Welcome another MedCram lecture we're talking about hypoxemia and specifically the mechanisms that cause it and today we're going to talk about VQ mismatch let's talk a little bit about what VQ mismatch means V stands for ventilation this is basically air going in and out of the lung and Q stands for perfusion this is basically blood circulating to that area of the lung and it kind of goes along with what they say about the lungs and learning about the lungs is basically air goes in and out and blood goes round and round and ventilation and perfusion occur simultaneously in the lung and how they occur and in what ratio can actually have a lot to do with the different pathologies and specifically hypoxemia let's give a little bit of background though on ventilation and perfusion the first thing to remember when we talk about ventilation perfusion is remember the heart okay here's my little diagram of the heart and there's the right side of the heart and the left side of the heart here we have the tricuspid and mitral valves you remember that the right ventricle pumps blood out to both the right and the left lungs through the pulmonary artery and then of course the left side pumps blood out through the aorta first of all you know the left side when it pumps blood to the aorta the pressures that we usually see there are anywhere from 80 to 120 millimeters of mercury not so on the right side the right side is very is used to pumping very low pressures in fact the typical what we call pulmonary artery systolic pressure ranges around 30 at most in terms of normal 20 to 30 is the general range why that is important is we have to realize the lungs are sitting here on both sides of the heart and this blood gets pumped to them because it's it's done at such a low pressure around 30 millimeters of mercury gravity actually has a very large determinants about where this blood goes and so what actually happens is that you have more profusion to the lower areas of the lung than you do the higher areas of the lung and areas in the upper part have lower profusion now there is also a very similar distribution of ventilation there's more ventilation in the lower areas but that that differential in ventilation between the upper and the lower is much less variable than it is in perfusion so there's a big difference and as a result of all of this what we see is when we look at a ventilation - perfusion ratio and what I mean by a ventilation - perfusion ratio is when we look at the alveolus and the capillary that goes along with it we look at how much ventilation is occurring in the alveolus and how much perfusion is occurring in that units so in this unit here specifically what is the perfusion to ventilation ratio now why that becomes important we'll get into but suffice to say at this point is that areas in the top part of the lung generally have high vq ratio and and the reason why that is the case is because there is more ventilation in the upper portions of the lung and there is less perfusion okay so perfusion is lower in the upper portions of the lung that makes the vq ratio higher where is the lower portion of the lungs the vq ratios are generally lower and why is that because there's more perfusion so you could probably see that if you have a lot of ventilation what's the extreme if you had a lot of ventilation but no perfusion that would be kind of like your trachea okay your trachea is got a lot of ventilation but there is no perfusion to that area in terms of capillaries and so we call that dead space so that would be an extreme that would be an infinitely high VQ ratio whereas the alternative is with a low VQ where basically ventilation is zero well we just talked about that type of a of a mode or mechanism of hypoxia and then we call that shunt just to kind of give you an idea what the extremes are okay but we digress a little bit let's get back to what a v/q ratio is so if we have an area of high VQ again here is our alveolus and here's our capillary pulmonary artery pulmonary vein what this means is that there's quite a lot of ventilation occurring but there's not a lot of perfusion this type of situation is going to cause for generally speaking a high oxygen situation now that is opposed to an area of low VQ in an area of low VQ we have here's our alveolus again we do have some ventilation but the majority of this here what we have is we've got a lot of perfusion as a result of that this ventilation is not able to saturate this area of perfusion and so we typically have low oxygenation okay notice however the ventilation is occurring in both these situations there is no shunt going on anywhere and there's no dead space the situation that we talked about here in this case is something called VQ mismatch and that's a situation where there are certain lung units that are high VQ and certain lung units that are low that is what we call VQ mismatch and that's what gets us into trouble in terms of hypoxia and now let me demonstrate what it is that happens in that situation okay so I'm gonna draw a diagram that we had when we talked about shunt but it's not shunt these are two options that the blood can go through and these represent two different areas of the lung the top part is going to represent an area where we have high VQ and the lower area is going to represent where we have low VQ now this is important areas of high VQ by definition are going to have low profusion that's why I've drawn this limb Guinea because there's not gonna be a lot of blood going through this area okay whereas by definition areas with low vq are gonna have quite a bit of perfusion going through this area okay so let's again look at our numbers your typical saturation of oxygen in the venous blood is 70% now what's gonna happen as this blood splits notice it's not 50/50 like it wasn't shot there is definitely a smaller area of blood going to the areas of high V Q versus air is going to the low V Q but anyhow the areas of high V Q will go and they will be fairly well oxygenated so let's say that after it goes through these alveolar units that the saturation here is 98% and why is that that's because we have little perfusion but very good ventilation so we're going to oxygenate these lung units very well let's go to the area of low VQ mismatch I'm sorry the of low V Q I should say the mismatch is the fact that these two are different and in that situation the saturation is going to be it'll still be better than 70% but it won't be much better let's say it'll be 80% saturation and why is that it's because we have a lot of perfusion going to this areas of lung units and we have low low ventilation and so we cannot oxygenate this very well so now when these two limbs come back together the question is what's the new saturation going to be and the thing you've got to realize here is that there's many more red blood cells that are 80% saturated then there are red blood cells that are 98% saturated and as a result the arithmetic mean of these two is not sufficient to tell the answer because it's always going to be weighted more toward the lower saturation area because that has the more perfusion so that's always the way it is by definition is that it's not going to be the mean of these two what would be the average of 98 and 80 well it would be around 89% it's not going to be 89 okay it's not gonna be 89% it's gonna be closer to 80% and so a more realistic number there would be 85% okay and that's the mechanism that gives us hypoxemia so let's think about this let's go ahead and give a hundred percent oxygen okay that 100 percent oxygen is going to go here and that hundred percent oxygen unlike shunt is also going to go here and so in fact what happens is the hundred percent oxygen is able to overcome this this inequality in VQ mismatch and as a result the eighty-five percent will correct to 95 percent plus depending on the amount of oxygen that you use and so as opposed to shunt we're a hundred percent oxygen doesn't really make a difference with oxygenation with VQ mismatch in this case on a percent oxygen does actually make a difference now let me go ahead and show you an example of VQ mismatch and for that I'll show you the prototypical now VQ mismatch is the most common if they ever ask you that on a test it is the most common cause of high pox amia and the reason for that is because of the causes are so common for this for instance pneumonia pulmonary embolism what about a COPD all of these are reasons for the patient to have EQ mismatch to the point where is if they ever ask you this on a question and you don't know the answer to it and you see VQ mismatch then I would select that one let's go ahead though and focus on one of these to sort of illustrate that let's focus on pulmonary embolism okay so you've got your lungs let's go ahead and show these schematically and of course you've got your pulmonary artery coming out and going to the different lungs and it splits off and splits off again this is your heart here so when you get a clot that forms the clot goes gets pumped in and it goes over to a specific area and lodges what happens at that point is that there is no more perfusion to this segment of the lung and as a result of that if there is no perfusion you're gonna have an area of high V Q because there's very low perfusion now the blood that would normally go to this area what happens to it it's got to go everywhere else so perfusion increases there it increases here it increases here it increases here it increases everywhere so in other words everywhere else you get a area of low V Q low V Q in comparison to this area here as a result of this the whole situation is is that you get low oxygen in the blood okay so let's summarize the situation with VQ mismatch alright number one it responds to a hundred percent o2 remember the only one that did not respond to a hundred percent o2 was shunting number two you have an increased a a gradient number three the causes so here they are COPD you can even get fibrosis causing this asthma pulmonary embolism pulmonary hypertension pulmonary hypertension and even pneumonia and as a result of all of these causes it is the most common form okay well thank you for joining us