. It's actually "during." So it's over a
certain time interval. During ejection. So ejection is when
blood is actually being ejected out of
the left ventricle. So on our graph, ejection
would begin there, and it would continue
to about there. So if I was to draw in red
which part of this is afterload, this part of the
curve is afterload. So I'm going to
just make it red. This entire bit is
considered afterload. So that's interesting because
before, with preload, we had a specific time point. But now we have many,
many time points. In fact, in a way,
you can say it's an infinite number of
time points, right? And all of these combined make
up what we define as afterload. So I want to refresh
your memory now on what wall stress is exactly. So you might be thinking,
well, I remember the term, but exactly what it
is, I don't remember. So wall stress--
and I'm just going to write EJ for
"ejection" because you have to remember that
afterload happens just during that part of the
pressure-volume loop, just during that chunk of
it, is equal to pressure during ejection times the
radius during ejection-- this is the radius of the
left ventricle-- divided by 2 times the wall
thickness during ejection. And now, if you
wanted to say, well, could we actually
figure out the value? Is there an actual number
we could figure out? Well, you could say, all right. Well, let's pretend for a
moment that this is 120, and let's pretend that
right there is about 75. So that would be that
spot maybe right here. So you could actually sit
there and calculate it. You could say, well, 120
times whatever the radius is. And remember, there's
a relationship between volume and radius. The radius equals the
cube root of the volume times a bunch of numbers. And, in fact, it's actually
that plus the wall thickness. Remember that. So you could say, well,
the radius equals all that. So if you can actually
figure out these letters, if you could figure out the
volume, which I said was 75, and if you could figure out
the wall thickness, which in a person that's about
70 kilograms, that's about my weight, they would
be around 1 centimeter, let's assume. So if you could make
these assumptions, you could actually
make a number for r. And if you have a
number for P and r, couldn't you just come
up with some answer for what wall stress
is at that point? And to you, I would say, yes. Yes, you could actually
come up with a number at that purple arrow. But then are you going
to go ahead and calculate this one and this one and
this one and this one? And there's an infinite
number because you have to calculate all the
time points in between. So are you really going to try
to calculate all those time points? And you could, using
a bit of fancy math. But if you're just
trying to eyeball it, it would be actually
kind of a tough thing to do, to calculate all that. So how do people actually
look at afterload? If I'm telling you
that it's this equation and that it's actually
during ejection, during that whole time point,
not at any one specific time, but during that entire time, how
do people calculate afterload? Well, here's a dirty little
secret-- people don't. They don't calculate afterload. Not usually, anyway. I mean, you could actually
go through the math and calculate it. I guess if you're
going to publish it, maybe you would do that. But people don't
usually calculate it. What they usually
do is the following. They'll say, OK, well,
this number right here, this wall thickness,
well, that's not really going to change. That's going to
be about the same. So let's just kind
of ignore that piece. And this radius
part, well, that's going to be some small
number because, remember, it's the cube root, and that's
not going to be very big. So at the end of the day, all
they really kind of look at is they're going
to look at this. They're going to
say, all right, well, let's just look at the pressure. And we will assume-- and it's
a pretty safe assumption. I don't want to make it sound
like that's a bad thing to do. It's a pretty safe
assumption that wall stress is proportional to pressure. And if you assume that, if you
buy that, that wall stress is proportional to pressure,
then, of course, you could say,
well, in that case, afterload is proportional
to pressure during ejection. So something like that. So let's go ahead
and test this out. Let's see if you buy
this, first of all, and if you can apply this,
and see if you can find value in this kind of shortcut. So I'm going to draw another
pressure-volume loop here just to test this out. So let's say we have a
kind of tiny one over here, and let's say this heart is
going to contract right there. And you're going to get
something like that. And if someone looked
at these two loops and said to you, hey, tell
me which one has a higher afterload, could you quickly,
just by eyeballing it, answer that question? I'm just going to
highlight the afterload on this loop, which
is right here. It's this entire time span. This part is the ejection part. So could you look
at it and identify which one-- the yellow
loop or the purple loop-- has a higher afterload? And if you look at
it, you could probably say pretty quickly
and confidently that, well, using this
rule that afterload is proportional to
pressure if it's related, then clearly this one
has a lower afterload. And you would be right. That's exactly right. You didn't have to go
through any fancy math or spend a lot of time on your
calculator to get that answer. You just kind of
quickly eyeballed it and figured it out. Now, let me do one
more just to make sure that we're all kind
of on the same page. Let's say I do
something like this, and I'm going to
draw this blue one. We're going to make it kind of
a megaloop, something like that, and a high amount of pressure. And now, compare this
one to the other two. Which one of these three then
has the highest afterload? And if you get the idea, you
would say very quickly, well, of course, this
blue one that I'm drawing has the
highest afterload. This one is obviously
higher than the other two. So that's how you figure it out. You just basically
kind of-- or that's how most people
figure out afterload. They say, well,
let's just assume that pressure and afterload
are related or proportional to one another. Even though we know
now technically the mathematical formula says
that there's other variables we should look at, like
radius and wall thickness, but most people just kind
of eyeball things and say, well, yeah. That's a higher afterload. So now, let me push you one
step further and say, OK. If you think that you've
mastered this little bit, let me now build
in an assumption. I'm just going to write it
very clearly because this is definitely not always true. But assume that the aortic
pressure is the same. And let's say during
ejection, aortic pressure during ejection, is the
same as the left ventricular pressure during ejection. So let's assume this is true. What does that mean? Well, if this is true--
and for many, many people, it is true, right? Most people don't
have any problem with their aortic valve. Or their aortic valve is
working normally, I should say. So their aortic pressure
is basically the same as their left
ventricle the pressure. So for most people,
if this is true, what does that mean for
our pressure-volume loop? Well, what it means is that,
if I'm saying that you can just look at the pressure on
that part of the curve to assume what
afterload is, well, that pressure is something
that we know more commonly. We actually have
another term for this. What is the more common term? Well, usually, we call this
"systolic blood pressure." That's usually
what we know it as. And we usually call this
"diastolic blood pressure." These are the blood
pressures that we generally record when you check someone's
arm for what their blood pressure is. You can actually get a
good sense for afterload simply by looking at
someone's blood pressure. It gives you a lot
of information. It may not be exact
because, of course, systolic and diastolic
blood pressure are usually checked where? They're checked
usually in your arm, and they're not checked
actually in the aorta itself. But if we assume that
there's a lot of similarity between those two spots--
and there might be-- then, we can say, well, we can learn
a lot about aortic pressure-- or sorry, we can learn a lot
about left ventricle pressure and, therefore, about
afterload simply by looking at your
blood pressure. And if your blood
pressure goes up, then there's a good chance your
afterload is going up as well.