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Current time:0:00Total duration:10:28

Video transcript

one of the really nice things about preload and afterload is that the two have so much in common so if you're trying to figure out after load then remembering what preload is about is a really good idea because the definitions are so similar so we have volume and pressure on this graph I'm actually going to start by sketching out very quickly a pressure volume loop and you remember that to do that well you always have to kind of start with the two lines the end diastolic pressure volume relationship which is here which basically tells you how pressure and volume relate to each other when the heart is completely relaxed so this is the the line that would form if we were to fill up a relaxed left ventricle and then we have another line that goes something like this and this one is called the end systolic pressure volume relationship and this is when the heart is completely contracted right something like that so these are our two lines and now we have to just draw in our loop I'm going to draw a loop that starts here and goes down this is of course during diastole where the heart is filling up the left ventricle is filling up anyway and then of course there's contraction and finally blood is ejected out into the aorta so that's what the pressure-volume loop looks like right so that's how we start and let me set up the definition of afterload and we'll actually start by looking at this pressure volume loop and what part of it is afterload because i think sometimes it's easier to just to see it so afterload the definition of afterload is again it's very similar to the definition of preload it's left ventricular wall stress so so far it's identical to the preload definition and this time it's during so this is a key word it's not at any specific time it's actually during so it's over a certain time interval during ejection so ejection is when blood is actually being ejected out of the left ventricle so on our graph ejection would begin there and it would continue to about there so if I was to kind of drawing red which part of this is afterload this part of the curve is afterload so I'm going to just make it read this entire bit is considered afterload so that's interesting because before with preload we had a specific time point but now we have many many time points in fact in a way you could say it's an infinite number of time points right and all these combined make up what we define is afterload so I want to refresh your memory now on what wall stress is exactly so you might be thinking well I remember the term but but exactly what it is I don't remember so wall stress and I'm just going to write ej for ejection because you have to remember that after load happens gesturing that part of the pressure-volume loop just during that chunk of it is equal to pressure during ejection times the radius during ejection this is the radius of the left ventricle divided by two times the wall thickness during ejection and now if you wanted to say well could we actually figure out the value is there an actual number we could figure out well you can say all right well let's pretend for a moment that this is 120 and let's pretend that right there is about 75 so that would be that spot maybe right here so you could actually sit there and calculate it you could say well 120 times whatever the radius is remember there's a relationship between volume and radius the radius equals remember the cube root of the volume times a bunch of numbers and in fact it's actually that plus the wall thickness remember that so you could say well the radius equals all that so if you can actually figure it out you know these these letters if you can figure out the volume which I said was 75 and if you could figure out the wall thickness which in a person that's about 70 kilograms that's about my weight that would be around 1 centimeter let's assume so if you could make these assumptions you can actually make a number for R and if you have a number for P and R couldn't you just come up with some answer for what wall stress is at that point in to you I would say yes yes you could actually come up with a number at that purple arrow but and are you going to go ahead and calculate this one and this one and this one in this one and there's an infinite number right because you know you have to calculate all the time points between so are you really going to try to calculate all those time points and you could using a bit of fancy math but if you're just trying to eyeball it it would be actually kind of a tough thing to do right to calculate all that so how do people actually look at afterload if if I'm telling you that it's this equation and that it's actually during ejection during that whole time point not at any one specific time but during that entire time how do people calculate afterload well here's a dirty little secret people don't they don't calculate afterload not usually anyway I mean you could actually go through the math and calculate it and I guess if you're going to publish it maybe you would do that but people don't usually calculate it what they usually do is the following they'll say okay well this number right here this wall thickness well that's not really going to change that's going to be about the same so let's just kind of ignore that piece in this radius part well that's going to be some small number because remember it's a cube root and that's not going to be very big so at the end of the day all they really kind of look at is they're going to look at this they're going to say all right well let's just look at the pressure and we will assume and it's a pretty safe assumption I don't want to make it sound like that's a bad thing to do it's a pretty safe assumption that wall stress is proportional to pressure and if you assume that if you buy that that wall stress is proportional to pressure then of course you could say well in that case after load after load is proportional to pressure is proportional to pressure during ejection so something like that so let's go ahead and test this out let's see if you can if you buy this first of all and if you can apply this and see if you can find value in this kind of shortcut so I'm going to draw another pressure volume loop here just to test this out so let's say we have another one a very kind of tiny one over here and let's say this heart is going to contract right there and you're going to get something like that and if someone looked at these two loops and said to you hey tell me which one has higher afterload could you quickly just by eyeballing it answer that question I'm just going to highlight the after load on this loop which is right here it's this entire time span this part is the ejection part so could you look at it and identify which one the yellow loop or the purple loop has a higher after load and if you look at it you could probably say pretty quickly and confidently that well using this rule that after load is proportional to pressure if it's related then clearly this one has a lower after load and you would be right that's exactly right you know you didn't have to go through any fancy math or spend a lot of time on your calculator to get that answer you just kind of quickly eyeballed it and figured it out let me do one more just to make sure that we're all kind of on the same page let's say I do something like this and I'm going to draw this blue one we're going to make it kind of a mega a mega loop something like that and a high amount of pressure and now compare this one to the other to which one of these three then has the highest afterload and if you get the idea you would say very quickly well of course this blue one that I'm drawing has the highest afterload this one is obviously higher than the other two so that's how you figure it out you just basically kind of well that's how most people kind of figure out after load they say well let's just assume that pressure and after that are related or proportional to one another even though we know now technically the mathematical formula says that there's other variables we should look at like a radius and wall thickness but most people just kind of eyeball things and say well yep that's a higher afterload so now let me push you one step further and say okay if you if you think that you've you know kind of mastered this a little bit let me now build in an assumption let's assume I'm just going to write it very clearly because this is definitely not always true but assume that the aortic pressure a or t'k pressure is the same and let's say during ejection a rhotic pressure during ejection is the same as the left ventricular pressure during ejection so let's assume this is true what does that mean well if this is true and for many many people it is true right most people don't have any you know problem with their aortic valve or you know their their aortic valve is working normally I should say so their air or duct pressure is basically the same as their left ventricular pressure so for most people if this is true what does that mean for our pressure volume loop well what it means is that if I'm saying that you can just look at the pressure on that part of the curve to assume what afterload is well that pressure is something that we know more commonly we actually have another term for this what is the more common term well usually we call this systolic blood pressure right that's usually what we know it as and we usually call this diastolic blood pressure right these are the blood pressures that we generally record when you kind of check someone's arm for what their blood pressure is so you can actually get a good sense for afterload simply by looking at someone's blood pressure it gives you a lot of information it may not be exact because of course systolic and diastolic blood pressure usually checked where they're checked usually in your arm and they're not checked actually in the aorta itself but if we assume that there's a lot of similarity between those two spots and there might be then we can say well we can learn a lot about a or t'k pressure or sorry we can learn a lot about left ventricular pressure and therefore about afterload simply by look at your blood pressure and if your blood pressure goes up then there's a good chance your afterload is going up as well