Changes in pressure-volume loops

Pressure- volume loops are graphs, where the pressure inside the left ventricle is on the y axis and the volume of the left ventricle is on the x axis.

Each loop represents one cardiac cycle, including both ventricular systole and diastole, or more simply, one heartbeat.The lower right hand corner is the end-diastolic point, and it’s the point in the cardiac cycle when diastole is over.

Αt this point, the mitral valve is closed and the left ventricle is filled with the maximum volume of blood, known as end-diastolic volume.

After that, the left ventricle contracts, and systole begins. This makes the pressure shoot up, but since both mitral and aortic valves are closed, the left ventricular volume doesn’t change.

This phase is isovolumetric contraction. Eventually the pressure inside the left ventricle exceeds aortic pressure, forcing the aortic valve to pop open, and that starts the ejection phase.

Blood leaves the left ventricle and goes into the aorta, decreasing left ventricular volume. The left ventricle continues to contract, so ventricular pressure keeps rises further, but then falls slightly and finally the aortic valve shuts when aortic pressure exceeds the left ventricular pressure.

That point, called the end-systolic point, marks the end of systole. At this point, left ventricular pressure is called end-systolic pressure, and left ventricular volume is called end-systolic volume.

The difference between end-diastolic volume and end-systolic volume, is the stroke volume. After that, the left ventricular muscle starts relaxing, so left ventricular pressure falls.

However, all valves are closed, so the volume remains constant. This phase is isovolumetric relaxation.

Eventually, the pressure drops below left atrial pressure, and that allows the mitral valve to open and blood to flow from the left atrium flows into the left ventricle.

As the left ventricle fills with blood, its volume rises back to its end-diastolic volume, and the pressure increases only slightly.

This relaxation phase continues until the mitral valve closes, letting the loop start all over again. All this happens during one heartbeat.

With every heartbeat, or stroke, the heart is doing work. And that’s called “stroke work” and it’s proportional to the area inside the loop.

In other words, the bigger the loop and the more the area inside of it, the more stroke work our heart does.Sometimes, you’ll see this simplified to look more like a box.

The vertical dimension of the box is the distance from the x-axis to end-systolic pressure, and the horizontal dimension is the stroke volume.

And we can draw it over the pressure volume loop - so that it looks like this. In terms of stroke work, it would be the area inside the box.

The box isn’t exactly the same as the loop, but it’s a good approximation. Okay, now that we’ve seen what the normal pressure-volume loop looks like, let’s see how it varies if we change some parameters.

First, let’s say we increase the preload by adding more blood to the left ventricle during diastole. That means that during diastole, this bottom line goes further, and that increases the end-diastolic volume.

Because of the Frank-Starling relationship, there’s a strong contraction. The isovolumic contraction phase doesn’t look any different, but once blood gets ejected, it’s clear that there’s going to be a larger than normal stroke volume.

The end-systolic pressure and volume end up being about the same as what they were before. So an increased preload leads to a larger pressure-volume loop, and that means there’s increased stroke work.

Now, let’s say we increase afterload by increasing the blood pressure in the entire arterial system, including in the aorta itself.

That means that during isovolumic contraction, the ventricular pressure has to rise higher-than-usual before it exceeds aortic pressure and makes the aortic valve open.

During the ejection phase, relatively high-pressured blood leaves until aortic pressure exceeds left ventricular pressure.

Since there’s elevated blood pressure, the end-systolic pressure ends up being high as well, and that means that there’s a relatively small stroke volume, and that more blood is left within the left ventricle.

In this situation, there’s a trade-off, a smaller stroke volume, with a higher pressure - which means that the stroke work remains relatively stable.

Finally, let’s say that we increase contractility, by giving a medication to the heart that makes it contract more forcefully.

That means that during the ejection phase, blood is under higher pressure, and therefore, the ejection phase goes on for much longer, eventually ending when the left ventricular pressure equates the aortic pressure at the same end-systolic pressure, but now that point occurs with a much smaller left ventricular volume.

In this situation, there’s a big increase in stroke work, because there’s a larger stroke volume and even though the end-systolic pressure is unchanged, the blood is under higher pressure, so it increases the size of the pressure-volume loop.

#SUMMARYAll right, as a quick recap, pressure- volume loops can be used to visualize the effects of changing preload, afterload, and contractility.

An increase in preload and contractility, increases the size of the pressure-volume loop and therefore increases the stroke work.

An increase in afterload, increases the pressure, but decreases the stroke volume, and that means that the stroke work remains relatively stable.

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 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 is more blood left in the ventricle at the end of cyst In this situation there's a bit of a trade-off because 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 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 end 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 An increase in preload and contractility increases the size of the pressure volume loop And therefore it increases the stroke work and increase afterload though increases the pressure but decreases the stroke volume And