 Ventilation perfusion mismatch in lungs is one of the most common cause of hypoxemia in clinical condition. Hypoxemia when we talk hypoxemia is decreased in the partial pressure of oxygen in arterial blood. So let us see that to what is ventilation perfusion ratio and how ventilation perfusion mismatch can cause hypoxemia. First of all you see total ventilation in the lungs it is how much and when we are talking about ventilation perfusion ratio we are interested in the ventilation of the alveoli and not like the total ventilation which is occurring throughout the respiratory passages as well as the alveoli. So you see suppose if the tidal volume is 500 ml to calculate alveolar ventilation we have to subtract the dead space volume from this. So normal dead space volume say suppose it is 150 ml and we have to multiply with the respiratory rate. So let us assume respiratory rate to be 12 per minute. Now let us calculate how much will be the alveolar ventilation with this. So it is 350 okay into 12 and how much it will come 12 5 6 8 3 6 40 200 ml that is 4.2 liters. So that is the alveolar ventilation and how much is the normal perfusion of the lungs? Normal perfusion of the lungs is the total cardiac output because the right ventricle is sending the full cardiac output to the lungs. So perfusion is 5 liters. So if we calculate ventilation perfusion ratio using this total alveolar ventilation and total perfusion it comes to 4.2 divided by 5 that is 0.8. So that is the ventilation perfusion ratio of the whole lung. But imagine a hypothetical condition in which suppose these are two lungs and what we imagine suppose that the whole ventilation is going to one lung say suppose there is a block in the bronchus here okay. So whole ventilation is going to one lungs and whole perfusion is going to the other lung. Okay very hypothetical situation but just to understand the concept we are taking this example. So here ventilation will be 4.2 liters and here the perfusion will be 5 liters. Now if we calculate the ventilation perfusion ratio still the value will be 0.8 but will it be any useful? No. No oxygen will be able to pass into the blood. Blood will simply flow from the lungs and reach right and here there is no blood. So it is not useful for the body. So it is very important that ventilation should match with the perfusion wherever ventilation is there perfusion should be there. So when we talk about ventilation perfusion ratio we are interested in the regional ventilation perfusion ratio of the lung that is in the various parts of the lung what is the ventilation perfusion matching which is happening. So what is the difference in the lungs? Is it same everywhere or it varies and what happens in some disease conditions? Well actually in physiological conditions also there is variation in the ventilation and perfusion from apex to base of the lung. So this is the base. First let us see what is the variation in the ventilation. You see ventilation is less at apex and more at base. Why is that? Well ventilation when we are talking we want that with each breath how much is the change in the air in the lungs. With each breath the air should go in and it should come out. So what happens that in normal state due to the effect of the gravity you see that the base of the lung is bit compressed okay. Base of the lung is compressed and you see it is there is one plural layer right. So base of the lung is compressed and thus it creates some positive pressure here, positive pressure. I mean this is not actually a positive pressure this is relatively positive compared to that of the apex. So intra-plural pressure here at the base is less negative, less negative intra-plural pressure at the base compared to that of the apex of the lung and when we talk about the expansion of the alveoli we always talk about the compliance and what causes the expansion of the alveoli. It is the difference between the intra-pulmonary pressure okay intra-pulmonary pressure minus intra-plural pressure. Now when this intra-plural pressure is less negative this difference between the intra-pulmonary and intra-plural pressure is less at base. So now it might have become very confusing let us see some numbers to understand this. Suppose at the apex the intra- plural pressure is minus 5 millimeter mercury and at base because of the gravity it is less negative so suppose here it is minus 3 millimeter mercury okay. Now the alveolar pressure say suppose at equilibrium is 0 millimeter mercury okay. So alveolar pressure is nothing but the intra-pulmonary pressure. So how much is the pressure difference? The pressure difference is plus 5 millimeter mercury that is the distending pressure that is the pressure which is trying to stretch the lungs okay. So why it is plus 5 it is 0 minus minus 5 so it becomes plus 5 millimeter mercury. On the other hand at base by the similar logic this distending pressure is only plus 3 millimeter mercury understanding. So because of this more distending pressure at the apex the alveoli and the apex are kept more distended here at the base they will be little bit compressed state okay compressed it and here they are at distended state. Now that means that the alveoli at the apex can change from this distended state to the maximum distended state possible maybe this one and here the alveoli at the base can distend from this particular small size to the maximum size of this one correct. And if you remember compliance remember that when the alveoli are almost fully distended it is very difficult to change their volume to the maximum size you see in the compliance graph in the end the graph is almost flat. By the way I have another video in compliance you can check that out for details on lung compliance. So I was talking about the change in the size of the alveoli or change in the volume of the alveoli which will be much difficult at the apex with each breath because the alveoli are already in distended state at the apex. So at apex ventilation is much less compared to that of the base okay that is with each breath. So don't think that at apex the alveoli are distended so volume of air is yes at all points of breath it is more but we want the change in the volume of the air so let us draw it graphically at apex the ventilation is lesser compared to that of the base okay so base I have drawn it more so this is the change in the ventilation from base to the apex we took only two points but gravity is acting throughout the lungs okay so the size of the alveoli at different points of the lung are different so that is about ventilation coming to perfusion again due to the effect of the gravity the perfusion at the base of the lung is much more and why is it so see perfusion depends on the hydrostatic pressure and hydrostatic pressure depends on the volume of the blood and the gravity okay so gravity will cause more hydrostatic pressure at the base of the lung compared to that of the apex of the lung so again at base perfusion is also more compared to that of the apex however if we see the variation you see the variation it is something like this okay perfusion is much more at the base this is perfusion line and it falls much steeply to the apex okay so this is the perfusion line which we are talking the variation in perfusion is much more from base to apex compared to that of the ventilation which is this is the ventilation line see if the fall was similar say suppose the fall was something like this right then you calculate the ventilation perfusion ratio it will be coming one everywhere but because of this steep fall ventilation perfusion ratio if we calculate it comes something like this okay so if we see at the base and here ventilation perfusion ratio is v by q ventilation is more but perfusion is much much more so you see the denominator is more in value and hence ventilation perfusion ratio is lesser apex what happens again ventilation perfusion ventilation is lesser but perfusion is much much much lesser so compared to perfusion ventilation that is the numerator is more and that is why the ventilation perfusion ratio at the apex is much more and it here comes at approximately three so what is the point of all this thing well as I told you before that it is the matching of ventilation and perfusion which is important here at the base yes ventilation is more but unnecessary extra blood is going which may not be needed okay and here at the apex ventilation is less and perfusion is much lesser so whatever oxygen is coming into the apex that is also not going into the blood because the perfusion is less now because of this physiological differences in the ventilation perfusion ratio there are differences in the partial pressure of oxygen at apex and at the base let us see this concept little bit in detail so what is happening that due to this difference in ventilation perfusion physiologically also there are changes in the partial pressure of oxygen at apex and base at apex the partial pressure of oxygen in the alveoli is approximately 132 millimeter mercury while at the base it is approximately 89 millimeter mercury understanding you see if there is alveoli and there is a blood vessel how much will be the partial pressure of oxygen in alveoli depends on two things one that how much is coming in from outside right that is how much ventilation is occurring because that is bringing the atmospheric oxygen whose partial pressure will be 150 millimeter mercury okay and it also depends on how much of this is diffusing into the blood so after equilibrium we say that the total partial pressure of oxygen in blood is how much 100 millimeter mercury correct yeah but you see if we see each alveoli in the lung it is different here it is 132 millimeter mercury much closer to that of the inspired air because ventilation is more right and diffusion into the blood is less because the blood flowing is less then here ventilation is lesser and whatever is there it is diffusing into the blood so here partial pressure of oxygen is less but we say that alveoli partial pressure of oxygen is equivalent to that of the arterial partial pressure of oxygen that is also we say that it is equal it is 100 millimeter mercury yes that is equal because that we are measuring in a different way what we do we ask the patient to exhale maximally and we collect the last bit of the air so whatever air is coming from all the alveoli mixes up here in the tracheobronchial tree and we are collecting the mixed alveolar air so we get a mix of air from all the alveoli so then it is 100 millimeter mercury but here what we are interested in the regional values understanding so normally also there is difference in the partial pressure of oxygen from fx to base of the lung however physiologically it doesn't affect the oxygen content much in blood why so you see when the blood is coming from here from the apex and getting mixed with the blood which is coming from the base definitely there will be decrease in the partial pressure of oxygen so how much will be the partial pressure of oxygen in blood it will not be a mean of this it will you cannot just add this and divide that cannot happen why because the blood flow here is less so contribution of oxygen which is coming from the apex is much less and here p2 is much closer to than that of the partial pressure at the base so that becomes 97 millimeter mercury okay so now you see here oxygen dissociation curve in oxygen dissociation curve what we see like this is partial pressure of oxygen and this is percentage hemoglobin saturation so by 60 millimeter mercury hemoglobin is almost 90 saturated okay 90 saturated and after that the curve is almost flat so that is why at 97 millimeter mercury the oxygen content is not affected that much because even if the partial pressure of oxygen say suppose becomes 110 millimeter mercury will they there be much effect on the oxygen content no they will not be much effect however suppose this ventilation perfusion mismatch becomes exaggerated that means suppose there is a block in this blood vessel what will happen there will not be any contribution of this apical alveoli to the partial pressure of oxygen and what we will get after mixing the partial pressure of oxygen will be much lesser why because it will be the venous blood you see what is entering is the venous blood where partial pressure of oxygen is 40 millimeter mercury and this will mix when it will return it will mix with this and the partial pressure of oxygen will fall much much more so let's try to understand this ventilation perfusion mismatch so here in this graph we have shown three alveoli where you see this is the normal condition okay so partial pressure of oxygen here in this is 100 millimeter mercury all right and partial pressure of carbon dioxide here is 40 millimeter mercury that is after equilibrium has been attained with the alveoli so this is a normal condition now here this extreme condition shown where there is blockage of the blood vessel and what will be the partial pressure of oxygen in this case in the alveoli it will be equivalent to that of the incoming air because nothing is diffusing into the blood as I told you that partial pressure of alveolar oxygen depends on two things ventilation and then diffusion so here it will become equivalent to that of the inspired oxygen so it will become 150 millimeter mercury right 150 millimeter mercury so in this diagram we are talking about alveolar oxygen this is O2CO2 diagram which depicts alveolar partial pressure of oxygen in different levels of ventilation perfusion mismatch so here total block of blood vessel partial pressure becomes 150 millimeter mercury and partial pressure of carbon dioxide approximately zero that is equivalent to that in the normal condition inspired here is very less carbon dioxide so here we are taking it as zero millimeter mercury and this is known as physiological dead space because the ventilation is occurring but it is not taking part in gas exchange actually it is pathological named as physiological so this is contributing to dead space and in this condition second condition what we are showing that alveoli is blocked or there is collapse of the alveoli so in that case there will be zero ventilation understanding zero ventilation and the blood is simply moving through the alveoli without getting oxygenated so this is known as shunt okay this is a shunt and in this case how much will be the alveolar partial pressure of oxygen well it will be same as that of the mixed venous blood okay because they might have become equilibrium might have occurred so PaO2 will be equivalent to 40 millimeter mercury and partial pressure of carbon dioxide will be 45 millimeter mercury same as that of the mixed venous blood so here this is 40 millimeter of mercury is the partial pressure of oxygen okay so these are two extreme conditions but if we follow this line we get to know that how much will be the partial pressure of oxygen carbon dioxide at various degrees of ventilation perfusion mismatch okay let us see how it affects the partial pressure of oxygen in case of various clinical conditions which cause ventilation perfusion mismatch say suppose here there is PaO2 of 132 millimeter mercury that we are taking normally and here PaO2 is 89 millimeter mercury right so how much will be the alveolar PaO2 again remember that we are taking the mixed alveolar partial pressure of oxygen so the person will exhale and the air from all the alveoli will come out right and it will mix so partial pressure of exhaled oxygen will be somewhere in between say suppose it is 100 millimeter mercury after mixing okay and what will be the partial pressure of arterial oxygen in normal condition as we saw that it comes to 97 millimeter mercury so there is not much difference maybe 3 to 4 millimeter mercury difference is there between the alveolar and arterial oxygen and this difference is known as alveolar arterial gradient so that is written as PaO2 minus PaO2 now suppose this blood flow is decreased okay it is very less so the blood which is returning from here the partial pressure of oxygen from this blood will be 132 millimeter mercury right but the amount of blood is very very less and the blood which is returning from here the amount of blood is quite a much compared to what is returning from here so the effect of this partial pressure on increasing this partial pressure of oxygen will be very very less right because the blood coming from the apical alveoli is quite less so maybe it reaches only 291 millimeter mercury understanding now what will be the alveolar arterial gradient so here it will become 100 because alveolar partial pressure of oxygen is same so it is 100 however arterial is decreased so what we see there is increased alveolar arterial gradient of oxygen in case of hypoxemia due to vq mismatch similar is the case when ventilation is blocked let us see another example for that so suppose here is two alveoli right and blood is flowing through them normally so if there is block here in the alveoli the blood returning from the apex what will be the p2 p2 will be like that of the mixed venous blood that is 40 millimeter mercury and while here even if it is getting oxygenated p2 will become only 89 millimeter mercury right 89 millimeter mercury and what will happen when these mix even the blood which is flowing from the apex is lesser but still it is going to decrease this partial pressure of oxygen to much lesser so again you see that this pa2 value is going to fall too much and then again there will be increase in the alveolar arterial gradient so hypoxemia due to vq mismatch increases this gradient however suppose there is another cause there is hypoventilation the person is not breathing itself the central respiratory drive is not there or the muscles are not working properly so the air going inside itself is less okay at the level of the lung the diffusion is occurring properly so in case of hypoventilation pa2 will decrease right so it may become like 90 millimeter mercury but there will be a similar fall in arterial pa2 so this may become 87 millimeter mercury so there is hypoxemia occurring due to hypoventilation also but there the alveolar arterial gradient is normal but that due to vq mismatch this gradient increases so that was about concepts on ventilation perfusion ratio and how in diseases it will lead to hypoxemia thanks for watching the video if you liked it do press the like button share the video with others and don't forget to subscribe to the channel physiology open thank you