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Current time:0:00Total duration:6:53

Video transcript

we've talked about a lot of ideas specifically on pressure and volume and what I wanted to do in this video is kind of tie them together and make sure that we see kind of 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 the lines that we've gotten so familiar with the first one is the end systolic pressure volume relationship this is the es PVR 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 in a purple color something like this and already you can kind of take a look at this and say well you know Arturo license just to kind of refresh your memory is the end systolic pressure over stroke volume so you can take a look at where it crosses the line these two points this point right here when it crosses the ESP VR line and then the baseline where 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 and 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 get your PV loop and actually if I was to know take this PV loop I could actually kind of shade it in and this whole bit would be the stroke work now this is kind of a new term I haven't talked about stroke worked before but the heart is doing work with every heartbeat and you can also call that every stroke and so what is 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 and sometimes you know these loops I've been drawing them kind of as 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 kind of allude to is the fact that you can actually kind of 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 kind of draw this one out to show you but if you took a box let's say you start at this kind of red point up here I'm going to draw the box and read the box would basically have a vertical line that is the distance of this this is the pressure and 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 and so 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 you know 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 is this corner for example this is in our PV box but did not quite make it into the PV loop right so there's 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 kind of 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 and 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 doesn't the PV box then isn't it bigger than the PV loop because it includes area under that yes I'm starting the IDI PVR 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 so the key idea here is that if you're drawing a box and if we're saying 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 okay there's three parameters you can kind of change here the first parameter is to swing in or swing out this would be kind of 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 is positive and this would be changing arterial elastance so EA is the second way and the 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 kind of 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 and 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