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Current time:0:00Total duration:10:03

Diffusing capacity of the lung for carbon monoxide (DLCO)

Video transcript

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 so it starts with a D for diffusion and then L because it's in the lungs and then C oh 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 being in our homes because it going to poison us and we'll get to in just a second why we use this gas it gets to get really technical diffusion we'll 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 now it's covered by this layer of blood vessels and we care about how diffusion goes here because usually it's job this whole area is to have one gas diffuse from the air space into the blood and another one from the blood to the air space so of course this one is oxygen and this one going from blood into the lungs is carbon dioxide and this test is able to answer the question of how well can the lungs move a gas into the bloodstream so to do this test we have our patient here let's call a mr. D for diffusion alright mister 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 also go down the airway so to prevent to isolate all the numbers and data we're getting his nose is going to be plugged to get only breathe through his mouth so 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 and in the first one where he's breathing from there it's full of carbon monoxide the gas that we mentioned is part of the name of this test so it takes a big breath as much as you can as much as he can breathe in so carbon monoxide goes down down his pipes into his lungs and it fills his lungs now certain amount gets absorbed here into the bloodstream and then when you can't breathe in anymore he holds it for a second for just a split second and he blows it all out as much as he can go keep going keep going until he casts no air left now the computer is able to calculate two things for us one is how much carbon monoxide you breathe in and then how much he breathed it back out you'll care about these two numbers because essentially how much you took in - how much came back out equal however much went into his bloodstream so that's the amount that was diffused across if we come back and look at this drawing here the carbon monoxide goes in here and 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's nowhere else for the gas to go so either one in the bloodstream or came back out therefore our equation here gives us an estimate of how much gas diffused across the reason we used carbon monoxide instead of the two gases that usually are in the loans that oxygen in the carbon dioxide is because of the hemoglobin hemoglobin is something in our blood it's part of the red blood cells and it's job is usually to carry these gases in our blood you can carry actually multiple gases first you can carry carbon dioxide which for our purposes is a waste product so the body makes carbon dioxide hemoglobin takes it up and then when it gets to the lungs it exchanges the carbon dioxide for oxygen so back here we talked about these two gases being won't go in while 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 a we breathe in it we just so happen to know the hemoglobin not only carries carbon monoxide but it actually has a huge preference for it it plays favorites this is like its favorite kid so here's why we use it in this test is because since it likes carbon monoxide so much we're able to maximize diffusion maximized okay diffusion because when the hemoglobin C 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 the reason we're so afraid of having carbon monoxide at home is if you can imagine if your air at home there's a leak and there's there's carbon monoxide in the air many many molecules of that and there's also a regular oxygen that we usually want so if your hemoglobin 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 so instead of carrying oxygen it's going to carry carbon monoxide like this now this is a problem because hemoglobin job is to take the oxygen all over you 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 okay 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 in this person's quickly their oxygen levels going to drop to the basically suffocating even though they're still moving air in and out that's why carbon monoxide poisoning is so serious so coming back to talk about diffusion what on earth exactly are we testing for in the lung so so what it can diffuse well but what does it mean if it does or does not diffuse well so let's look at an equation of how gas behaves and what are the things that affect diffusion so the volume of gas that can move across a barrier is equal to equal to the area surface area is going across divided by thickness of the membrane or I'm sorry the barrier multiplied to a constant this constant does it's experimentally found is related to the gas so we're not gonna worry too much about that but it's also related to the partial pressure of one of the first place mine is 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 the yes that's really talking about the nature of the septum the nature of the barrier that we have to move across so in our case assuming the blood vessels are okay we're really looking at the membrane of the alveolus here so 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's in the top of the fraction if area goes up then volume goes up but if thickness goes up volume goes down so let's say something we can affect the surface area will be a disease such as emphysema where the lung tissue is being destroyed you literally get less surface area for diffusion so in emphysema the area goes down an area goes down the volume is going to go down too because this is at the top of the equation it's another example something that might affect the thickness would be lets say fibrosis where the lung tissue gets scarred and thickened there's too much connective tissue the thickness will go up and because that's at the bottom of the equation that actually drives the volume down too so do you see how both these diseases would drive down the amount of fall that the gas that goes across and that's how they both impair the lung function now for partial pressures I I find the concept a little confusing let's try to look at it this way so say there's a barrier and there you're a certain gas 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's a ton of particles in the first area and only one or two 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 so 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's not that much force pushing it over so that's the same concept here p1 minus p2 and asking what's the difference in the in the airspace versus in the blood so 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 I mean P 2 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 P 1 minus P 2 will be large the more of a difference between T 1 P 1 and P 2 the higher the volume for diffusion so the diseases that affect this 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 so as you can see for this test for diffusion many different diseases in many different mechanisms can affect the diffusion so this is not really a diagnostic test as much as it tells us how severe somebody's disease is