PV Loops Explained: Contractility, Ea, and Preload Effects

[00:00:02]We've worked really, really hard to understand PV loops.

[00:00:05]Now I want to show you how PV loops-- and more specifically, PV boxes-- can be helpful in understanding what's going on in our heart.

[00:00:13]These are going to be three diagrams.

[00:00:16]We're going to write out three diagrams.

[00:00:17]And in all three, we're going to see how PV boxes change shape if we tweak one of three things.

[00:00:25]So the first thing we can tweak is contractility.

[00:00:30]The second thing is arterial elastance-- or sometimes we just call that "Ea."

[00:00:37]And the third thing we can tweak to change our PV box is preload.

[00:00:41]So these are going to be the three ways that we can actually change how the box looks.

[00:00:46]And I want to actually walk you through exactly will happen if we change it.

[00:00:50]So let's do contractility first.

[00:00:52]Let's see how contractility can change our PV box.

[00:00:55]And to start out, I actually kind of want to show you how I think about these things.

[00:01:00]I'm drawing a little cement block here.

[00:01:03]And this cement block is to remind me that the ESPVR line-- remember, this end-systolic pressure-volume relationship-- is going to be fixed in terms of the volume at which it hits the bottom.

[00:01:18]And the reason for that is that you know that there's a certain minimum volume that you need to be able to get pressure in your left ventricle.

[00:01:27]And that isn't going to change.

[00:01:28]So I think of that line as being fixed.

[00:01:31]And then, there's a second line.

[00:01:34]Let's say I draw it out right here.

[00:01:36]And this is my arterial elastance line.

[00:01:39]And at the bottom of that line-- instead of just letting it hit the baseline-- I'm actually going to show you how I think about it.

[00:01:45]I think about it as kind of having a wheel.

[00:01:47]A little wheel.

[00:01:48]And the reason for that drawing a wheel is to show you that, if I wanted to move it, I could.

[00:01:56]In this particular case, we're going to leave the wheel alone.

[00:01:59]We're not going to move it.

[00:02:00]And we're only going to change contractility.

[00:02:04]Just try to think about what change in contractility means exactly.

[00:02:08]Well, what that's going to do is that's going to pivot-- I'm going to write it down here-- pivot the ESPVR line.

[00:02:16]It's going to cause changes to the ESPVR line.

[00:02:21]Let's now draw that out.

[00:02:22]Let's say we actually increase-- so I'll do increase first.

[00:02:25]Well, actually, maybe, even before doing that, let's actually draw what the pressure volume loop looks like to start out with.

[00:02:31]I just have to take the two corners.

[00:02:33]These will be the two corners.

[00:02:34]And I draw a box that connects the two corners.

[00:02:37]Right?

[00:02:37]These are the two corners of my box.

[00:02:40]It actually looks more like a rectangle than a box, but that's OK.

[00:02:44]We call them "boxes" even though they're sometimes rectangles.

[00:02:46]And the height of the rectangle is the end-systolic pressure.

[00:02:56]The width of my box is stroke volume.

[00:03:00]So stroke volume is kind of how wide it is, and end-systolic pressure is how tall it is.

[00:03:05]So what's going to happen if I actually now increase-- let's start with increase-- my contractility?

[00:03:10]Well, increasing contractility means that I pivot that way.

[00:03:13]And I'm going to write a little plus sign to mean increase.

[00:03:16]And I have to start at that cinder block because I said it's always fixed.

[00:03:19]It's not going to move.

[00:03:21]And I just kind of draw it.

[00:03:22]Like that.

[00:03:22]So this is my new line.

[00:03:25]And to draw the box, I just have to say, well, where does it cross the Ea line-- the arterial elastance line?

[00:03:32]It crosses at the blue dot.

[00:03:34]The other corner is going to have the same point as my wheel.

[00:03:38]Right?

[00:03:39]I just have to draw a vertical line down and a horizontal line across, and I've got my box.

[00:03:47]There's my box.

[00:03:48]And it's a little bit bigger than my green box.

[00:03:51]And it's increased both the width and how tall it is.

[00:03:55]Right?

[00:03:56]So now it's taller and wider.

[00:03:58]And that means that my stroke volume went up because that's my wideness.

[00:04:03]That went up.

[00:04:04]And also, my end-systolic pressure went up.

[00:04:06]Right?

[00:04:07]So this length is higher.

[00:04:09]Both the end-systolic pressure and the stroke volume went up just by increasing the contractility.

[00:04:15]That's good and easy to remember.

[00:04:16]Just imagine that line pivoting.

[00:04:19]And if I wanted to know what happens if I decrease contractility, I could just draw a third line.

[00:04:24]This would be a decrease in contractility because it's pivoting down.

[00:04:29]And now, I draw a new dot where the red line and the purple line cross.

[00:04:33]And I draw a box from there-- just as before-- to my wheel, which is right there.

[00:04:40]And I say, well, wow.

[00:04:42]Now my box is smaller.

[00:04:43]So the amount of stroke work or the area in my box went down, and both dimensions of my box went down.

[00:04:49]Again, the end-systolic pressure-- I'm not going to write it out, but-- well, I guess I could write it out over here.

[00:04:54]This is now smaller than the green value.

[00:04:59]And my stroke volume is actually smaller.

[00:05:01]This is smaller than it was as well.

[00:05:03]So both the stroke volume and the end-systolic pressure went down.

[00:05:07]These are the changes you see with changes in contractility.

[00:05:10]And remember, it all goes back to pivoting the ESPVR line.

[00:05:16]By increasing or decreasing contractility, what you're doing is increasing or decreasing the size of the box.

[00:05:23]Let's move on to arterial elastance.

[00:05:25]I think the first example is pretty easy.

[00:05:27]I think you've got it.

[00:05:28]Let's move on to the second example, and I'm going to draw it kind of the same way with a little cinder block here and a line coming off of it.

[00:05:35]Let's say the line is something like that.

[00:05:37]I'm trying to draw it very similar to the first time but probably not identical, I guess.

[00:05:43]I'll draw a purple Ea line coming like this with the little wheel at the bottom.

[00:05:49]And again, I'm not going to move the wheel, but I want you to always remember that it could be moved if I wanted to.

[00:05:54]But in this case, we're not going to move it.

[00:05:56]What we're going to do instead is we're going to change arterial elastance.

[00:06:01]And what that does is it pivots-- so just as the other one pivoted, this one also pivots-- this one pivots-- I should probably write an "s," "pivots"-- the arterial elastance line.

[00:06:13]So now, instead of pivoting the yellow line, we're going to pivot the purple line.

[00:06:18]Let me start by drawing my first box, kind of our standard box, just as a point of reference.

[00:06:22]Right?

[00:06:22]You need that just to see how things change.

[00:06:24]You've got to know how things started.

[00:06:26]So this is my initial PV box.

[00:06:29]And if I was to pivot things-- let's say I now moved the arterial elastance up.

[00:06:35]The two ways I could do this, remember, are to increase the heart rate or increase the resistance.

[00:06:40]Now, if I moved it up like that, then my new line would look something like this.

[00:06:45]Let's say that would be my new line.

[00:06:48]Remember, there's still a wheel down here.

[00:06:51]So that's my new line.

[00:06:52]And what would my new box look like?

[00:06:55]The blue line and the yellow line cross right there.

[00:06:57]So I just have to draw my box using that as my example, and I could do this.

[00:07:04]This is my new box.

[00:07:06]And now, if I was to shade the new area, this is kind of the increase area.

[00:07:13]But there's also kind of a decrease.

[00:07:15]Right?

[00:07:16]I also lost a little bit of stroke volume on this side.

[00:07:18]So I just want to point that out to you.

[00:07:21]So you do lose some stroke volume, but you gain some end-systolic pressure.

[00:07:27]So if I was to write the new variables in, you can see the stroke volume's a little tinier than it used to be, but the end-systolic pressure has gone up.

[00:07:37]So this is different than the first example.

[00:07:39]In the first example, the entire box basically just got bigger or smaller as contractility went up or down.

[00:07:45]But now, you're seeing that one dimension goes up-- in this case, the pressure went up-- but the other dimension goes down.

[00:07:52]That was the stroke volume.

[00:07:53]And if I was to actually shift it the other way-- let's say I kind of shifted things this way-- now my line pivots that way, and the arterial elastance is lower.

[00:08:04]Now my new box is actually going to do the opposite.

[00:08:09]The stroke volume actually increases.

[00:08:11]And I could finish off this box like that and like that.

[00:08:16]Now my stroke volume increases because look at this big stroke volume over here.

[00:08:21]Right?

[00:08:21]This is a bigger stroke volume than it ever used to be, but the end-systolic pressure went down.

[00:08:28]This is actually lower than the original green box or the blue box.

[00:08:33]Here, it's actually a little different.

[00:08:35]When you pivot the arterial elastance line-- which is what we're doing-- you can actually see that now you're kind of trading off.

[00:08:42]On the one hand, you can increase pressure if you are willing to get a smaller stroke volume.

[00:08:48]Or you can do the opposite.

[00:08:50]You can actually decrease pressure and get a larger stroke volume.

[00:08:54]So this is actually kind of different than what was happening in the first example.

[00:08:58]Now, let's move to our third example and see how the pressure volume box will change with preload.

[00:09:05]So here I'm going to start out the same way as before, kind of drawing my cinder block just to kind of remind myself that this ESPVR line never really shifts.

[00:09:14]It always stays at one point, even though we know it can pivot.

[00:09:19]It doesn't roll.

[00:09:22]I'm going to finally give you an example of what rolling would do.

[00:09:25]So let's say you have our Ea line with a little wheel here.

[00:09:29]What preload does is it rolls.

[00:09:34]Actually, let me change the color there because that's kind of a weird color.

[00:09:39]Let's do that.

[00:09:41]It rolls the Ea line.

[00:09:43]So that's different than what was happening with the pivoting of the Ea line.

[00:09:48]So when I roll it, what I mean-- I'll show you in just a second-- it's actually going to move the entire line.

[00:09:53]So this is my initial PV box.

[00:09:57]Right?

[00:09:57]Something like this.

[00:09:59]And if I now decide I want to increase-- let's start with increase my preload-- then I would basically kind of move things this way.

[00:10:07]You want to draw a plus sign here.

[00:10:09]Now, my new line.

[00:10:11]Let's say it's over here.

[00:10:12]And I'm going to have to draw this as best I can to make sure that I maintain the exact same slope because the slope does not change.

[00:10:20]And that's my new line.

[00:10:22]And actually, I should probably-- well, yeah.

[00:10:25]Maybe I should do that in a different color just to make it very, very clear because I don't like having two purple lines.

[00:10:31]It looks too similar.

[00:10:33]So this would be my new blue line.

[00:10:37]And I can also do the opposite and move it in.

[00:10:41]And actually, that would be a decrease in preload, and that would actually look like this.

[00:10:45]So you can see that, basically, what happens is that the line shifts.

[00:10:50]The entire line shifts, but the slope stays the same.

[00:10:54]So that's what I mean when I say "rolling" the line.

[00:10:57]Actually, let's draw out our box to see what that looks like.

[00:11:01]So if I increase preload, then my new box basically gets much bigger.

[00:11:08]So my new box is a larger area than my green box.

[00:11:11]Right?

[00:11:11]You can see it's much, much larger.

[00:11:14]Even though there's a little sliver of green on the left side, I'm going to show you that we have to at least identify.

[00:11:22]You have this bit right here that you lost.

[00:11:24]Right?

[00:11:25]This little bit.

[00:11:26]But overall, you gained much more than you lost.

[00:11:29]So the blue box is definitely bigger.

[00:11:31]And in fact, it's not only bigger in stroke volume-- this is definitely larger-- but it's also a bigger in end-systolic pressure.

[00:11:38]They both went up.

[00:11:40]A larger preload-- it's not like it would happen forever, but within certain limits-- it basically increases your stroke volume and increases your pressure.

[00:11:50]And the opposite is true, too.

[00:11:52]So if you decrease your preload, you get a tinier box.

[00:11:56]So basically, this is my small box here.

[00:12:00]And this box I'm going to kind of draw in for you.

[00:12:08]Maybe I'll just color the whole thing in just so you can kind of see the area of the small box.

[00:12:12]This red box is obviously much smaller than the green box used to be.

[00:12:16]So stroke volume has gone down here.

[00:12:18]It's gotten smaller.

[00:12:19]And end-systolic pressure has also gone down.

[00:12:23]What happens with preload is actually, in some ways, kind of similar to what happened with contractility.

[00:12:28]Basically, as you go down in preload, the entire box gets smaller, and both dimensions of the box get smaller.

[00:12:36]And as preload goes up, the opposite happens-- both dimensions, stroke volume and end-systolic pressure, go up.

[00:12:42]And the entire box, therefore, goes up.

[00:12:44]And this is quite different than what happened in the middle example with arterial elastance, where it was more of a trade-off between the two.

[00:12:52]Right?

[00:12:52]In one case, you had a higher stroke volume.

[00:12:54]In the other case, you had a higher end-systolic pressure.

[00:12:57]So this is how you can kind of put it together.

[00:12:59]Just kind of think about two lines.

[00:13:01]It's actually quite simple.

[00:13:03]One of them is fixed.

[00:13:04]The other rolls.

[00:13:05]And then, you can very easily draw out your box.

[00:13:07]And then you can just see what the differences would be.