Changes in pressure-volume loops

Definitions & Key takeaways

Pressure-volume loops are graphs used to study the effects of changing preload, afterload, and contractility of the heart. The pressure inside the left ventricle is plotted on the y-axis, whereas its volume is on the x-axis. A loop presents one cardiac cycle or one heartbeat comprising diastole and systole. When preload and contractility increase, it leads to an increase in the size of the pressure-volume loop. When the afterload increases, it leads to an increase in pressure and a decrease in stroke volume. This helps to keep the stroke work stable.

Chapters:

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Introduction0:00–0:22

If you've already watched the first video on pressure volume loops, then you'll know that the pressure inside the left ventricle is on the Y axis and then the volume of the left ventricle is on the X axis.
And each loop represents one cardiac cycle. So that's ventricular systole and diastole.
It's basically just a heartbeat. The bottom right hand corner of the graph is called the end diastolic point, and you guessed it, that's the point in the cardiac cycle when diastole is over.

Cardiac Cycle0:22–3:18

So at this point, the mitral valve is closed and the left ventricle is filled with the maximum amount of blood that can be inside it.
And that amount is known as the end diastolic volume. After that, the left ventricle contracts, and that's the beginning of systole.
When the ventricle contracts, it makes the pressure shoot up, but because the mitral and aortic valves are still closed, the left ventricular volume doesn't actually change.
So this phase is called isovolumetric contraction because iso means the same and volumetric refers to the volume. Eventually, after the pressure's been getting higher and higher, it ends up being higher than the pressure inside the aorta, so that forces the aortic valve to pop open, and that's the start of the ejection phase.
In the ejection phase, the blood is squirted out of the left ventricle into the aorta. And so as this happens, the volume in the left ventricle goes down.
So the left ventricle continues to contract and ventricular pressure keeps on getting higher and higher, but then it starts to fall a bit when the aortic pressure exceeds the pressure in the left ventricle.
At this point, the aortic valve shuts, and this is the end of systole, which is really imaginatively named end systolic point.
So to continue our theme of really imaginative names, the pressure in the left ventricle at this point is called the systolic pressure.
And the volume in the left ventricle at this point is called the systolic volume. Now I know what you're thinking, that's all too exciting, but if you think that's exciting, then get this.
So the difference between the end diastolic volume and the end systolic volume is called the stroke volume. After all of this, the left ventricular muscle starts to relax, and so the left ventricular pressure starts to fall.
But the aortic and mitral valves are still shut, so the volume remains the same. So you remember how when the ventricle was contracting, but the volume remained the same, we called that isovolumetric contraction.
Well now it's isovolumetric relaxation. After a while, the ventricular pressure drops so low that it's lower than the atrial pressure, so that causes the mitral valve to open and then blood rushes from the left atrium into the left ventricle.
As the ventricle fills up with blood, the volume rises back to what it was at the end diastolic point. But the pressure only increases a little bit.
This relaxation phase goes on until the mitral valve shuts, and then the loop can start all over again. All of this stuff happens during just one heartbeat, and with every heartbeat or stroke, the heart is doing work, and that is called stroke work.

Stroke Work3:18–4:03

And it's proportional to the area inside the loop. So in other words, the bigger the loop and the more air you're inside of it, the more stroke work is being done.
Sometimes this is shown to look like a box, which is basically just drawn over the top of the pressure volume loop, and it looks like this.
The vertical dimension of the box is the distance from the X axis to the N systolic pressure, and the horizontal dimension is the stroke volume.
So in terms of stroke work, it would be the area inside the box. And the box isn't exactly the same as the loop, but it's a good approximation.
OK, so now we know what a normal pressure volume loop looks like, but let's go ahead and change some stuff up and see what it looks like now.

Increasing Preload4:03–4:57

First of all, let's increase the preload by adding more blood to the left ventricle during diastole. So that means that during diastole, this bottom line goes further and that increases the end diastolic volume.
And then because of the Frank Starling relationship, there's a nice big strong contraction. And the isovolumetric contraction phase doesn't look any different, but once the blood gets ejected, it becomes pretty clear that there's going to be a larger than normal stroke volume.
And then the end systolic pressure and volume end up being about the same as what they were before. So overall, an increased preload leads to a larger pressure volume loop, and that means increased stroke work.

Increasing Afterload4:57–5:58

OK, now let's try out another change. Let's say we increase the afterload by increasing the BP in the entire arterial system, including the aorta itself.
That means that during isovolumetric contraction, the pressure in the left ventricle has to get higher than it was before because it needs to exceed the aortic pressure which is higher.
That's the only way we're gonna get the aortic valve to open. During the ejection phase, relatively high pressured blood leaves the ventricle until the aortic pressure is higher than the ventricle pressure.
And since there's elevated BP in the aorta, the end systolic pressure ends up being higher as well, and that means that there's a relatively small stroke volume.
It also means that there's more blood left in the ventricle at the end of systole. In this situation there's a bit of a trade off cos there's a smaller stroke volume, but there's higher pressure.
So it means that the stroke work works out about the same, remaining relatively stable. So we've tried out two changes.

Increasing Contractility5:58–6:47

Let's go for one last change and up that contractility by giving a medication that makes the heart contract more forcefully.
Increased contractility means that during the ejection phase, the blood is under higher pressure, and so the ejection phase goes on for much longer.
The ejection phase comes to an end when the left ventricular pressure is the same as the aortic pressure. So they have the same end systolic pressure, but now that point occurs with a much smaller left ventricular volume.
So in this situation of increased contractility, there's now a big increase in stroke volume, so there's a larger stroke work.
And even though the systolic pressure is unchanged, because the blood is under higher pressure, it means that the size of the pressure volume loop is bigger.
OK, so that was quite a lot of information to take in, but let's just quickly go over the main points. So pressure volume loops can be used to visualize the effects of a changing preload, afterload and contractility.

Review6:47–7:19

An increase in preload and contractility increases the size of the pressure volume loop and therefore it increases the stroke work.
An increased afterload, though, increases the pressure but decreases the stroke volume, and so that means that stroke work remains relatively stable.