 Hypoxia is basically deficiency of oxygen at the tissue level. Normal partial pressure of oxygen in arterial blood is around 100 millimeter mercury and this partial pressure of oxygen at 100 millimeter mercury ensures that hemoglobin is fully saturated with oxygen. If you know about hemoglobin oxygen dissociation curve you might be aware that binding of oxygen to hemoglobin depends on the partial pressure of oxygen in arterial blood. So for oxygen to be effectively carried to tissues we should have a normal partial pressure of oxygen, normal hemoglobin for oxygen to bind to it. Then this oxygen both in dissolved form and bound to hemoglobin should be carried via blood to the tissues and tissue should be able to effectively utilize it. So problem in any of these that is if there is a decrease in partial pressure of oxygen in blood which we will see how what are the causes of that. Then if there is a decrease in hemoglobin available for binding with oxygen or oxygen is not being carried to the tissues and tissue is not utilizing oxygen all of these can lead to various types of hypoxia that is decrease oxygen level at the tissues. Now this hypoxia is actually a very dangerous condition especially for the brain tissue. So neurons are very sensitive to hypoxia and in a very short duration the cells may start dying. Okay so now let's see that what are the types of hypoxia based on that where the problem is occurring. Hypoxia is basically of four types that is hypoxic hypoxia also known as hypoxemic hypoxia then there is anemic hypoxia third one is ischemic or a stagnant hypoxia and finally there is histotoxic hypoxia. Let's see the causes of each of them one by one and then we will see the features also. So we will use that diagram to see what are the causes. So first one is hypoxic hypoxia in which there is decrease in partial pressure of arterial oxygen. Now there is a whole apparatus which is responsible for delivering oxygen from atmosphere to the blood and if there is a problem in this apparatus or there is a problem in the atmosphere itself then it will lead to decrease in partial pressure of arterial oxygen. So if there is a decrease in oxygen in the atmosphere only see normally at sea level that is the partial pressure of atmosphere being around 760 millimeter mercury 21% of the atmospheric gases is oxygen. So the partial pressure of oxygen in atmosphere is calculated like 21% of 760 millimeter mercury. So it comes to around 160 millimeter mercury. Now at high altitude what happens this atmospheric pressure decreases. So if the total atmospheric pressure decreases but the percentage composition of oxygen is still 21% this value see this value is going to decrease. So say suppose at high altitude if the pressure is 600 millimeter mercury. So how much will be the partial pressure of oxygen. So let's round it off and see. So it comes to 120 millimeter mercury. So from 160 millimeter mercury it has dropped to 120 millimeter mercury. The point is that if atmospheric oxygen content is less then obviously less oxygen is going to reach to the lungs which brings in oxygen closer to the blood vessels. Less will diffuse into the blood and then less is going to reach to the tissues. So first cause is high altitude. Second reason of decrease in oxygen in atmosphere may be closed spaces. So suppose in a closed space there is no ventilation and there is some fire. So what will happen the oxygen is being used up and after some time it will create a condition of lower oxygen content. So in this case the percentage oxygen of the room will be decreasing. So in first case high altitude see total atmospheric pressure is reducing but in second case closed spaces percentage of oxygen is decreasing. So here instead of 20% it may become like 12% 15% like that. Then there may be that outside oxygen is normal but there is a problem with the lungs. Either there is a decrease in ventilation that is hypoventilation the bringing in and moving out of air is decreased. So there is hypoventilation. Then there may be problem in diffusion. So suppose there may be thickening of alveolar lining. So if there is thickening then the diffusion will be affected. You might be aware about Fick's law that thickness of the membrane is the one thing which is responsible for the rate of the diffusion. So that also can cause hypoxic hypoxia because the delivery of oxygen to the blood itself is being decreased causing to decrease in partial pressure of arterial oxygen. Apart from this there can be ventilation perfusion defects. What is that that maybe some alveoli are ventilated but the other alveoli are not ventilated like it's happening in covid that some alveoli are affected and ventilation in them decreases. So in these kind of alveoli you see the oxygen will be able to diffuse into the blood but in the other alveoli which are affected oxygen will not diffuse into the blood. So this blood coming from the other alveoli will remain deoxygenated. So finally for going into the left side of the there will be mixing of the oxygenated blood coming from the properly ventilated alveoli and deoxygenated blood coming from the non-ventilated alveoli. This will decrease the partial pressure of the final oxygen. So this is basically what this is basically shunting of blood is occurring when there is a blood which is going but it remains deoxygenated because of decreased ventilation or no ventilation in the alveoli this is known as a shunt. So in case of these kind of shunts also hypoxic hypoxia may occur. Now apart from this there is another kind of shunt which may exist at the level of the heart. So suppose there is some communication between the right side and left side of the heart I will not go into the pathophysiology of that but what happens that deoxygenated blood from right side starts entering into the left side of the heart. So what happens some blood reaches into the lungs and some blood directly goes into the left side of the heart. So basically again there is a mixing of oxygenated blood coming from the lungs and deoxygenated blood which has directly entered into the left heart. So again the partial pressure of oxygen in blood will be less. So all these can lead to hypoxic hypoxia. So just a mini revision high altitude and closed spaces basically they will decrease the oxygen content in the atmosphere. Then if there are problems in the lungs which can be ventilation defects diffusion defects or they can be ventilation perfusion defects that also can lead to hypoxic hypoxia and then they can be right to left shunts in the heart itself. So basically we have seen that in this kind of hypoxia partial pressure of oxygen in arteries is decreased. Okay now let's come to second type of hypoxia that is anemic hypoxia. So again let's go back to our original diagram. So what has happened that our apparatus for delivering the oxygen to the blood has worked properly and partial pressure of oxygen in blood is normal but there is non-availability of hemoglobin in blood. So even if oxygen is able to enter into the blood it is not able to bind with the hemoglobin and you see that most of the oxygen in blood is transported in the bound form. So approximately like 20 ml of oxygen is being carried per 100 ml of blood. So out of that only 0.3 ml is being carried in dissolved form which is due to the partial pressure of oxygen while rest of it is carried bound to hemoglobin. So you can imagine that if there is non-availability of hemoglobin how oxygen levels will decrease in blood despite normal partial pressure of oxygen. So what are the causes? See here I am using the word decreased availability of hemoglobin. That's important because you will understand the causes based on that. First is obviously the quantity of hemoglobin there is anemia. So hemoglobin itself is less or RBC count is less that will lead to anemic hypoxia. But there is another cause carbon monoxide poisoning. See carbon monoxide gas has 210 times affinity to bind with hemoglobin compared to oxygen. So if there is carbon monoxide poisoning what will happen that most of the hemoglobin will be bound to carbon monoxide and not with oxygen. So despite the presence of hemoglobin despite the presence of oxygen hemoglobin is actually bound to carbon monoxide. So oxygen is not being carried that is why I have used the term availability of hemoglobin. So either the quantity may be decreased or the available hemoglobin may be bound to carbon monoxide. So these are the causes of anemic hypoxia. So we saw that in hypoxic hypoxia partial pressure of oxygen in blood was less but in anemic hypoxia partial pressure of oxygen in blood is normal but still oxygen content as such is low. Okay so suppose if these both are normal that is the oxygen content is normal in blood the partial pressure of oxygen is normal also and hemoglobin also is normal but there is ischemia or what is called as stagnant hypoxia. So what is this kind of hypoxia? See heart is responsible for maintaining the blood flow and for delivering the contents of the blood to the tissues. So oxygen is also the content in the blood only. We need that heart should continuously pump and blood should flow through the vessels so that blood reaches to the tissues. So suppose if there is heart failure so cardiac failure has occurred and the heart is not able to pump. So this will decrease the blood flow to the tissues. Secondly there can be other causes of shock there is circulatory shock. So in this shock also basically what is the main thing BP has decreased actually heart failure is also a cause of circulatory shock. So BP has decreased and it will affect the blood flow to the tissues. Then there can be even local block in the blood vessels. So if there is a local block in the blood vessels say suppose coronary vessels then particular tissue that particular tissue in whose blood vessel there is a block that tissue will suffer from hypoxia. So if coronary vessel is blocked then heart will have the hypoxia and that we know that we know it is known as myocardial infarction. So there can be local block. Finally there can be vasoconstriction that is suppose there is excessive cold weather is there in that case the blood vessels of the fingers may undergo excessive vasoconstriction. So that also leads to tissue hypoxia because the delivery of the oxygen to the tissues is affected. So these are the various causes of ischemic or stagnant hypoxia local vasoconstriction and we said there can be local block. Another one is that tissue edema also can cause the local block. See when there is a swelling in the tissues what happens there is external compression of the blood vessels. So too much edema is also dangerous that it will compress the blood vessels and will decrease the supply of the blood to the tissues. So that also can lead to ischemic or stagnant hypoxia. So again we see that PO2 in this kind of hypoxia is also normal. Okay. Final type of hypoxia is histotoxic hypoxia and here histo word is related to tissue. Histo is used for tissue. So there is some toxin which has affected the tissues. So there is decreased utilization of oxygen by the tissues. Everything else is normal. The system is working normally. Delivery of the oxygen is normal but the tissue themselves are not able to utilize the oxygen. So this happens in cyanoy poisoning basically basically it inhibits the enzyme cytochrome oxidase which is a which is important in electron transport chain in mitochondria for generation of A2P. Similarly there is a diphtheria toxin which can also inhibit some enzymes in the electron transport chain. So these are the various causes of hypoxia. Now let's come to some of the features of hypoxia. I think we have mostly dealt it. So let's suppose PO2. PO2 here is decreased anemic normal. Schemic stagnant normal. Histotoxic normal. Then what about oxygen carrying capacity? See this oxygen carrying capacity is basically defined for hemoglobin. 1.34 ml of oxygen is carried by 1 gram of hemoglobin. So suppose there is a hemoglobin of 15 gram per 100 ml of blood. Then we calculate the oxygen carrying capacity something like this 15 into 1.34. So this is the oxygen carrying capacity of 100 ml of blood. So basically we can say that in hypoxic hypoxia oxygen carrying capacity is normal. In anemic hypoxia it is decreased because hemoglobin is not available. Schemic or stagnant hypoxia again it is normal and histotoxic hypoxia again it is normal. What about the O2 content of blood? See beginning only I told that oxygen content in blood depends on two things. One is a partial pressure of oxygen in arterial blood which is directly responsible for dissolved oxygen and then the oxygen bound with hemoglobin. So you see that in hypoxemic hypoxia oxygen content will decrease since partial pressure of oxygen is decreasing. So obviously dissolved oxygen is going to be less but dissolved oxygen contributes very less to the oxygen content. But since partial pressure of oxygen decreases the binding of oxygen to hemoglobin also decreases. So basically oxygen content decreases. Anemic hypoxia obviously because hemoglobin is less so oxygen content is also less then ischemic hypoxia oxygen content will be normal. Only the blood flow is reduced and histotoxic hypoxia again the utilization of oxygen is affected and oxygen content as such is normal. Okay now why do we need to know all this because of a very important question which we have to answer for the treatment of a patient with hypoxia. Will increase FIO2 be helpful? Here FIO2 is increased fraction of inspired oxygen. So normal fraction as I told in atmospheric air our oxygen is 21%. You might be noticing that in COVID patients oxygen support is required. So what is that basically? Basically we are increasing the fraction of inspired oxygen. So for the treatment we need to decide whether in this case of hypoxia increased fraction of inspired oxygen will be helpful. So suppose if we increase the fraction of inspired oxygen to 40% will the patient respond? So for this we have to go back to our causes and then we will see that in which types FIO2 will be helpful. Okay let's start with the easier ones first. See in histotoxic hypoxia we are saying there is decreased utilization of oxygen. So basically even if we increase the FIO2 from 21% to 40% that means this is going to increase the partial pressure of oxygen in blood. Okay but the problem is in utilization you may keep on increasing the partial pressure of oxygen in blood but the tissue is not going to utilize it. So obviously in histotoxic hypoxia increased FIO2 will not be helpful. Okay what about ischemic? So in ischemic will increase FIO2 or increase partial pressure of oxygen in arterial blood be helpful? Again no because that is not the problem. Oxygen content itself is not the problem. Problem is in the delivery to the tissue. So you may keep on increasing the oxygen content but it is not going to be helpful. So we have to treat the main cause. Okay what about anemic hypoxia? See anemic hypoxia is answer is both yes and no depending on the cause. See if the problem is in hemoglobin even if you increase the partial pressure of oxygen in blood the amount of oxygen content which will increase is not much. As I told you before that only 0.3 ml is being carried in dissolved form and partial pressure of oxygen in arterial blood is responsible for this dissolved form. So if we increase this partial pressure only this dissolved form is going to increase. So even if you increase the partial pressure of oxygen in blood say suppose six times. So say suppose it becomes 600 millimeter mercury. So this will only increase to 1.8 ml. So this may not be adequate. So in case of anemia increased FIO2 may not be helpful. Rather we should focus on increasing the hemoglobin concentration in blood. But in carbon monoxide poisoning yes increase FIO2 is helpful because what happens that increase in partial pressure of oxygen in blood basically displaces carbon monoxide from hemoglobin and once it happens once hemoglobin becomes free it can bind to oxygen. So it's kind of a competition which goes on between oxygen and carbon monoxide. The increased affinity which carbon monoxide has for hemoglobin can be overcome by increasing the partial pressure of oxygen in blood. So in anemic hypoxia yes in case of carbon monoxide poisoning increase FIO2 is helpful. Now let's come to the last one that is hypoxic or hypoxemic hypoxia. So answer in this case is definitely yes in most cases. See high altitude you might have heard that those who people go to very high altitude sometimes they carry these oxygen cylinders with them so that they can inhale the oxygen with higher percentage. So even if atmospheric pressure goes from 760 millimeter mercury to say suppose 600 millimeter mercury if we increase the oxygen content from 21 percent to 40 percent. So basically we are compensating for decrease in the atmospheric pressure because now the oxygen content of the gas which we are delivering is something like this. So definitely it will help. Then closed displaces will we need increase FIO2? But why do we need increase FIO2? Precaution is better than cure. We have to keep the places ventilated but for other parts that is ventilation defects, diffusion defects and VQ defects which are happening at the level of the lung. Increased FIO2 is definitely helpful because with these defects what is the problem? The problem is decrease in partial pressure of oxygen in blood. So when we inhale oxygen at higher partial pressure basically the partial pressure at the level of the alveoli increases and if that increases then obviously the diffusion will increase. So again the fixed law which says that partial pressure difference is one factor which is responsible for weight of diffusion. So by increasing the alveolar partial pressure we can increase the arterial partial pressure of oxygen. So definitely it will work. So in simple terms we see in ischemic and histotoxic hypoxia it will not help. In hypoxic hypoxia and anemic hypoxia it will be of help. So I hope that with this video you have understood the concept of hypoxia properly. If you like the video do press the like button, share the video with others and don't forget to subscribe to the channel Physiology Open. Thank you.