Health and medicine
- What is preload?
- What is afterload?
- Increasing the heart's force of contraction
- Reimagine the pressure volume relationship
- What is contractility?
- Getting Ea (arterial elastance) from the PV loop
- Arterial elastance (Ea) and afterload
- Arterial elastance (Ea) and preload
- Stroke work in PV loops and boxes
- Contractility, Ea, and preload effects on PV boxes
- Pressure-Volume Boxes
Watch how PV loops can be "morphed" into PV boxes to make drawing them and thinking about them much simpler. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.
We've talked about a lot of ideas, specifically around pressure and volume. And what I wanted to do in this video is kind of tie them together and make sure that we see the big picture-- how everything relates to one another. So I'm going to set up our pressure and volume axis here. And we're going to draw out the lines that we've gotten so familiar with. The first one is the end-systolic pressure-volume relationship. This is the ESPVR line. And the next one is going to be the end-diastolic pressure-volume relationship. I'll just draw it kind of like that. And coming across these two is going to be the arterial elastance. I'll do it in a purple color. Something like this. And already, you can take a look at this and say, well, you know arterial elastance-- just to kind of refresh your memory-- is the end-systolic pressure over stroke volume. You can take a look at where it crosses the line. These two points-- this point right here where it crosses the ESPVR line and then the baseline where it hits the volume axis. And you could say, well, based on those two points, you can kind of make some pretty educated guesses on what the pressure-volume loop would look like. It's going to probably come down like that. It's going to follow the curve. And at this point, where it goes back up, this is ejection. And finally, at some point-- it's hard to say without having any kind of other information-- but at some point, it's going to start ejecting. And it's going to close out the loop like that. So you've got your PV loop. And actually, if I was to now take this PV loop, I could actually shade it in. And this whole bit would be the stroke work. Now, this is a new term. I haven't talked about stroke work before. But the heart is doing work with every heartbeat. You can also call that every "stroke." And so the area inside of this PV loop is sometimes called "stroke work." So a bigger loop with more area inside of it would mean that your heart is doing more work, and a smaller loop would mean the opposite. And I'm shading it in green just to make it very clear that this entire area is going to be your stroke work. And of course, stroke work is, therefore, related to the stroke volume and also the pressure at the end of systole. These loops, I've been drawing them as you see here, but sometimes you see PV loops-- instead of being drawn as an actual loop, you see them drawn as a box. And that's the next thing I want to allude to-- the fact that you can actually equate the two. You could say, well, a PV loop and a PV box are actually quite similar. They're not exactly the same. And I'm actually going to try to draw this one out to show you. But if you took a box-- let's say you start at this red point up here. I'm going to draw the box in red. The box would basically have a vertical line that is the distance of this. This is the pressure at end systole, of course. That would be the vertical distance. And it would have a horizontal distance that would be the stroke volume. That would be our box. Right? And I would draw it kind of coming straight across like that and coming straight down like this. You can see already that the PV loop and the PV box are pretty close. Right? They're not exactly the same, but they're pretty close. And I could actually sketch out which part of this would be the PV box. I'm going to color the PV box in red. And you can see part of this is actually excess. This corner, for example, this is in our PV box but did not quite make it into the PV loop. Right? So there is some little red bits that are extra. And you're actually going to have some red bits at the bottom as well that are extra. But the key idea here is that we use PV boxes to simplify how things look. You can draw a box much more easily than drawing the actual loop out. So a lot of times you'll see these kind of sketched out in books or in articles as well. So just keep in mind that, when people draw a PV box, they're basically just kind of saying that there's a PV loop that's very close in area. Now, let me show you the extra red bit down here. This is all extra red. And so you might be thinking, well, wait a second. The PV box, isn't it bigger than the PV loop because it includes area under the EDPVR line-- this yellow line? And in a way, it does have a little bit extra area here. That's true. But there is a little bit of extra green up top. In fact, this whole part did not get included in our PV box. Right? So there's a little extra green up top and a little extra red around the corners and on the bottom. So overall, most people kind of just assume that they're quite similar to each other. And especially, if you're thinking in terms of stroke work, that the area of the green loop and the area of the red box are going to be basically the same. So people kind of equate them to each other. But now you can see there is a ton of overlap, but they're not identical. The key idea here is that, if you're drawing a box and if we're seeing that the box is basically the same as the loop in terms of area anyway, and area tells us about stroke work-- then you can change the box size by really changing three things. OK. There's three parameters you can kind of change here. The first parameter is to swing in or swing out. This would be swinging out. I'll put a plus sign. Or you could swing in with a negative sign. Contractility. So that's the first way you can change the size of the box. So contractility would change the size of the box. Another way you could change the size of the box-- a second way-- would be to change the slope of the arterial elastance line. This would be a second way. And again, to go down-- let's say this might be negative, or you could think of this as positive. And this would be changing arterial elastance. So Ea is the second way. And a third way you could actually change the size of the box is if you could actually move the Ea line to the left or to the right. And if you can shift the entire line over-- not changing the slope, but shifting the line over-- we would call that "changing preload." So these are the three tricks we have to changing the size of our box-- contractility is one, changing arterial elastance is another, and preload is a third way. We're going to go through all three in another video. But I just wanted to point out that these are the three ways-- and really the only three ways-- that I want you to think about changing the size of this box and the two dimensions of it, which are end-systolic pressure and stroke volume.