Systolic heart failure pathophysiology

- [Voiceover] So systolic heart failure, your heart can't pump as hard as it used to and so it doesn't squeeze as much blood out and it doesn't meet the body's demands. It's called systolic heart failure because we're talking about systole, which is the phase of the cardiac cycle where the heart contracts and ejects blood to either the body or the lungs. You can sort of think of it like squeezing a water bottle and squeezing the water out. It's the same sort of thing except with the heart it's actually squeezing blood instead. So systolic heart failure, this force of contraction, or this squeezing force is a lot lower. And sometimes we say that has lowered inotropy which just means the force of muscle contraction. And we're just going to talk about the ventricular inotropy, which is the strength of these ventricles or these bigger chambers. So now think about squeezing that same bottle, but just pinching it instead with two fingers. You can imagine it's going to be a lot harder to squeeze the water out, right? Well it's the same thing with weakened muscles. If they're smaller and weaker, it's going to be way harder to squeeze blood out and your stroke volume is going to be reduced and your body's not going to get as much blood as it used to. And this is all due to this weakened heart muscle. But how does that muscle get weakened. Well it's usually caused by some kind of underlying disease that causes the death of cardiac muscle cells or cardiomyocytes. So I'm going to draw a few of these cardiomyocytes and basically they're kind of holding hands, so when they contract they end up pulling on each other. And when they pull on each other they get a little smaller and this is what causes that squeezing motion, but what if a couple of these cells died? I mean, obviously they're not going to pull as hard and you're not going to get as good of a squeezing motion and your heart's not going to eject as much blood as it used to. So if you just look at this heart and compare it to a healthy heart, you'll see that these walls are way thinner and these ventricles are way bigger. These are classic signs of a heart with systolic heart failure. So what are some of these underlying diseases? Well you've essentially got four categories. And the first category is heart muscle diseases and a lot of times we're just going to call those cardiomyopathies. You can essentially break that down into cardio which means heart, myo which is muscle, and pathy which means disease. But typically with these, the cause is going to be unknown, but in general they tend to weaken the heart muscle and make it harder to pump blood. The second category isn't as straight forward. It's going to be reduced blood supply, but I'm not talking about the blood supply to the body, I'm actually talking about to the heart itself. And so coronary artery disease is a huge cause of systolic heart failure because the coronaries are what supply your heart with blood. So let's get some of this stuff out of the way and move these guys over here and then I'm going to draw some vessels going to the heart muscle cells. These are going to be our coronary arteries, well I guess there's just one, so it's a coronary artery. So usually you have blood coming down and giving oxygen to these muscle cells, but with coronary artery disease you have this narrowing of the vessel because of a buildup of plaque and that buildup reduces the blood supply to the muscle cells and those cells start to die off and as we're well aware, this is not a good thing because when muscle cells die off your muscle gets weaker and makes it harder to pump blood. If we go one step further and that plaque builds up so much that it completely obstructs or blocks that vessel then none of those cells get any oxygen at all and really they all end up dying off. This is really serious because you can get whole areas of cells that die and also why heart attack is a major contributor to systolic heart failure. Alrighty, so the third class of diseases are actually valve diseases. So your heart has four valves. Two valves that separate the upper and lower chambers and then two valves that separate the lower chambers from wherever the blood's going to, either the body or the lungs. So if we start with the valves that separate the two chambers, we can talk about something called regurgitation. And this just means that the lower chambers are letting blood go back into the upper chambers. So if we just watch this heart beat, usually those valves close when the blood's pumped out, but regurgitation means that the valves stay open just a little bit and let some of the blood go back into the upper chambers. Now blood is still pumped out, but some is allowed to go into the upper chambers and so naturally you lose the blood that's going into the upper chambers and less blood is pumped to the body. If we think about our favorite water bottle analogy again, but this time there's this like hole on the side, every time you squeeze it, most of the water goes out, but some's going to leak out the hole. And since now we're pumping less blood, the heart has to work harder to maintain that same level as it had before and more work costs more oxygen, but with heart failure, that's the whole thing, we can't supply more oxygen and more cells die off as a result. The other valve disease has to do with these other two valves. It's called stenosis and that means that the blood is being pumped out through this smaller opening, which makes it a lot harder to pump blood. Well I guess, let's look at our water bottle, but this time let's have our nozzle being super small. Just think about trying to squeeze it through that smaller nozzle. It's going to be a lot harder, right? Well it turns out it's also a lot harder for your heart muscles to squeeze blood through a smaller opening. And the same way as regurgitation, now the heart has to work harder to maintain the same blood supply and more work equals more oxygen that can't be supplied, so more cells die off. Alright, last one. This one's arrhythmias, which means abnormal heart rhythm. This could be a rhythm that's either too slow or maybe too fast or even uncoordinated. And uncoordinated, that just means that the chambers don't contract at the same time. Usually they would contract at the same time, but maybe now the left one's going before the right or the right one's going before the left. Either way, all these lead to a serious reduction in pumping efficiency, which means less blood is pumped to the body. And by kind of the same mechanisms as we talked about before, the more work means more oxygen demand and more cell death. So we start to see this pattern, right? Where each one reduces the pumping ability and this makes it a lot harder for the heart to maintain the same supply. And those cells that work harder will demand more oxygen, but the oxygen can't be supplied and those muscle cells die off. And when they die off the heart gets weaker and your pumping ability is lowered even more. So we see that pumping ability is really important. And one way we can measure pumping ability is by the ejection fraction, which is the percentage of blood ejected with each beat. So if we look at our water bottle, we can take the total volume filled and then we can also take the total volume ejected and then if we divide the volume ejected by the total volume filled, and then I guess multiply it by 100 to get the percent, you get your ejection fraction. And usually we just talk about the ejection fraction for the left ventricle, when it is possible to look at your right ventricle ejection fraction too. A normal range is about 55 to 70% and abnormal range would be considered anything between 40 and 55% and then evidence for heart failure is anything under 40%.