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Current time:0:00Total duration:7:43

Video transcript

let's talk about the pressure-volume curve and we're actually going to use this to figure out how preload fits into this story so let's do volume going that way and pressure going up and I'm going to use our normal lines our end-systolic pressure-volume relationship something like this and I'll also use the same color to show the end diastolic pressure volume relationship and I wanted to keep an eye on this line because this is the one that's going to be of interest to us right now this is the end diastolic pressure volume relationship and I'm also going to sketch over all this the arterial elastance remember we talked about arterial elastance that was e with a little a and the formula was pressure at the end of systole divided by stroke volume and I'm going to quickly sketch out where those points would be this would be our end systolic pressure and our stroke volume would be basically if we drop this dot something like this right so this would be our stroke volume right here so these two numbers kind of help make our slope our arterial elastance and we can actually also use these to help sketch out our pressure volume loop so it basically would go down just like that and at some point kind of pick up our curve and we would pass over the purple line keep going and right at the same mark where the volume is we go up and we have contraction so the heart is contracting here and it continues to contracting and then finally blood gets ejected knocked out and that blood goes into the aorta so that's our pressure volume loop now I want to point out to you a couple things this one in particular this is our end diastole point remember when we talked about end diastole it it should remind you of preload in fact I'm going to jot the preload equation down here remember preload we said is this is from mr. Laplace this is wall stress at the end of diastole and I'm going to actually go ahead and write out the whole equation just to make sure that we're completely on the same page that we remember this equation this is pressure times the radius at the end of diastole divided by two times the wall thickness at the end of diastole so all of this stuff is happening at that point the yellow arrow point so here's the question what would happen if we actually increased the pressure or in other words at the end of diastole or in other words increase the preload so the pressure right now if I was to kind of figure out would be somewhere on there that's the pressure at the moment at the end of diastole but let's say I increased I wanted to increase it to let's say something like this and I can kind of sketch out that that would be let's say right here and immediately you can see what would happen you'd have an increase or it seems like you'd have an increase in stroke volume right because now you have extra blood entering so contraction would happen only at this point right here and I'm going to start contraction I'll finish off the loop in a moment but I want to actually jump back to our elastance equation I want to make sure we follow the math on this one because it's going to be helpful and make sure we don't get stuck in any kind of mental traps so remember that's the equation for elastance arterial elastance and that we also said previously that there's heart rate times resistance and that should all equal each other right that's that's how we wrote it out now I did not mention changing the heart rate and I didn't mention changing resistance I left all that the same and if all that's the same and now I'm increasing stroke volume I want you to take a guess as to what would happen to pressure now if it's going to have to remain the same the the overall equation has to be the same then you know the only way to do that is to also increase the pressure right there's no other way to do that and that's the the math that's the beauty of the the numbers then it helps us kind of figure out what's going on so if the pressure is going to go up then I can actually draw that on my equation and I can say well my equation is going to have maybe a point right here where the new pressure is so this is my new pressure at the end of systole so let me go over that one more time why I knew that had to happen I first said my systolic or my stroke volume was going up and so if my stroke volume goes up and I don't change the heart rate or the resistance then I know that the pressure the end of systole has to go up as well so that's why it mathematically makes sense but I also want to make sure to intuitively make sense in other words if you put more blood in the aorta then it makes sense that the pressure at the end of systole which is the same as the pressure in the aorta at that point would be higher so more blood in the aorta will create more pressure and that makes sense right because you have more molecules kind of bouncing around more blood molecules and so of course pressure would go up so this is really interesting because I know there's a temptation to say well if you have an increase like this and this is what I didn't finish drawing it there's a temptation to kind of say well then that just means that your stroke volume is going to go up in the end of the story in fact I see it drawn like this quite a lot and it's really confusing and to put it bluntly it's actually wrong it should not be drawn like this the truth is that the stroke volume goes up and it also increases the end systolic pressure so the right way to draw this is to say okay well it goes up up up up up and then you have ejection and something like that and then of course it goes down from that point goes down from that point and it meets up like that now you probably notice something right away which is that if I'm saying stroke volume is going up because of all this then isn't stroke volume going down because you've lost a little bit of stroke volume on this side and the answer is yes yes you do have a little bit less stroke going because of this loss over here but because you have so much gain on this side you actually make up for it so overall stroke volume still goes up so the old stroke volume which is right here is still less than the new stroke volume which is right here so that's something you should definitely keep in mind and you also have a couple other changes which is that you have an increase in the after load right here so after load is actually going up as well and so here I want to show you that if you have a change that's driven by just preload right if that's the main change that the loop entirely changes but the slope of the line this purple line that we've been drawing actually stays about the same and so I'm just going to try to make sure I draw it about the same and it looks like that so this is what the new line would look like for preload and now you can actually get an appreciation for the fact that if you increase preload you're basically shifting the line this way and if you decrease preload I could show you the exact opposite and it would go the other way so this is what increasing and decreasing preload does to our line it does not change the slope of the line so the elastance or the slope stays the same but where the line is located on the volume axis actually shifts over