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Tetralogy of fallot

Created by Amy Fan.

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Video transcript

- For Tetralogy of Fallot, don't worry about what Fallot means. That's just the name of the guy, this French physician, who named this disease. So we wanna focus on the first word here, which is tetralogy. And going back to Latin here, tetra, tetralogy. The word is telling us that there are four defects that we care about, that make up this particular heart disease. Remember that this is congenital, so people are born with these four defects in the heart. But first let's quickly review what a normal heart does. The blood from the body that returns to the heart initially is gonna be blue because it's lower in oxygen. The muscles have used up the oxygen. So it returns to the heart into the right atrium, the receiving chamber. From there it goes to the right ventricle, which pumps it into this blue vessel, which is the pulmonary artery. Remember that any vessel that receives blood going out of the heart is an artery. So even though it's an artery, it's still blue, de-oxygenated blood. From the lungs, red oxygen in the blood returns here via the pulmonary veins, into the left atrium. From there it goes to the left ventricle, and it goes out this big red structure, the aorta, to the body. So in tetralogy, the first defect that we worry about, some would argue it's the defect that governs how this heart functions, is called pulmonary stenosis. So pulmonary, you know, has to do with the lungs. In this case, it's referring to the valve that leads blood to the lungs. So the pulmonary valve is right here. Stenosis is a stricture or narrowing, so this pulmonary valve here is thickened. So as the right ventricle is pumping blood into it, it is harder to pass through it because it's literally just thick and restricting. So the degree of pulmonary stenosis determines how hard is it to pump blood into the pulmonary arteries, and determines how severe this heart is functionally damaged. Now our second defect is a direct result of this, and also depends on the degree of stenosis, and that is right ventricular hypertrophy. So imagine trying to pump blood against a narrow opening, day in and day out. Hypertrophy is when the muscle is over exercised and over used and it literally becomes bigger. So the right ventricle, this chamber right here, becomes thicker to assume the extra workload that it has for pumping against this narrow valve. So a lot of times this hypertrophy is enough to alter the shape of this right ventricle here. So do you see in the way I've drawn it before, it has this nice slope. So if our normal heart follows a curvature kind of like this, in tetralogy the shape can actually look more like this. It comes down here, right angle. And that's why some people actually say, on x-ray, when just looking at the growth shape of the heart, that it'll look like a boot. So the boot sign is something that we associate with tetralogy because of the severe right ventricle hypertrophy that can happen. Alright, let's move on to the third one. So usually there is a hole between the right and left ventricle, the bottom chambers. I'm gonna draw this hole right here. It can actually be anywhere along this septum. And we call it a VSD, which stands for ventricular. It tells you where it is, it's between the ventricles. Septal, which is the wall, the septum between them. Defect, we have a hole. OK, I'm gonna draw this right here. So now in the machinery of the heart, we have a connection between the left and right. And now for the last defect, we have an overriding aorta. What is the overriding? So the aorta is usually plugged into the left ventricle, but overriding means that it actually receives blood from both left and right now. And from the place I've drawn as the VSD right under the aorta, I could almost just leave that like this, but just to drive the point home, I wanna redraw this part, that the aorta comes over to the right and is now this kind of central structure. Now from here, now it can receive blood both from the right and the left. So now let's think about if I'm a drop of blood, where I want to go in this new heart with these four defects. So from the right atrium, go to the right ventricle, that's normal. So if I'm here, I have a choice, right? I can either go up this pulmonary artery, through this very narrow opening in the valve. That's gonna be difficult. Or I can easily go into this aorta with a valve that's more welcoming, that's less restricted. See I draw these two arrows with different calibres 'cause I'm leading you to the answer, which is that I want to go this way, into the aorta. Also in general, blood in the right ventricle is gonna be pushed toward the left because look at how big the right ventricle is now. It's this huge powerful muscle, and it serves as the motor that drives this right to left shunt. And therein you have the answer for why this is a cyanotic disease, because blue blood is being forced to the left. Because A, it's hard to go through the pulmonary artery where it's supposed to go, and B, the right ventricle is so powerful, it's gonna push it across the VSD. Remember, blood follows the easiest path. So the left ventricle is still pumping, and the blood is basically gonna go into the aorta. So now in the aorta going out to the body, we have red blood and we have blue blood. So what actually goes out to oxygenate our body is this purplish mixture. Purple, some blue, some red. And at any given moment, how blue it is or how red depends on the tug of war between all these factors. How restricted is this pulmonary artery? How much resistance is coming from the lungs? Remember the pulmonary arteries are plugged into the lungs. How hard the right ventricle is pumping blood across the VSD. So these are all factors that determine how much our purplish mixture is red versus blue. Sometimes kids who have tetralogy can have a sudden tet spell, which is when they get acutely worse, and they can't breathe. That's because their pulmonary artery pressure or resistance from the lungs, has suddenly increased. All that does is create more back pressure in the pulmonary artery. It's already hard to get blood in here, and the tet spell makes it even worse. So a tet spell shunts more blue blood into the aorta. This mixture becomes even bluer. This is a life-threatening emergency in tetralogy. And lastly, I also wanna talk about the fact that before people here knew what tetralogy was, or what the heart actually looked like, we noticed that kids who have it, or this type of kid, would suddenly squat onto the ground, especially when they've been playing or running around. They suddenly feel a lot worse, they'll squat, and they'll feel better. So here we have a kid- I always say kid because, for the most part, this is still a pediatric disease. We correct it in the patients when they're kids. So they're running around, let's say they're four years old. And suddenly they'll stop and they'll squat onto the ground. How do you draw a squatting stick figure? Does that make sense? That looks more like they're sitting. Let me try again. So they're squatting. There we go. And as they get into this position for a few seconds, they feel better. So let's think through this step by step. OK, two things happen when somebody with tetralogy- Or I guess anybody is running around. The first one is that the O2, or oxygen saturation, in their veins drops because their muscles are working hard, and they're extracting more oxygen from it. So the blood going back to the heart here and here have a lower oxygen content. And secondly, what happens is that their vessels in their legs, or whatever muscles they're using a lot, they vasodilate, so the body can get more blood there. Literally their vessels in their blood, the diameter goes from this to this, it just gets bigger. So let's look at this one at a time. So if the O2 in the veins drop, what happens? So the blood returning to the heart is essentially bluer. So if the blood returning to the heart has a lower oxygen concentration, and some if it goes right back out through the aorta, then running around has made our blood bluer in the aorta. All this does is make our cyanosis worse. That's gonna make our patient more blue in the face, and they don't feel good. For vasodilation, what happens is this stricture, or the calibre of the vessels, determines how much resistance is pushing back on our aorta. So the vasodilation really lowers the resistance in the aorta. So in the competition between the right and left, between the blue and the red, again this makes us bluer, making this red structure have lower resistance means more of the blue blood will get into it. So again, we're bluer and more cyanotic. So now this child is really not feeling well. And look when they squat, both of these things are reversed. First of all, they stopped running around, so the oxygen content in their veins can go up. So here they've got slightly higher oxygen content in their veins just by stopping and trying to take deeper breath. And secondly, they're literally taking their vessels in their legs and squeezing them, which increases their systemic resistance. This is the resistance of all the arteries in the body. Systemic resistance. And since they're all connected to the aorta, this increases the resistance that the aorta is pumping against. So as these two colors of blood rush at this general area, when the resistance in the aorta increases, it literally just forces more blood into the pulmonary artery. So the competition is, where is it harder to go? And when this child has squatted, it is harder to go into the aorta at this point, because it's like they've taken the end of a hose and they've squeezed it down. So this just forces more blood into the pulmonary artery. It's still restricted, but the difference between the two has lessened. So more blood goes into the pulmonary artery. This child will feel like it's easier to take some good breaths, and this will also increase the return of oxygenated blood into the left side of the heart. So the whole effect is that we reverse the shunt a little bit. We get more blood out to the lungs and the pulmonary artery, and we just increase the overall oxygen content in our mixture a little bit. So when we think of tetralogy, the things that most determine how bad or how severe a person's symptoms are. One, the pulmonary stenosis, how severe is it? And two, where is the VSD? So I would like of tetralogy overall as a constant fight, a tug of war, for the heart to pump blood into either the pulmonary artery or the aorta. And the state of the mixture between the red and blue blood determines this person's symptoms at any given moment.