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Current time:0:00Total duration:13:11

Contractility, Ea, and preload effects on PV boxes

Video transcript

we work really really hard to understand PV loops and now I want to kind of show you how PV loops and more specifically PV boxes can be helpful in understanding what's going on in our hearts so these are going to be three diagrams we're going to write out three diagrams and in all three we're going to kind of see how PV boxes change shape if we tweak one of three things so the first thing we can tweak is contractility and the second thing is our turtle elastance or in sometimes we just call that EA and the third thing we can tweak to change our PV box is preload so these are going to be the three ways that we can actually change how the box looks and I want to actually walk you through exactly what will happen if we change it so let's do contractility first let's see how contractility can change our PV box and to start out at you kind of want to show you how I think about these things and I'm drawing a little cement block here and this cement block is to remind me that the ESP VR line remember this end-systolic pressure-volume relationship is going to be fixed in terms of the the volume at which it hits on the bottom 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 and that isn't going to change so I think of that line is being fixed and then there's a second line and this little let's draw out right here and this is my arterial elastance line and at the bottom of that line instead of kind of just letting it hit the baseline I'm actually going to show you how I think about it I think about it as kind of having a wheel a little wheel and the reason for that reason for kind of drawing a wheel is to kind of show you that if I wanted to move it I could so in this particular case we're going to leave the wheel alone when I can move it and we're only going to change contractility and just try to think about what contractility what changing contractility means exactly well what that's going to do is that's going to pivot I'm going to write it down here pivot the ESP VR line so it's going to cause changes to the ESP VR line so let's now draw that out so let's say we actually increase I'll do increase first well actually maybe even before doing that let's actually draw what the pressure-volume loop looks like to start out with so I just have to take the two corners these will be the two corners and I draw a box that connects the two corners right these are the two corners of my box actually looks more like a rectangle than a box so that's okay we call it boxes even though there are sometimes rectangles and the height of the rectangle is the end systolic pressure right this height is end systolic pressure and the width of my box is stroke volume so stroke volume is kind of how wide it is and end systolic pressure is how tall it is so what's going to happen if I actually now increase let's start with increase my contractility while increasing contractility means that i pivot that way and I'm going to write a little plus sign to mean increase and I have to start at that cinder block because I said it's always fixed it's not going to move and I just kind of draw it like that so this is my new line and to draw the box I just have to say well weight is across the EI line there are chiral elastance line it crosses that the blue dot and the other corner is going to have the same point as my my wheel right and so I just have to kind of draw a vertical line down and a horizontal line across and I've got my box there's my box and it's a little bit bigger than my green box and it's increased both the width and how tall it is right so now it's taller and wider and that means that my stroke volume went up because that's my wideness that went up and also my end systolic pressure went up right so this length is higher so both the end systolic pressure and the stroke volume went up just by increasing the contractility so that's good and easy to remember just imagine that line kind of pivoting and if I wanted to know what happens if I decrease contractility I could just kind of draw a third line and this would be a decrease in contractility right because it's pivoting down and now I draw a new dot where the red line and the purple line cross and I draw a box from there just as before to my wheel which is right there and I say well Wow now my box is smaller so the amount of stroke work or the area in my box went down and both dimensions in my box went down right again the end systolic pressure I'm not going to write it out but well I guess I could write it out over here this is now smaller then the green value and my stroke volume is actually smaller right this is smaller than it was as well so both the stroke volume and the an systolic pressure went down so these are the changes you see with changes in contractility and remember it all goes back to pivoting the ESP VR line and so by increasing or decreasing contractility what you're doing is increasing or decreasing the size of the box let's move on to our chiral elastance I think that one the first example is pretty easy I think you got it 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 let's say the line is something like that I'm trying to draw it very similar to the first line but probably not identical I guess and I'll draw a purple ei line coming like this with a little wheel at the bottom 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 but in this case we're not going to move it what we're going to do instead is we're going to change arterial elastance 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 so now instead of pivoting the yellow line we're going to pivot the purple line so let me start by drawing my first box kind of our standard box just as a point of reference right you need that just to see how things change you got to know how things started so this is my initial PV box and if I was to pivot things let's say I now moved the arterial elastance up so the two ways I could do this remember are to increase the heart rate or increase the resistance now if I moved it up like then my new line would look something like this let's say that would be my new line remember there's still a wheel down here so that's my new line and what would my new box look like well it crosses the blue line in the yellow line cross right there so I just have to draw my box using that as my example and I could do this this is my new box and now if I was to shade the new area this is kind of the increase area but there's also kind of a decrease right I also lost a little bit of stroke volume on this side so I just want to kind of point that out to you so you do lose some stroke volume but you gain some end systolic pressure so if I was to write the new variables in you can see the stroke volume is a little tinier than it used to be but the end systolic pressure has gone up so this is different than the first example in the first example the entire box basically just got bigger or smaller as contractility went up or down but now you're seeing that one dimension goes up in this case the pressure went up but the other dimension goes down that was the stroke volume 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 Archer elastance is lower now my new box is actually going to do the opposite now the stroke volume actually increases and I could finish off this box like that like that now my stroke volume increases because look at this big stroke volume over here right this is a bigger stroke volume than it ever used to be but the pressure and systolic pressure went down this is actually lower than the original green box or the blue box so here it's actually a little different 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 on the one hand you can increase pressure if you are willing to get a smaller stroke volume or you can do the opposite you can actually decrease pressure and get a larger stroke volume so this is actually kind of different than what was happening in the first example now let's move to our third example and see how the pressure-volume box will change with preload so here I'm going to start out the same way as before kind of drawing my cinderblock just to kind of remind myself that this ESP VR line never really shifts it always kind of stays at one point even though we know it can pivot it doesn't roll and now I'm going to finally can be an example of what rolling would do so let's say you have our EI line with a little wheel here what preload does is it rolls it rolls actually let me change the color there because that's kind of a weird color let's go to there let's do that it rolls the EI line so that's different than what was happening with the pivoting of the EI line so when I roll it what I mean I'll show you just a second it's actually going to move the entire line so this is my initial TV box right something like this and if I now decide I want to increase let start with increase my preload then I would basically kind of move things this way I'm going to draw a plus sign here and now my new line let's say is over here 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 and that's my new line and actually I should probably well yeah maybe I should do that at a different color just to make it very very clear because I don't like having two purple lines it looks too similar so this would be my new blue line and I can also do the opposite and move it in and actually that would be a decrease in preload and that would actually look like this so you can see that basically what happens is that the line shifts the entire line shifts but the slope stays the same so that's what I mean when I say rolling the line and actually let's draw out our box to see what that looks like so if I increase preload then my new box basically gets much bigger so my new box is a larger area than my greenboxe right you can see it's much much larger 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 you have this bit right here that you lost right this little bit but overall you gained much more than you lost so the blue box is definitely bigger and in fact it's not only bigger in stroke volume this is definitely larger but it's also bigger in in systolic pressure they both went up and so a larger preload it within certain limits is not not like it would happen forever but within certain limits it basically increases your stroke volume and increases your pressure and the opposite is true too so if you decrease your preload you get a a tinier box you get a tinier box so basically this is my small box here and this box I'm going to kind of draw in for you has this maybe I'll just color the whole thing in just so you can kind of see the area of this small box this red box is obviously much smaller than green box used to be so stroke volume has gone down here it's gotten smaller and end systolic pressure has also gone down so what happens with preload is actually in some ways kind of similar to what happens contractility basically as you go down and preload the entire box gets smaller and both dimensions of the Box get smaller and as preload goes up the opposite happens both dimensions stroke volume and and systolic pressure go up and the entire box therefore goes up and this is quite different than what happened in the middle example with our chiral elastance where it was more of a trade-off between the two right in one case you had a higher stroke volume in the other case you're a higher end systolic pressure so this is how you can kind of put it together just kind of think about two lines it's actually quite simple just two lines one of them is fixed the other roles and then you can very easily kind of draw out your box and then you can just see what the differences would be