in our bodies there's a relationship that exists when it comes to oxygen levels in our blood this relationship looks at the ties between how much oxygen is actually bound in our blood cells at various oxygenation levels and it really gives us a representation of how readily oxygen is available to be used we often express this relationship as a graph something that we call the oxygen hemoglobin dissociation curve which i'll discuss now [Music] all right you guys welcome back to another video lesson from icu advantage my name is eddie watson and my goal is to give you guys the confidence to succeed in the icu by making these complex critical care subjects easy to understand i truly hope that i'm able to do just that and if i am i do invite you to subscribe to the channel down below when you do make sure you hit that bell icon and select all notifications so you never miss out when i release a new lesson as always the notes for this lesson as well as all the previous videos are available exclusively to the youtube and patreon members you can find links to join both of those down in the lesson description below also don't forget to head over to icuadvantage.com or follow that link down in the lesson description to take a quiz on this lesson test your knowledge while also being entered into a weekly gift card as well as don't forget that you can help support this channel through the purchase of an icu advantage sticker again those are found at the website icuadvantage.com forward slash support link down in the description now understanding the oxygen hemoglobin dissociation curve is important especially in times when our patient's physiology can be altered in ways that have an impact on this dissociation curve given that this curve represents the availability of oxygen to be used alterations in this availability aka alterations in this curve can have real effects on our patients so before we get into talking about the curve itself we do have to talk about a couple things first the first thing is going to be saturation and when we're talking about saturation what we're really talking about is the amount of oxygen that is bound to hemoglobin in the blood most commonly we use a pulse ox and get a spo2 or we can also use the saturation from an arterial or venous blood gas if you remember from our anp days we have our red blood cells now this cell here is without much of the normal machinery of other cells including its nucleus and its main purpose is really to transport oxygen the way that most of the oxygen is transported is through the help of the hemoglobin molecule now the hemoglobin molecule here actually consists of four subunits so we have two alpha and two beta and they have iron at the core of each subunit and iron is the binding site for an oxygen molecule each subunit combined with one oxygen molecule making the maximum possible of four oxygen molecules bound per molecule of hemoglobin and then each one of these red blood cells contains something like 270 million hemoglobin molecules so that's a lot of oxygen molecules the structure of the hemoglobin molecule isn't constant though its initial stage where it has no oxygen is something that we refer to as tense once we have oxygen that binds to the first subunit it causes a conformational change in the next subunit something that we call relaxed this change actually increases the affinity or the attractiveness of oxygen to bind when oxygen then binds to that next subunit it again changes the next one making it even more relaxed which further increases the affinity for oxygen until we eventually have the last subunit bound which also has the greatest affinity and this changing affinity is something that is referred to as cooperativity so as more oxygen is bound there's a greater affinity to bind with more oxygen once we have all four subunits that have an oxygen molecule bound this is something that we refer to as saturated and when we look at the blood as a whole we can evaluate what percentage of hemoglobin molecules are saturated with oxygen and this is our spo2 reading that we get and this can range anywhere from zero to a hundred percent so for an example if we have every hemoglobin molecule that has three of the four subunits bound the saturation of our patient would be 75 percent all right so the next thing we need to talk about is partial pressure at its most basic level this is the pressure of a gas in our example here the gas that we're looking at is oxygen the partial pressure tells us essentially how much oxygen is available the more oxygen that's available to the blood the higher the partial pressure this amount or partial pressure here that is available is going to be roughly equal to the amount of oxygen that's dissolved in the blood but not bound to hemoglobin and this is what's reflected as our pao2 on our abg now important to know that while dissolved oxygen in the blood does contribute to overall oxygen availability it actually contributes just a fraction of the oxygen when compared to hemoglobin to understand this more i'm going to link to a lesson above where i do talk about oxygen delivery all right so now that we got those two concepts out of the way let's actually talk about the oxygen hemoglobin dissociation curve so this is also something that you'll hear sometimes refer to as the oxyhemoglobin dissociation curve or the oxygen dissociation curve if we look at the name dissociation means to separate or disconnect so just by looking at the name of this we can infer that this is about disconnecting or separating oxygen from hemoglobin all right so here is the curve here and for reference along the x-axis we have the partial pressure of oxygen and along the y-axis we have the percent of hemoglobin saturation now the curve itself has an s-shaped or a sigmoid appearance and this is something that i'll explain more in just a minute what this curve represents is that for any given availability of oxygen aka our partial pressure at the bottom how much hemoglobin is going to be saturated with oxygen so for example if we look at a partial pressure of 50 millimeters of mercury we would expect if we go up here a saturation of 85 now towards the right of the curve here you can see that we have this plateau as we really approach 100 saturation and what this means is that at this point as we increase the partial pressure more and more we really don't see much of an increase in the saturation and obviously once fully saturated at 100 it's impossible to go any further than that so our max limit would be at 100 here now just like i talked about with the hemoglobin molecule as it becomes more bound with oxygen it has a greater affinity for binding more oxygen and thus in an oxygen-rich environment the hemoglobin is going to have a greater affinity for that oxygen increasing saturation and we can really think about this if we think about the physiology and the capillaries of the alveoli in the lungs so here co2 is being offloaded and due to the hopefully large availability of oxygen we should come pretty close to fully saturating hemoglobin now as we start to move left along the curve we start to see the curve dropping off and it gets quicker the further left we move so what this tells us is that as the availability of oxygen drops off so as our pao2 is dropping we begin to see less and less hemoglobin being saturated with oxygen and so if we think about this what this really means is that the oxygen is being unloaded off of the hemoglobin molecule in an environment where there isn't much oxygen available obviously the oxygen can't be used if it's bound to hemoglobin so it needs to be unbound and released in order to be used wherever it's needed and we can think of this as being at the site of capillaries that are nrn tissue that need the oxygen for cellular metabolism there isn't much oxygen available here as it's being used and thus the hemoglobin will begin to unload its oxygen to make that available hopefully this makes sense and from the perspective of our physiology this should make sense that we do want this to happen as mentioned as hemoglobin has less oxygen bound to it it's going to have less and less affinity for oxygen so it actually wants to unload this oxygen easier and this is what accounts for this increasing drop in the curve as our oxygen levels get lower and lower the unloading becomes easier and easier increasing the rate of the descent so knowing all this we can expect that at a particular amount of oxygen availability aka partial pressure we should have a particular level of hemoglobin saturation the curve itself does not account for how much oxygen our patients are getting into their lungs this is just looking at the oxygen that's available at the site of gas exchange if you want to better understand the relationship between the amount of oxygen delivered and the amount of oxygen that's available watch the previous lesson i did talking about the pf ratio i'll link to that up above all right so hopefully that makes sense about what this curve is this isn't the whole story though with all this we can have shifts in our body's physiology that really alters this curve and we call these either right shift or left shift so first let's talk about our right shift so here is an example of a curve and as you can guess it's shifted to the right when compared to our normal curve what a right shift means is that hemoglobin will have less of an affinity for oxygen and it will unload the oxygen easier so we can see this by looking at our example of a pao2 of 50 millimeters of mercury from earlier so normally if you remember we'd expect 85 percent saturation but in this example here we can see that we have somewhere around 75 saturation so for the same given amount of oxygen availability we're going to have less bound in hemoglobin and thus more available to be used when compared to our normal curve so what would cause this right shift so we can think about this in terms of where or when we want oxygen to unload easier to be available to be used and so this is going to be at muscle at n tissue as well as in the placenta there are a couple physiologic things that do drive this and each of these that i'm gonna list out below cause changes in the structure of the hemoglobin molecule that's gonna give it less affinity for oxygen and cause it to unload its oxygen molecule easier so first it's gonna be an increased co2 so at our tissue we expect there to be high levels of co2 as this is the byproduct of cellular respiration another thing is also going to be increased acidosis so here think a lower ph and so increased co2 is obviously going to lower our ph as well but potentially also have the presence of lactic acid and oxygen starved cells clearly we'd want more oxygen to be available to these cells at this point another thing is also an increased 2-3 dpg and this is the byproduct of glycolysis and the presence of hypoxemia our 2-3 dpg levels are going to be increased signaling the need for more oxygen and then finally increase temperature so increased temperature is the result of increased cellular work which requires oxygen and thus we would want to have more available here so all of these examples are going to shift that curve to the right giving oxygen less of an affinity for that hemoglobin meaning it's going to be more available to the tissue that needs it so now let's talk about the left shift so as you can see here we have a curve that is shifted to the left of our normal curve and this has the opposite effect of the right shift here we have hemoglobin that has a greater affinity for oxygen to be bound for any given level of availability so oxygen is not going to be unloaded as readily as before so again for our example here a partial pressure of 50 normally we'd expect 85 in the example of our right shift we were seeing 75 but now here in our left shift we're seeing something that's around 90 so what causes this left shift well again we can think about where this would make sense to happen so in the lungs we want the oxygen to have the greatest affinity to be bound to hemoglobin to maximize the amount of oxygen that can then be delivered to the rest of the body so the physiologic changes that we would see would be the opposite of our right shift so again each of these are going to cause changes to that hemoglobin molecule giving it greater affinity for oxygen so these are going to be things like decreased co2 decreased acidosis so here think increased ph or alkalosis decreased 2-3 dpg decreased temperature and then separately fetal hemoglobin is actually something that has a much greater affinity for oxygen at lower partial pressures and thus it creates a left shift obviously in the womb this is favorable to pull as much oxygen to the baby as possible now one thing to understand about these shifts is they actually happen both directions normally in our body so in the lungs where co2 is low thus typically less acidic has less metabolic activity aka temperature and less 2-3 dpg we would normally see a left shift causing the oxygen to be more readily bound with hemoglobin then when we get to the tissues where all the work is taking place the elevated co2 levels that decreased ph increased temperatures as well as dpg levels this is where we're gonna see a right shift leading to more unloading of oxygen from the hemoglobin so it can be more available for work and all that said we also have various disease processes and changes to our patients physiology that can also shift these curves more one way or the other therefore we would expect to see changes in our patients ability to either release oxygen or to bind it in the first place so one example would be if someone is extremely acidotic systemically then hemoglobin is going to have less affinity for oxygen in the lungs where we want it to bind as much as possible and thus it could impair oxygen delivery to the rest of the body another example would be if we had a septic patient who's in a metabolic acidosis due to an imbalance in oxygen availability at the tissue if we correct their acidosis we shift that curve to the left making it harder for oxygen to unload at the tissue where it's desperately needed so these were two basic examples but i just wanted to get you thinking about the consequences of these shifts on our patient and obviously decisions that are made can and do have impacts on the delivery of oxygen something that is very vital to life so hopefully after all that the oxygen hemoglobin dissociation curve makes a little bit more sense to you guys and perhaps is a little less intimidating when you're looking at it or you're thinking about the concepts of it especially as we talk about those right and left shifts because they are something that's always happening within our patient's body and then especially for the critically ill patient we're going to often have different physiologic changes that are additionally shifting these curves further left or further right which are going to have impacts on either oxygen availability oxygen delivery the binding of oxygen to hemoglobin and really how much of that oxygen is going to be available to the n tissue so i hope that you guys found this information useful if you did please leave me a like on the video down below it really helps youtube know to show this video to other people out there as well as leave me a comment down below i love reading the comments that you guys leave and i try to respond to as many people as i can make sure you subscribe to this channel if you haven't already and a special shout out to the awesome youtube and patreon members out there the support that you're willing to show me and this channel is truly appreciated so thank you guys so very much if you'd be interested in showing additional support for this channel you can find links to both the youtube and patreon membership 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