Shunting in the heart

- I wanted to spend just a tiny bit more time on the concept of shunting, just because it's so important in what we're talking about. Shunting is used in a lot of ways in medicine, and it just means pushing something from where it's suppose to be or used to be to a new place. Here we're talking about blood going from its natural place of flow to a different place. And I wanted to introduce the anatomically correct heart. I didn't want you to sit here and watch me draw it, so I drew it in advance and here it is. So our heart doesn't actually look like the cartoon. This is how it actually looks. And we'll just go through this really quickly and label everything. So this is still the right side. And this is still the left side. The right side of the heart receives blood, from the body that's used up. It's low in oxygen. So here we have the right atrium. And blood goes into the right ventricle. And here this blue thing that I've drawn connected to the right ventricle, do you see how the blood in the right ventricle goes this way, this is our pulmonary artery. Now every vessel that receives blood, going out of the heart is an artery. So even though this is de-oxygenated blue blood, it's still an artery because it's coming out of the heart. Then we have our left atrium going to the left ventricle. And here this big red structure, this is our aorta. Actually, let's just write out the whole thing, aorta. So if I'm a drop of blood or if I'm the Magic Schoolbus, this is how I would go through this whole system. So coming in here or here, so the veins don't blood into the right atrium. And I go through here into the right ventricle. There's a valve here and let's not worry about what it's actually called for the moment, just remember that it's there. So I'm in the right ventricle and from here I get squeezed into the pulmonary artery. Again, this is the valve here. From the pulmonary artery, I go to our wonderful lungs, and draw the lungs like this like a tree. It's the right and left branches of the lungs. Here we pick up oxygen. Now our blood has oxygen. The lungs return the blood to the heart through, to the left atrium. Oh, I didn't draw those here. So here we have pulmonary veins. Even though it's carrying red blood now, since it's going back into the heart, it's a vein. So pulmonary veins return blood here and here, into the left atrium. So now I'm in the left atrium and I'm going to the left ventricle through this valve. And this big left ventricle pumps me into the aorta. And from the aorta, I go all over the body, these little branches. And that's basically my path, through the heart and the lungs. So look at these little discrete chambers and paths, the blood, even though it's in all very close together, follows this pattern and the de-oxygenated blood is separated from the side that has oxygen. So shunting, here again, we're talking about right to left shunting because we are worried about people turning blue, cyanotic, so this blue blood on this side is going this way. And for shunting to happen, there are two things that need to happen. One, there needs to be a path. And two, there needs to be somehow this force or pressure behind it to make it want to go that way. This is just like plumbing. Not that I know anything about plumbing, but basically blood follows the path of least resistance, just like water does. So let's think about how we can create a path for that mix in the first place. So, one obvious one when we look at these big chambers, we're gonna have a hole here. Also, people are born with different holes in their heart. And sometimes they heal up, sometimes they don't depending on what exact disease or structural abnormality they have, so they're gonna have a big hole there. The next place we're gonna have a hole is between the atrium. Oh, I didn't draw the septum here. So, there it is. And, of course, we're gonna have a big hole there. Interestingly, everybody is born with a hole here, but it usually closes within the first few seconds, first few breaths you take. Or sometimes there's a hole that's always gonna be there. And then for babies, especially when they're in the uterus, there is a vessel here called the "ductus arteriosus". Okay, let's write that up. Ductus. Arteriosus. And basically they need this when they're in their mom's belly because we don't breathe in the belly. And since we get blood from the placenta, this is a conduit to help us get blood to the body. It has to do with fetal circulation. So the ductus actually begins to slowly close as soon as we're born. So adults don't have this. In fact, even toddlers don't have this. But in the first few weeks of life, this is a very real path for blood to still go back and forth. It's really cool that we have medicine to keep it open or close it faster depending on the situation. But just remember that it's there. Oops, I accidentally erased part of my aorta here earlier. That's not supposed to be there. Okay, so now pressure. So I'm just gonna put over here pressure equals resistance, resistance for r, times flow. That's just our little physics for the day. All I want you to remember from this is that pressure and resistance are related. So the left and right side of the heart actually have huge differences in pressure and that all depends on what they're pumping against. See the left ventricle is pumping into the aorta, into the body, just pumping against the resistance of our whole body's worth of vessels. So the left side, the pressure actually equals our blood pressure. So jumping out of the baby's mentality for awhile, for adults, what is our perfect blood pressure? It's around 120, right. That's the systolic pressure or the pressure the heart is pumping against when it's squeezing. So the left ventricle, when it squeezes, it's pushing against the 120 or so millimeters mercury. I'm throwing numbers out there just to, so we have the comparison. Because on the right side, the ventricle is pushing, what, into the lungs. And the resistance in the pulmonary vasculature is much, much lower than our whole body. So the right side is actually pushing against-- The range is usually 9 to 18 millimeters of mercury. All this to show you that, look at how much harder the left ventricle has to work compared to the right ventricle. That's why in adults, in functionally normal hearts, the left ventricle is a lot bigger and stronger. It's just a stronger muscle than the right ventricle. So if both of them squeeze together and even if we have a hole here, you would think that the normal shunt goes from left to right because this side has so much more power and you'd be right because we certainly have lot of diseases with left to right shunts. So when we have right to left shunts, which you think about why is the right side overpowering. One important one is because pulmonary vasculature in newborn babies have high, high, high resistance. So initially the right side is pushing against a lot more pressure than the left side. So that in a newborn babies, it's very easy to go right to left. Because given the choice, the blood would rather go into left ventricle than the pulmonary artery because the resistance in the lungs is so high. I'm gonna say that again just because it's really important. So if you're a drop of blood, go back to, let me use orange, if you have the choice to go in here or in here, in a newborn, going into the pulmonary artery is hard because the lungs are stuffed and full of resistance. While going to the left is a lot easier, so it's gonna go this way. That's one important way of right to left shunting. And that's why it's so prevalent in newborns. In fact, in adults this can still happen. Adults can develop pulmonary hypertension. So the resistance can increase because of some disease in adults and can still go right to left. So that's one way to go right to left, is the lungs giving us too much resistant. The other way is, look at this valve here. This is called the pulmonary valve that connects the right ventricle to the pulmonary artery. This valve can be too tight. This artificially increases the resistance here. So, again, blood given the choice between here and here, is gonna want to go to the left side. So that's also how you get shunting. And sometimes in some cases, we don't even have a left ventricle, it just didn't develop. In that case, the pulmonary arteries and aorta can be plugged over here. They can be both receiving blood from the right ventricle. I guess that's not really shunting as much as blood from the right side and left side just mixing together and going out. So this is not an exhaustive list of all the ways we can shunt, but the one thing I want to use is to illustrate is that when you talk about the specific defects in the heart, always be thinking about the path the blood wants to go and the pressure, I guess, and resistance that's making it go that way. We got fluid and we got muscle and ducts. So no matter how it gets structurally messed up, and then coming back to this picture, will help us understand why this baby is blue or cyanotic.