Diffusing capacity for carbon monoxide

Transcription

Voiceover: There's a test that can tell us how well diffusion is going in the lungs. It's got one of those acronym names that are really hard to remember. It starts with a D for diffusion and then L because it's in the lungs and then CO, standing for carbon monoxide, let me write that out, carbon monoxide. That's the gas we use to do this test. And you're right, it's the same scary gas that we're afraid of of being in our homes because it can poison us. We'll get to in just a second why we use this gas. I guess to get really technical, diffusion will be talking about moving from a high concentration to a low concentration, but for our purposes, let's think of it as a gas moving across a barrier from place A to place B. In this case in the lungs, we have an alveolus, which is the end of the airway in the lungs. This is where gas exchange will take place. That is covered by this layer of blood vessels. We care about how diffusion goes here because usually, its job, this whole area, is to have one gas diffuse from the airspace into the blood and another one from the blood to the airspace. Of course, this one is oxygen, and this one going from blood into the lungs is carbon dioxide. This test is able to answer that question of how well can the lungs move a gas into the bloodstream? To do this test, we have our patient here. Let's call him Mr. D for diffusion. Mr. D here, if we look at his airway, it's connected to his mouth, and it's also connected to his nose up here, so here through his nostrils. Theoretically, that could go also down the airway. To isolate all the numbers and data we're getting, his nose is going to be plugged, so he can only breathe through his mouth. We put a mouthpiece into his mouth, and I can't draw what the machine really looks like, but just to get the idea here, we have one reservoir there. He breathes in through here, and then when he blows it out, the air goes to another machine. Let's draw it like this. In the first one where he's breathing from, it's full of carbon monoxide, the gas that we mentioned. It's part of the name of this test. He takes a big breath as much as he can breathe in, so carbon monoxide goes down, down his pipes into his lungs, and it fills his lungs. Now a certain amount gets absorbed here into the bloodstream, and then when he can't breathe in any more, he holds it for a second, for just a split second, and then he blows it all out as much as he can go, keep going, keep going until he has no air left. Now the computer is able to calculate two things for us. One is how much carbon monoxide he breathed in and then how much he breathed back out. We care about these two numbers because essentially, how much you took in minus how much came back out equal however much went into his bloodstream. That's the amount that was diffused across. If we come back and look at this drawing here, the carbon monoxide goes in here. It fills this airspace. A certain amount, if it crosses in the bloodstream and then all that's still left in the airspace, when he breathed out, it comes out here, so there is nowhere else for the gas to go. It either went in the bloodstream or it came back out. Therefore, our equation here gives us an estimate of how much gas diffused across. The reason we use carbon monoxide instead of the two gases that usually are in the lungs, the oxygen, the carbon dioxide, is because of hemoglobin. Hemoglobin is something in our blood. It's part of the red blood cell, and its job is usually to carry these gases in our blood. It can carry actually multiple gases. First, it can carry carbon dioxide, which, for our purposes, is a waste product. The body makes carbon dioxide, hemoglobin takes it up, and then when it gets to the lungs, it exchanges the carbon dioxide for oxygen. Back here, when we talked about these two gases being one going in, one going out, the vehicle is hemoglobin carrying it. Now a third gas it carries is, of course, carbon monoxide. This is not usually part of what we breathe in. We just so happen to know that hemoglobin not only carries carbon monoxide but actually has a huge preference for it. It plays favorites. This is like its favorite kid. Here's why we use it in this test, is because since it likes carbon monoxide so much, we're able to maximize diffusion, maximize diffusion. Because when the hemoglobin in the blood sees the carbon monoxide, it grabs all of it up and gives us that maximum value of how well the diffusion is happening. But the reason we're so afraid of having carbon monoxide at home is if you can imagine your air at home, there is a leak and there's carbon monoxide in the air, many, many molecules of that, and there's also regular oxygen that we usually want. If you're hemoglobin and you're picking out of these gases to pick up, you're going to choose all the carbon monoxide because you just like it better. Instead of carrying oxygen, it's going to carry carbon monoxide like this. Now this is a problem because hemoglobin's job is to take the oxygen all over. It can take the carbon monoxide to the same places, but our body can't use carbon monoxide. It's useless to us. Now it's OK for Mr. D to breathe in one breath of this, but if you do this for a couple of minutes and you're breathing carbon monoxide instead of oxygen, then this person is quickly, their oxygen level is going to drop, so they're basically suffocating even though they're still moving air in and out. That's why carbon monoxide poisoning is so serious. Coming back to talk about diffusion, what on earth exactly are we testing for in the lungs? So what, it can diffuse well, but what does it mean if it does or does not diffuse well? Let's look an equation of how gas behaves and what are the things that affect diffusion. The volume of gas that can move across a barrier is equal to the area, surface area that's going across divided by thickness of the membrane, or I'm sorry, the barrier. Once applied to a constant, this constant, as experimentally found, is related to the gas, so we're not going to worry too much about that. But it's also related to the partial pressure of the first place minus the partial pressure of the second place, with respect to whatever gas. OK, let's tackle this one thing at a time. This first glob here, the area over thickness, that's really talking about the nature of this septum, the nature of the barrier that we have to move across. In our case, assuming the blood vessels are OK, we're really looking at the membrane of the alveolus here. The tissue of the lungs, what is the condition of that? For the gas to go through here, what is the surface area? What is the thickness? Now remember, for fractions, whatever is in the top of the fraction, if area goes up, then volume goes up, but if thickness goes up, volume goes down. Let's say something that can affect the surface area would be a disease such as emphysema, where the lung tissue is being destroyed and you literally get less surface area for diffusion. In emphysema, the area goes down. And area goes down, the volume is going to go down too because this is at the top of the equation. Another example, something that might affect that thickness would be, let's say, fibrosis, where the lung tissue gets scarred and thickened, there's too much connective tissue. The thickness would go up. And because that's at the bottom of the equation, that actually drives the volume down too. You see how both these diseases would drive down the amount of the gas that goes across, and that's how they both impair the lung function. Now for partial pressures, I find the concept a little confusing. Let's try to look at it this way. Say there's a barrier, that you're a certain gas and you want to go from area one to area two. Now how willing this gas is to make its way over here depends on how much of it is on either side of this barrier. Say, in the first scenario, if there is a ton of particles in the first area and only one or two or here, let's say four particles there, that's going to have a huge drive to push it over this way because the partial pressure of P1 is really high, and the P2 is really low. This absolute difference between the two is a driving force. In the other case, if we have about this much P1 here and then P2 is about the same, then this drive is going to be very small. There is not that much force pushing it over. That's the same concept here, P1 minus P2. In asking what's the difference in the gas in the airspace versus in the blood, really, the question is how much gas did we get into the area? How much gas was Mr. D able to breathe into his alveolus? P2 here should be about zero. There should be no carbon monoxide in your blood, so if you breathe a lot of it in, then the P1 minus P2 will be large. The more of a difference between P1 and P2, the higher the volume for diffusion. The diseases that affect this part of the equation are the diseases that make it hard for air to get into the lungs, let's say chronic bronchitis. There's all this mucus and blockage, so the air can't get into the alveolus. The lack of that air pressure in the airways, that's going to have a lower P1 minus P2. Actually, also with fibrosis and also with emphysema, they're also bad for the partial pressure difference because it's hard to get air in. As you can see, for this test for diffusion, many different diseases in many different mechanisms can affect the diffusion. This is not really a diagnostic test as much as it tells us how severe somebody's disease is.