Full transcript
0:05Hi everybody and welcome to our module
0:07on pressure volume loops. Shown on the
0:10left side of the screen are the changes
0:11in left ventricular volume and left
0:13ventricular pressure over time during
0:16the cardiac cycle. The left ventricular
0:18volume curve is on the bottom. This is
0:19shown right here. And the pressure curve
0:21is on the top shown right here. And
0:23these two curves are both elements of
0:25Wigger's diagram which I talk about in
0:27the video on Wigger's diagram. Now, if
0:29you took the volume at each point in
0:30time and the pressure at each point in
0:32time and you plotted the volume along
0:34the x-axis and the pressure along the
0:36y-axis, you would get a square shape
0:39like what I've shown on the right side
0:40of the screen. And this is called a
0:42pressure volume loop. It shows you the
0:44changes in the left ventricle during one
0:46cardiac cycle. There's no time in a
0:49pressure volume loop. So you just have
0:50to know that as the cardiac cycle moves
0:52forward in time, the pressure volume
0:54loop will move in a counterclockwise
0:56manner like this around and around. So
0:58what we're going to talk about in this
1:00video are all the different things that
1:01you can read off of a pressure volume
1:03loop and how it changes in various
1:05cardiac conditions. The first thing I
1:08want to point out is that there is a
1:09short portion of the left ventricular
1:11volume versus time curve that is flat,
1:13meaning that volume is not changing. At
1:15the same time, the pressure in the left
1:17ventricle is rising. So anytime the
1:19volume is not changing that's called
1:21isovalumic and the reason that the
1:23pressure is rising is because the left
1:25ventricle is contracting. So this
1:27portion here between the two green lines
1:29represents isovalic contraction and that
1:32is denoted on the pressure volume loop
1:33by this green line here on the right
1:35side. The volume is constant. So it's a
1:37straight line but the pressure is
1:39rising. So we're moving up the pressure
1:41on the y ais. And the reason the volume
1:43is not changing is because at this
1:45portion of the pressure volume loop both
1:47the mitro and aortic valves are closed.
1:49So no blood can get in or out of the
1:51ventricle. Therefore volume cannot
1:52change. There's also a point on the
1:55volume versus time curve where the
1:56volume is flat but the pressure is
1:58falling. This is isovalic relaxation.
2:01It's denoted by the red line on the
2:03right side of the screen. What's
2:04happening here is that once again the
2:06mitro and aortic valves are closed. So
2:08no blood can enter or leave the
2:10ventricular chamber but the pressure is
2:12falling because the mioardium is
2:14relaxing. You may have noticed this
2:16curved line at the bottom right of the
2:18pressure volume loop and this straight
2:20line at the top left. Let's talk about
2:21what those lines mean now. So this point
2:24right here is the end of cy. We'll talk
2:26more about that in a minute. And this
2:27point down here is the end of diastily.
2:30So let's talk about the end of cy first.
2:32This is the end of cy where the
2:34ventricle has finished contracting and
2:36ejecting blood. The mitro and aortic
2:38valves are closed. So there is a closed
2:40chamber of the ventricle that no blood
2:42can get in or leave. I want you to
2:44imagine now that if we stuck a needle
2:46into the left ventricle and we injected
2:48just a little bit of blood at this point
2:49in time at the end of cy that would
2:52slightly increase the pressure and
2:53volume in the left ventricle to a point
2:55that's let's say right here. If we once
2:58again injected some blood we'd get
2:59another point right here. And if we do
3:01this same thought experiment and imagine
3:03removing a little bit of blood at the
3:05end of cy when all the valves are
3:06closed, we would lower the pressure and
3:08volume to a little point like down here
3:10and we could remove some more blood and
3:11we get another point down here. So from
3:14this thought experiment you can see that
3:15the addition of a little bit of volume
3:17at the end of cy or the removal of a
3:20little bit of volume creates a series of
3:21points and that makes a line and that
3:23line is called the nsystolic pressure
3:25volume relationship and that line is
3:28determined by characteristics of the
3:29myioardium and we'll talk later about
3:31some things that can change that line.
3:34We could do the same thought experiment
3:35here at the end of diastilly. If we add
3:37a little bit of blood to the ventricle
3:39when all the valves are closed and it's
3:41a sealed chamber, we would create
3:42another point here. And if we added some
3:44more, we'd create another point here.
3:45And we could remove some blood and
3:46create a few points down here. And then
3:48we would get a curve that looks like
3:50what I've drawn on the screen. And
3:51that's called the end diastolic pressure
3:53volume relationship. And that also is
3:55determined by characteristics of the
3:56ventricle. And we'll talk about things
3:58that can change the end diastolic
4:00pressure volume relationship later in
4:02this video.
4:03Now what I want to do is go through the
4:05pressure volume loop and point out where
4:07the mitro and aortic valves open and
4:09close. So I told you before that this
4:11flat portion of the left ventricular
4:13volume versus time curve represents
4:16isovalic contraction. In order for
4:18isovalic contraction to occur the aortic
4:21and mitro valves must be closed.
4:23Therefore at the beginning of isovalumic
4:25contraction that is when the mitro valve
4:27closes. And I've denoted that on the
4:29left and I've also denoted it on the
4:30right on the pressure volume loop. We
4:33now enter the period of isolic
4:35contraction where the pressure in the
4:36ventricle rises. At some point the
4:38pressure gets so high that the aortic
4:40valve opens and that's what occurs right
4:42here on the left. You know the aortic
4:44valve is opening because this is the
4:46beginning of the point where volume
4:47falls in the left ventricle. That means
4:49blood must be leaving and therefore the
4:51aortic valve must be opened. And I've
4:53pointed where the aortic valve opens on
4:55the pressure volume loop as well. Now at
4:57some point all the blood that is going
4:59to leave the left ventricle has left and
5:01we now enter this section down here
5:03which I told you was isovalic
5:04relaxation. So at the beginning of that
5:07section the aortic valve must close and
5:08that's what happens here on the left
5:10side of the screen and I've pointed it
5:11out at the right side of the screen as
5:13well. And I told you before that this is
5:15the end of cy. So naturally that's where
5:17the aortic valve closes at the end of
5:19cy. Then we go through that brief period
5:21on the left where we have isovalic
5:23relaxation and then the mitral valve
5:26opens and that's shown now on the left
5:28side of the screen and also on the
5:29right. So it's important you know which
5:31points in the pressure volume loop
5:33represent the opening and closing of the
5:35aortic mitro valve and now we've gone
5:36through all of those in this slide. I
5:39want to also point out that when the
5:40mitro valve closes that is one component
5:42of the first heart sound S1 and when the
5:45aortic valve closes that is a component
5:46of S2. So therefore you can identify S1
5:49and S2 on a pressure volume loop from
5:52knowing where the valves close. And on
5:54this slide I've pointed out where cy and
5:56diastily are found on the pressure
5:58volume loop. I told you before that this
6:00is the end of diastily when the left
6:03ventricle begins to contract. This is
6:05isovalic contraction here. This is when
6:07blood is being ejected from the left
6:08ventricle here. But everything shown in
6:11red on this drawing represents cy. Right
6:14here we have the aortic valve close and
6:15then we have isovalumic relaxation
6:18followed by left ventricular filling
6:19from the left atrium and that all
6:21represents diastily. And if you
6:23appreciate that it's easy to identify
6:25some of the important volumes and
6:27pressures inside the left ventricle.
6:29This point here is the end of diastily.
6:32Therefore the corresponding point on the
6:34x-axis is the end diastolic volume. The
6:36corresponding point on the y- ais is the
6:38end diastolic pressure. This point here
6:41that I'm circling with my pen is the end
6:43of cy. So therefore the corresponding
6:45volume at the end of cy is obviously the
6:47end systolic volume. And now we can
6:50identify even some more important
6:52parameters of the left ventricle. So the
6:54difference between the ends systolic
6:55volume here and the end diastolic volume
6:57here is the stroke volume. This is the
6:59volume of blood ejected from the left
7:01ventricle with each heartbeat. The wider
7:03the pressure volume loop, the more
7:05volume that the heart is pumping. This
7:07point down here at the bottom is the
7:09volume at the end of diastily. That's
7:11the amount of filling of the left
7:12ventricle and that represents the
7:14preload of the heart. This point up here
7:17is the point at which the aortic valve
7:18opens. That represents the afterload. In
7:20other words, the left ventricle has to
7:22raise the pressure up to a point such
7:24that it can open the aortic valve. The
7:26more afterload, the higher the pressure
7:28will have to become in order for the
7:30aortic valve to open. So the afterload
7:32is always found here in the top right
7:33corner of the pressure volume loop. For
7:36the next few slides, I'll go through how
7:38the pressure volume loop changes when
7:40various important left ventricular
7:42parameters are modified. So for example,
7:44we'll go through what happens to the PV
7:46loop when there's a change in preload,
7:47afterload, contractility, or compliance.
7:50Now, in reality, all of these parameters
7:52are interrelated. You cannot change one
7:54without affecting the others. For
7:56example, if you've watched the video on
7:57stling curves, you know that when the
7:59preload goes up, the contractility goes
8:01up via the Frank Starling mechanism. So
8:04all of these parameters are
8:05interrelated. But what we're going to do
8:06in the next few slides is consider the
8:08artificial scenario where we modify one
8:11parameter like preload but keep the
8:13other parameters like afterload,
8:14contractility, and compliance constant.
8:17Let's start by talking about what
8:18happens when there's an increase in
8:20preload. Shown on the screen are two
8:22pressure volume loops. The blue loop is
8:24the baseline state. The red loop is from
8:26a heart with increased preload. And this
8:28is very easy to understand if you
8:30remember what I told you before. the end
8:31diastolic volume which is the volume at
8:33this point right here in the blue loop
8:35that represents the preload. So if we
8:37increase preload we simply move to a
8:39point a little bit further up the end
8:41diastolic pressure volume relationship
8:42which is the black line right here.
8:44Therefore this represents a higher end
8:46diastolic volume and a higher preload in
8:49the red loop. So I've summarized these
8:50points at the top of the screen. The end
8:52diastolic volume goes up when you
8:54increase preload. You'll note that the
8:56width of the red pressure volume loop is
8:58greater than the blue. Therefore, we've
9:00increased stroke volume. And then
9:01finally, we've slightly increased the
9:03ejection fraction by doing this. Why is
9:05that the case? Well, the stroke volume
9:07is the end diastolic volume minus the
9:09nsystolic volume. And I told you that we
9:11raise the end diastolic volume when we
9:13increase the preload. This means that
9:15the ejection fraction, which is
9:17diastolic volume minus n systolic volume
9:19over n diastolic volume, will go up
9:21slightly when we do this. And remember
9:23this is very artificial. Normally when
9:25you do this, you would change
9:26contractility. But here we are keeping
9:28the afterload constant and the
9:29contractility constant just to show the
9:31isolated effects of an increase in
9:33preload. And if we decrease preload we
9:36get the opposite of the changes we
9:38described on the last slide. The end
9:39diastolic volume falls, the stroke
9:41volume falls and the ejection fraction
9:43falls slightly. If you look at the red
9:45pressure volume loop on the screen, you
9:46can see that its width is narrower than
9:48the blue one. That tells you that the
9:49stroke volume is lower. You can also see
9:51that the end diastolic volume is here
9:53for the blue loop and it's here for the
9:55red loop. That tells you that the end
9:56diastock volume has fallen. If we
10:00increase after load, we go from the
10:01baseline blue loop shown in the screen
10:03here to the red loop. I told you earlier
10:06that after load is the point where the
10:07aortic valve opens. That's right here
10:09for the blue pressure volume loop. It's
10:11been raised to a higher point here in
10:13the red loop. The consequence of this is
10:15that when we eject blood in the red
10:17pressure volume loop, we hit this black
10:19line sooner. In other words, we hit it
10:21at a higher volume. So therefore the n
10:23systolic volume shown right here is much
10:25higher than it is in the blue pressure
10:27volume loop. So the major consequence of
10:30raising the afterload is that it's
10:31harder for the ventricle to eject blood.
10:33So it ejects less blood. Therefore at
10:36the end of cy there's more blood left
10:38behind and the ends systolic volume is
10:40higher. Because the nsystolic volume is
10:42higher, the stroke volume is lower. You
10:44can see that the red pressure volume
10:45loop is narrower than the blue one. And
10:47when the stroke volume falls, the
10:49ejection fraction falls because stroke
10:50volume is the top term in the ejection
10:53fraction equation. Now let's talk about
10:55contractility changes. Contractility, it
10:57turns out, is the major determinant of
10:59the nsystolic pressure volume
11:01relationship. Those are the two lines
11:03shown in blue and red here. So when
11:05contractility decreases, it shifts the
11:08nsystolic pressure volume relationship
11:10from the dashed blue line to the dashed
11:12red line. That changes the pressure
11:14volume loop from the blue loop to the
11:16red loop. So let's talk about the
11:17consequence of that. First of all, you
11:20can see that the ncstolic volume here
11:21for the blue loop is much lower than the
11:24ncstolic volume for the red loop. So in
11:26other words, the nsystolic volume goes
11:28up significantly when there's a fall in
11:30contractility. And that should make
11:32sense to you. When there's less
11:33contractility, less blood is pushed out
11:35of the ventricle. Therefore, there is
11:37more left at the end of cy. Therefore,
11:39there is a rise in the endstolic volume.
11:41when there's a fall in contractility
11:43because the nsystolic volume goes up the
11:45stroke volume goes down and it's very
11:47easy to see that the red curve is
11:49narrower than the blue curve and the
11:50fall in stroke volume makes the ejection
11:52fraction lower. An increase in
11:54contractility has the opposite effect.
11:56It moves the ends systolic pressure
11:58volume relationship to the left. That
12:00lowers the endstolic volume. More blood
12:02is being squeezed out with higher
12:04contractility. So the endstolic volume
12:06is lower. This leads to a rise in the
12:08stroke volume and a rise in the ejection
12:10fraction. Lastly, let's talk about
12:13compliance. Compliance, it turns out, is
12:15the major determinant of the end
12:16diastolic pressure volume relationship.
12:19And it's very hard to talk about changes
12:21in compliance without also talking about
12:23changes in preload. One of the major
12:26determinants of preload is the
12:28compliance of the left ventricle. In
12:30order to fill the ventricle normally,
12:32you need a flexible stretchy myioardium.
12:34If the mocardium is thick and stiff,
12:37then that will decrease the ability for
12:39the ventricle to fill and that will
12:40lower the preload. So what I've shown on
12:43the screen here is what happens when
12:44there is decreased compliance. And this
12:46is the most important change in
12:47compliance that you should understand.
12:49When there's decreased compliance, the
12:51end diastolic pressure volume
12:53relationship shifts up and to the left
12:55as I've shown in going from the dashed
12:57blue line to the dashed red line. This
12:59has two important effects on the
13:01pressure volume loop. First of all, the
13:03end diastolic volume which is here in
13:05the blue curve falls slightly to be here
13:07in the red curve. That's because the
13:09ventricle is stiff and it cannot fill as
13:11easily. In addition, the end diastolic
13:14pressure which is here in the blue curve
13:15is much higher in the red curve. That's
13:17because the ventricle is stiffer. So
13:19those molecules are packed into a
13:21stiffer chamber and therefore the
13:23pressure in the ventricle at the end of
13:24diastly goes up. So I've highlighted
13:26these two important changes here that
13:28you should know. When compliance
13:29decreases, the end diastolic volume
13:31falls because of impaired ventricular
13:33filling and the endiastolic pressure
13:35rises because of a stiffer ventricle.
13:37And if compliance were to increase, you
13:39would see the opposite of all these
13:40changes.
13:42Let me make a comment about the work of
13:44the heart. The area of a pressure volume
13:46loop represents the work of the heart.
13:48The area represents how much volume it's
13:51pumping times how much pressure it's
13:52generating. And that determines how much
13:54work the heart is doing. So, if you look
13:56at the two pressure volume loops on the
13:58screen, you should be able to tell that
13:59the blue one is doing more work than the
14:01red one. The red one has much less area
14:03and therefore that ventricle is doing
14:05less work. For the last few slides, I'll
14:08show you some commonly tested pressure
14:10volume loops. These are loops that tend
14:11to come up on your exams. We'll go
14:13through all of these conditions shown on
14:14the screen here and talk about what they
14:16do to the pressure volume loop. So, the
14:18first one is aortic stenosis. Aortic
14:20stenosis raises the afterload. So
14:22therefore it moves us from the blue
14:24pressure volume loop whose afterload is
14:26right here to a higher afterload in the
14:28red pressure volume loop. Because after
14:30load is higher less blood can be ejected
14:32from the left ventricle and therefore we
14:34have a lower stroke volume and that is
14:35also what is seen in aortic stenosis.
14:38Mitro regurgitation shifts from the
14:40normal blue pressure volume loop shown
14:42on the screen to the red one. It should
14:44be easy to note with your eye that this
14:46line here is crooked and slanted to the
14:48left compared to normal. The reason for
14:50that is because isovalic contraction is
14:52disrupted. This line is supposed to be
14:55straight up and down meaning that volume
14:56is constant. However, in mitro
14:58regurgitation, blood is leaking out of
15:00the ventricle into the atrium.
15:02Therefore, during isolic contraction,
15:04the volume is falling, which should not
15:06be happening. So, it's very easy to
15:08identify the mitro regurgitation
15:09pressure volume loop because this line
15:11here is not straight. In contrast, in
15:13aortic regurgitation, the major finding
15:15in a PV loop is that isovalic relaxation
15:18is disrupted. If you look on the screen
15:20here, the blue loop is a normal PV loop.
15:22And isovalic relaxation is a straight
15:25line. That means it's isovalumic. In the
15:27red loop, which represents a case of
15:29aortic regurgitation, isovalic
15:31relaxation is no longer a straight line.
15:33It's curved. In other words, it's not
15:34isovalic anymore. And the reason it is
15:37not isovalic is because blood is filling
15:39the ventricle during relaxation. And
15:41that's because blood is leaking across
15:42the aortic valve and into the left
15:44ventricular chamber. And then finally,
15:46mitro stenosis produces the red pressure
15:49volume loop shown on the screen here.
15:51It's very similar to the blue one. It's
15:53just a little bit smaller, and that's
15:54because the ventricle can't fill
15:56properly. There's an obstruction to
15:58inflow from a stenotic mitro valve. So,
16:00this slightly decreases the stroke
16:02volume and makes a pressure volume loop
16:04that is just slightly smaller compared
16:05to normal. And that concludes our video
16:08on pressure volume loops.