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Drawing a pressure-volume loop

Use the left ventricular pressure and volume information to put together a cool new graph. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

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  • leaf green style avatar for user Maria Moores
    So how would these segments correlate the ekg waveforms, keeping in mind that the qrs (0.04s) is much shorter than the pr interval (0.12s)?
    (2 votes)
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    • leaf blue style avatar for user dysmnemonic
      Looking at the separate pressure and volume graphs, we can figure out how they correlate to electrical activity seen on ECG and to the heart sounds, bringing our whole cardiac cycle together.

      Let's start with the P wave. The P wave is atrial depolarisation, and it happens just before the little pressure bump at around 0.10 s. That pressure increase in the LV is coming from atrial contraction, stretching out the muscle fibres to increase contractility.

      The PR interval happens while the atria are contracting, and is the delay of the cardiac AP across the AV node. That leads us into our next big moment of electrical activity, which is the QRS complex.

      For pressure and volume, the main thing we're interested in from the QRS complex is the depolarisation of the ventricles. That depolarisation happens just before the ventricles contract. Ventricular contraction produces the big change in pressure, and the first heart sound happens at the start of the isovolumic contraction.

      We don't see any more electrical activity on the ECG until the T wave, because the depolarised ventricular cells are all at the same potential. The second heart sound happens at the beggining of the isolumic relaxation, and this is when the cardiac myocytes relax and repolarise, producing the T wave.
      (3 votes)
  • blobby green style avatar for user davidelha
    how come you don't get to see a rise in the volume when the left atrial contracts?
    (3 votes)
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    • piceratops ultimate style avatar for user ILoveToLearn
      The volume remains the same, but the blood is being expelled from the ventricle during contraction. That's why it's called isovolumetric contraction. Iso- means "same," -volumetric means "of volume," so the contractions have not changed the volume of blood. Volume of blood only changes in diastole when the atria are filling.
      (2 votes)
  • blobby green style avatar for user baig.fatima.1
    why is PV loop only depicting the left ventricle and not the right ventricle?
    (2 votes)
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Video transcript

So what do you see here? You see two figures, right? On the top figure, you've got pressure and time, and this is actually the pressure of the left ventricle over some period of time. In this case, I guess that's about 1.2 seconds. And on the bottom, you've got volume over time. And again, I've drawn it also over 1.2 seconds so you can kind of see exactly what's happening in the left ventricle as time passes. So let's actually do it step by step. We'll kind of go through this very, very carefully and figure out for any given point, like this point right here, what is the pressure and what is the volume. So here, the pressure looks like it's about, I would say, I don't know, around 10 millimeters of mercury. I'm just going to write a circled 10. And at that same point in time, if I was to kind of just go down that same point in time, I would find that the volume is about 125 millimeters, milliliters. So that's the volume, and that's the pressure at that particular time point, right. Now let's do another one. Let's do this one up here. This is about 80 millimeters of mercury, and if I was to drop that line down, that gets me to about here. And of course that is the same volume. Really nothing has changed in the volume even though the pressure has shot up. So those two are really easy, but let's do a slightly tougher one. Let's do something up here where the pressure is about 120. Pretty high pressure. In fact, it looks on this graph like the highest pressure it ever goes to. And dropping the line down, I'm going to get to, let's say, here. And I'm going to say that x is 75 milliliters. So at the peak pressure of 120, the volume is 75 milliliters. And I'm just going to keep going. This is, let's say about 100, pressure of 100. And you'll see how we can actually use this information to make another graph. So we're just going to kind of collect information from these two graphs using volume and pressure, and we're going to make another graph of our own. This I'm going to say it's about 20, and 20 drops down to here. And this looks like it's about 50 milliliters. Right? And you can kind of start getting the idea here. This is actually probably the lowest pressure. Here the pressure is about five, and dropping it down, it looks like it gets to about here. I'm just going to say 75 again just to kind of round off. And then here, you have kind of a slightly elevated pressure. This is about 12, I'm going to say. And then this last little bit right here is 10, back where we started. And if you drop those down, the pressure right before it kind of maxes out on the volume, let's say it's about 123, slightly less than completely full. And then that final x is going to be back at 125. Right? So this kind of how the volume changes over time and how the pressure changes over time. And actually I'm going to take now our two graphs, and I'm going to try to merge them together. I guess you can kind of think of it as a super graph. So this one is going to have volume down here, using the same units as before. We usually measure volume in milliliters. And we'll do 50 over here, and I'm going to estimate that's about 125 over there. And on this side we're going to do pressure. So pressure we measure usually in millimeters of mercury, although of course you could use pounds per square inch, or something else. But we're going to do millimeters of mercury because that's what we usually do. And I'm going to say this is, just kind of estimating, I'm going to say that's about 120. So these are the two new axes we're going to use, and we've got to pick some point to begin at. And I'm going to assume that the first point I started at, this one over here, is a good place to begin again. So we can start there. And the pressure there was about 10, and the volume was about 125. So I'm going to make a little red X there. That's our starting point on our new graph. And from there, it went up to a pressure of 80. Well, 80 is about over here, and the volume did not change. So I'm going to do a little red X there, and I'm going to connect the two lines like this, basically just kind of show the two connecting like that. And we know that over time, changes are happening, changes happening between the first and second X. And it takes a little bit of time, and how much time exactly I'm going to write out. Remember it was about 0.05 seconds. That's about how much time it took to go from the first spot to the second spot. And let's just keep track of that. So if you go forward in time you should go up on our graph, at least in the beginning. So what happens after that? Well, then you get to a point where the pressure is really high, 120, and the volume is about 75. So I'm going to draw a little red X there, somewhere there, right. And that's when the blood is actually leaving the left ventricle and entering the aorta. So this is going to be the next spot, right. And then the pressure falls again. It goes down to about 100. And the volume is still about 50. So volume has gone down. Pressure has gone down too. So it goes back down like that. So this yellow chunk, that I've divided into two just to kind of make sure I include the very, very high pressure of 120, but that entire yellow chunk takes time, of course. And how much time does it take? Let me mark it. So this whole bit takes some time. And we said that whole bit combined takes about 0.25 seconds. So about a quarter of a second to do that bit. So it's interesting, right, because the first bit happened really, really fast, 0.05 seconds, and the second bit takes five times as long, but on our graph it's not like you see a line that's five times longer, right? Because again, this is pressure and volume, and these graphs do not actually show you the time, which is why I have to separately show it to you, just to convince you that some segments do in fact take longer than others. Now the next part, the volume stays the same, but the pressure falls, and we said it goes down to a pressure of about 20. So let's say 20 is about there on my graph. So it's going to fall. And I'm going to try to keep the colors consistent. So we've got green now. This is when the left ventricle is relaxing. So the pressure falls at that point, right. And how long does that take? Well, if you remember this bit right here takes about 0.15 seconds. So, again, compare that to the contraction, the first part, which took only a third of the time, but the two lines in many ways look very similar, right? One's going up, one's going down, but the time is different. Now, let's continue and see how the pressure falls really low, right, about five. And we said the volume at that point is about 75, so the pressure falls down to somewhere like this. And then it starts to rise again. So let me show that. So you've got kind of a decline here in pressure, and then you've got a rise. So then it starts rising again. And remember, there's a point where it hits 12. So it's going to do something like that, rise, and then it's going to go up just slightly at the end and then go like that. So that would be our pressure-volume loop. Now, most times when you see it drawn, you'll see it drawn something like this, but almost never do you see this last little bump. So I'm just going to draw it, even though you know that it's there and I know that it's there, just to kind of show the way that most people draw it in books is like that. So that's what the pressure-volume loop looks like in most places. And actually, let me put the last little bit of time in there. So let's do this little bit right here. And this takes how long? This takes about 0.55 seconds. So the majority of the time is spent on that last bottom part of the loop. And so in sum, this is our pressure-volume loop.