in this video i'm going to finally teach you about pressure volume loops now i say finally teach you because i'm operating under the assumption that you're really overwhelmed by this overly complicated topic and if you've seen pressure volume loops before they do have a lot of information on them and they are a little bit complex but part of the ambiguity related to this topic stems from the fact that this gets taught so poorly in medical school i don't feel as though this topic is adequately taught so my goal today is to re-teach this to you and oversimplify things as if i'm talking to a fifth grader and i'm doing that not because i don't think you're intelligent enough to handle the topic i'm doing that because if you can understand this like a fifth grader would you're gonna get all the free points that you need on test day so let's take this topic which is not so scary and boil it down to something that's easily understandable so when it comes to pressure volume loops obviously the place to begin is to talk about where they get their name and shape from and so the reason that they're called pressure volume loops is because we graph them and the y-axis always has pressure and the x-axis always has volume right pressure volume loops pressure over volume now this is a graph and on this graph we have a tendency to look at the left ventricle so it's left ventricular pressure and left ventricular volume so when you hear somebody say pressure volume loop what they're actually saying without really saying it is left ventricular pressure volume loop because when we look at these loops we're doing that within the left ventricle specifically now the shape of the pressure volume loop looks like this and where that comes from doesn't really matter what matters is that you understand the shape now some people think that this looks like a piece of bread and i can see that a little bit but on test day chances are they're going to give this to you they're going to give you this shape and so you don't really have to worry about reconstructing this what you do need to know are four points on the curve and i've shown them here color coded to make things easy for you so that orange point represents where the aortic valve closes that red point represents where the mitral valve opens that blue point represents where the mitral valve closes and the green point represents where the aortic valve opens okay so you want to just memorize maybe two of these points and my recommendation would be the orange point and the blue point because if you understand that orange is aortic closes and blue is mitral closes then you can kind of figure out what green and red are just by working through the normal physiology of the heart and specifically the left ventricle as you go from systole to diastole okay and what that that'll make more sense in just a moment i'm going to walk you through that example of how you're going to use your normal understanding of how the blood flows through the heart to figure out what points the green and red point would be if you didn't actually remember this on test day so let's think about this let's start at the blue point the blue point is when the mitral valve closes if that's where the mitral valve closes then what that tells you is that from the blue point up to the green point and then from the green point over to the orange point we're talking about systole and i want you to think very critically about this this isn't that complicated let's just talk about normal physiology and i'm going to put a picture of the heart in systole on this slide so you can see what i'm referring to now look at that picture of the heart if the mitral valve is closing that means that the left ventricle is done filling which means that the blue point where the mitral valve closes tells you that on this curve you're in the beginning of systole now if you were taking a test and they said what stage of you know systole versus diastole where are you when the mitral valve closes you'd have to think for a second and be like okay well if the mitral valve's closed the left ventricle is filled if the left ventricle is filled then we've just completed diastole and so it's for that reason that the blue point signifies the beginning of systole now likewise if the aortic valve opens at the green point we're still in systole because the aortic valve only opens during the ejection phase when that left ventricle squeezes all of that blood over the aortic valve into the systemic circulation okay now what that means is that from the green point to the orange point when the aortic valve opens up until the point at orange where the aortic valve closes this top part of the graph shown in that green dotted line represents our ejection phase because that's when the blood is ejected out of the left ventricle over the aortic valve up until a point where the aortic valve then closes now if you look at the axes of our graph here this should also make some sense to you you can see look at the look at the x-axis left ventricular volume going from the green point to the orange point you see that volume drops and that volume is dropping because you're ejecting all of that blood out of the left ventricle that's why this is the ejection phase and when the aortic valve opens at the green point and ejects all the way to the orange point when it closes it reaches a critical top left ventricular pressure during that ejection phase but then on the second half of the ejection phase as that blood is almost fully emptied out of the left ventricle that pressure comes back down just a little bit and that's why you have that unique shape going from green to orange where it initially increases up to its maximum and then it dips back down a little bit as you approach the second half of the ejection phase approaching that orange point where the aortic valve closes so i hope that that makes sense to you starting from blue mitral valve closes you're now starting systole you go up to the aortic valve opening and then you do the ejection phase going to the aortic valve closing now let's talk about diastole so diastole begins at the orange point when the aortic valve closes and it includes when the mitral valve opens at the red point and then it goes to the blue point where the mitral valve then closes and again i'm going to put up this image of the heart and the chambers of the heart during diastole so if we're starting from that orange point when the aortic valve is closing if the aortic valve is closing then that means the left ventricle is no longer ejecting blood into the systemic circulation and it's ready to be filled that's why you see the red point the mitral valve opening because the left atrium is going to fill blood over the mitral valve into the left ventricle lastly going from the red point to the blue point you have that diastole and that filling occurring until you get to the point where the left ventricle can no longer accommodate any more filling because that volume is increasing so much going from red to blue that eventually the mitral valve will have to close and it will close to conclude diastole so i just spent a lot of time explaining the normal physiology of the heart and what's happening at the left ventricle and the various valves around it and my goal in doing so as you just heard was to illustrate to you that the pressure volume loop looks really scary but if you know two points on the loop and you can kind of work through the normal physiology in your head all of this should make a lot of sense now something that test writers like to do is ask you about the iso volumetric phases of the pressure volume loop and i want you to pause for a second the name is iso volumetric so the volume doesn't change look at this if you're going from the blue point up to the green point although your pressure is increasing in that initial systolic phase your volume didn't change and i've marked off where you are on left ventricular volume when you go from blue to green notice that that doesn't change so that's an iso volumetric phase the same is true going from the orange point down to the red point when the aortic valve closes and you begin diastole and go down to the red point where the mitral valve opens although the pressure is decreasing because you're in the filling phase now the volume doesn't change again i put that little tick there where you go from orange to red so these are two isovolumetric phases and more specifically we name going from blue to green isovolumetric contraction because it's isovolumetric meaning volume stays the same but it occurs during contraction it occurs during systole and going from orange to red we call that isovolumetric relaxation because although the volume stays the same hence isovolumetric we're in diastole or relaxation and therefore pressure decreases so the reason that we name these areas of the pressure volume loop isovolumetric contraction isovolumetric relaxation ejection phase all of these names can also be inferred from simply looking at the axes of the graph what's happening to pressure and what's happening to volume if volume stays the same it's iso-volumetric and simply knowing contraction versus relaxation would require you to either a think about what's happening in the normal physiology of the heart or b simply look at the y-axis and say hmm is my pressure going up or down and if it's going down it's relaxation if it's going up it's contraction hope that makes sense to you the last really high yield thing that i just want to slide in because for whatever reason test writers like to go after this point is you need to know that during those iso volumetric phases whether it's contraction or relaxation all heart valves are closed so look briefly at isovolumetric contraction the mitral valve closes at the blue point and it remains closed until the green point so that dotted line that represents iso volumetric contraction mitral valve is closed but aortic valve is also closed because it hasn't yet opened and then look at iso-volumetric relaxation so at the orange point the aortic valve closes and then you go through isovolumetric relaxation but the mitral valve is also closed because you haven't yet reached the red point and so the high yield point here the takeaway is that whether we're talking about isovolumetric contraction or relaxation in all isovolumetric phases all the heart valves are closed and that's really unique because there's no area on this graph where all of the heart valves are opened this is the only area where all of the heart valves are doing the same thing and in this case they're all closed so very important to understand so here's where we are so far in the first part of this video we've talked about how we draw this graph what the axes represent what the various points on this graph represent which phase is systole which phase is ejection which phase is diastole and we talked about isovolumetric phases and which one was contraction versus relaxation the next part of this video is going to focus on using this graph to infer different types of cardiac physiology and so where the test writer is going to go is they're going to ask you questions about things like stroke volume after load contractility etc and so let's start by talking about stroke volume and how you get stroke volume from looking at this graph so recall from the first part of this video we said that at the blue point we start systole and saying that systole starts at the blue point is the same thing as saying that diastole ends at the blue point now look at the orange point we said that at the orange point diastole starts and saying that diastole starts at the orange point is the same thing as saying that systole ends at the orange point and this is really important because knowing the normal physiology of the heart but also understanding how we draw this graph we can draw in the end diastolic volume and the end systolic volume so look at the names guys end diastolic volume end systolic volume we're talking about volume so we're on the x-axis because again our x-axis measures volume now where does diastole end if diastole ends at the blue point right before systole begins then that's our end diastolic volume in other words this is going to sound obvious but let's really think about this like fifth graders if it's the volume in the left ventricle when diastole ends it's our end diastolic volume so we can plot that on our x-axis likewise at the area shown in purple if that's the volume in the left ventricle when systole ends it's the end systolic volume so the names that you've been used to using when you learned this in class and regurgitated it for exams should make a lot of sense if you say them slowly and think about what you're talking about so in our pink in on the x-axis the point shown in pink is our end diastolic volume and the point shown in purple is our end systolic volume and recall i hope they taught you this but recall from your lectures that end diastolic volume minus and systolic volume is our stroke volume so stroke volume is merely the difference in the volume from the pink point minus the purple point and that should make sense to you because stroke volume is the volume of blood that gets stroked out by the left ventricle when you go from the pink point to the purple point or when you go in systole from green to orange and looking at this graph and thinking about what stroke volume means in laypersons terms but also thinking about what that means in terms of a change in volume this should make sense to you so high yield takeaway here stroke volume equals end diastolic volume minus end systolic volume we can plot those things we can plot e dv on the x-axis and we can plot esv on the x-axis and knowing where to plot them should make sense if you either one look at the graph and look at where the various valves are opening or closing or two think about what happens in systole versus what happens in diastole and then put that on top of the graph to help you plot the pink point and the purple point so that's stroke volume now in the final part of this video what i want to do is clear out the graph but leave it on the slide and we'll talk about what happens with changes in contractility after load and preload and what my goal is for this remaining part of the video is to show you how these graphs change with a change in afterload preload or contractility and more specifically point out how you look at those changes and infer changes in things like end systolic volume or stroke volume etc now i want to make you aware that i'm not going to list every possible cause of an increase in contractility or an increase in afterload or an increase in preload that's going to be on you to understand so before watching the last part of this video make sure you understand how the heart changes and what is responsible for an increase in contractility what is responsible for an increase in preload what is responsible for an increase in afterload and once you're comfortable with that material then watch the last part of this video because now you'll see once that happens how do these graphs change so let's start by talking about an increase in contractility so the graph shown with the solid line was the normal graph but if we increase the contractility of the heart we get a change from the solid graph to the dotted graph and when we get an increase in contractility we get an increase in stroke volume an increase in ejection fraction and a decrease in the end systolic volume and what i'm going to do is color code all of these changes and just quickly walk you through how you could look at this graph to interpret that data so we increase our stroke volume because remember that the stroke volume is the difference between end diastolic volume and end systolic volume in other words it's the distance from the right and left side of these graphs so we started with the stroke volume shown there in blue at point a so blue a is the initial normal stroke volume but when we increased our contractility and the graph slid to the left with the dotted line our stroke volume increased because now there's a greater difference between our end diastolic volume or the right border of the graph and our end systolic volume and or the left border of the graph and that new stroke volume or that difference now between the edv and the esv changes from blue a to blue b so our stroke volume increases our ejection fraction also increases because the ejection fraction i'm over simplifying things but you can think about it on this graph as the ejection phase of the graph and you see that we went from that solid black line at the top of the graph to that dotted red line at the top of the graph and now there's a greater ejection fraction lastly our end systolic volume decreased because we started at point a which is the volume at the end of systole and we went to purple point b which is the new volume at the end of systole so when you increase contractility and the graph gets stretched to the left we decrease our end systolic volume because if you're looking at the x-axis which is our volume we went down from purple point a to the new purple point b so high yield summary increased contractility means increased stroke volume increased ejection fraction and decrease end systolic volume you can either know that by memorizing like what you probably did when you were initially taught this material you could do that by thinking about in your head what a heart would look like if it was contracting more and how stroke volume might change how ejection fraction might change etc or you can simply look at this graph and see that the numerical or the pictorial depiction of this would change as you see here so that's an increase in contractility now let's talk about an increase in afterload when we increase our afterload we go from the normal solid graph to the new dotted graph and an increase in afterload causes a few things one it causes an increase in aortic pressure which you can appreciate by simply looking at the graph and seeing that in the aortic section of the heart pressure goes up we can also note a decreased stroke volume which i've shown in red so remember stroke volume is the difference between the right and left border of the graph it's the difference between the end diastolic volume or the right border of the graph minus the end systolic volume or the left border of the graph and so initially we started at a stroke volume shown by the red a and it was much larger but when we increased our afterload we went from the red a now to the red b and so the difference between the right and left side of the graph went down so our stroke volume decreased lastly we have an increase in end systolic volume remember end systolic volume was initially shown at the purple point a but when we increased our afterload we went from purple point a to purple point b the purple point b is the new end systolic volume also known as the volume in the left ventricle at the end of systole and because we went from a to b we went up on the x-axis so our end systolic volume increased so these are the changes with an increased afterload lastly let's conclude this video by talking about what happens when we increase preload so an increase in preload changes the graph from the solid graph to the dotted graph and when we do this there's really only one major change that you need to know and that's that our stroke volume goes up remember stroke volume is the difference between the right border and the left border of the graph in other words it's the difference between the end diastolic volume and the end systolic volume so initially our stroke volume was illustrated by the red point a right the distance between the right and left side of the graph but when we increase our preload and the graph gets stretched to the right there's a greater difference between the end diastolic volume and the end systolic volume and therefore our stroke volume increased so we went from the red point a to the red point b now that's it guys that concludes this video remember this video had three distinct parts one how do you set up and label the graph two what do the various phases in the graph represent and three how does the graph stretch and change with changes in contractility preload and afterload so this is a complex topic there is a lot of information to know but i hope that i was able to simplify this for you