[Music] so in the last lesson i did talking about ards i specifically mentioned about our pf ratio and that it was being used to categorize the severity of arts in this lesson i wanted to quickly review over what the pf ratio is and really what it means so essentially our pf ratio is a shortened way to say our pao2 to fio2 ratio so pao2 as we know is our partial pressure of arterial oxygen and our fio2 is the fraction of inspired oxygen or the percentage of oxygen that we're delivering to our patients really what this means is that we're comparing the concentration of oxygen in arterial blood to the amount of oxygen that's being delivered to our patient what this does is this allows us to assess the severity of hypoxemia and we can really use it as a tool to be able to trend the progression of respiratory failure with it we can quickly assess which patients are oxygen stable who needs intervention to try to avoid intubation and really who needs to be intubated early so how is it that we calculate it it's actually pretty simple so we just take our pao2 and then divide it by the fio2 when you do this though you want to make sure that for the fio2 that we're using it as a decimal and not a percentage so for example we have room air which is 21 oxygen this would be 0.21 now in a normal healthy individual our pf ratio is going to be usually greater than 400 so normally we would say we would expect a pao2 of 100 on room air which is 21 oxygen so to figure out our pf ratio here we would take our pao2 of 100 divide that by our fio2 of 0.21 which comes out to 476. now let's quickly diverge here and just talk a little bit about our pao2 versus our spo2 so our pao2 is essentially basically measuring dissolved oxygen that's in the blood and this requires an arterial blood gas in order to get this so what we can do in a pinch is we can actually use our patients spo2 and then estimate what their pa 02 would be and we do this by there's conversion charts that are out there and i'll kind of put one up here just to kind of show you some of the values here but these are basically just derived from moving along the oxyhemoglobin disassociation curve and kind of figuring out what for each spo2 level what the pao2 would be so here's this chart here with those associated values and really for the spo2 if you can remember this is a measure of the percentage of hemoglobin that's bound by oxygen now like i just talked about with that oxyhemoglobin dissociation curve that this is that relationship between spo2 and pao2 as our pao2 drops we begin to see a pretty sharp drop in our spo2 but once we're towards the top right so as that pao2 is increasing it doesn't necessarily mean that we have a higher spo2 especially once we reach full saturation because at that point you can continue to drive the pao2 up and the oxygen can only be 100 saturated now knowing that we have this relationship between our pao2 and our spo2 and that this is really going to be directly driven by the patient's ability to have that gas exchange take place as well as the availability of oxygen that we're providing to them if we have a patient that is receiving oxygen and has a sub-optimal pao2 the only way that we could really see the impact of changes in our ph o2 would be to reduce them or take them off of oxygen which is probably not a good idea so in comes the true value of our pf ratio and what the pf ratio allows us to do is it allows us to assess the changes in hypoxemia without actually removing our patient from oxygen so i want to go through a couple examples here do some pf calculations give you guys an opportunity to go through this as well just to help cement that and then kind of talk through what it is that this does for us so we're going to take our first patient here so this is a normal healthy patient they're on room air they have a pao2 of 100 and their satin 97 98 which if you look at our oxy hemoglobin dissociation curve this is what we would expect for a pao2 of 100. now here we already did this calculation we know that their pf ratio is 476. so now let's take a look at patient number two this patient is on 50 oxygen so 50 percent fio2 they also have a pao2 of 100 and they are also sating 97 on the spo2 so we need to figure out what this patient's pf ratio is so again we do pao2 divided by fio2 and that gives us our pf ratio so here we would insert our pao2 of 100 divide that by our fio2 of 0.5 which would be the decimal version of 50 and then doing this math we come up with a pf ratio of 200. now based on this information we can actually calculate what his pao2 would be if he was just on room air so i'm not going to bore you with the details of why this works but if we just use some basic algebra and flip our equation around we can say that our pf ratio times our fio2 equals our pao2 so here we plug in the values that we know so our pf ratio of 200 multiplied by our fio2 and our case here we want to figure out how they are in room air so this is going to be 0.21 so this equals a pao2 of 42 so that's pretty low so if we actually go to our chart or our oxyhemoglobin dissociation curve we can see that a pao2 of 42 would actually give us a spo2 of just 75 so if this person was on room air they would be sadding 75 this patient is extremely hypoxic we would not want to reduce their o2 and we really need to keep close eye on this person so let's take a look at a third patient so this patient here is on 30 fio2 they also have a pao2 of 100 and they also have a sat of 97 so i want you guys to do this one what would their pf ratio be so we would know that we would take our pao2 of 100 divide that by our fio2 of 0.3 for 30 and this equals a pf ratio of 333. hopefully you guys were able to get that now we want to calculate out what their pao2 would be on room air so i'm going to give you guys a minute to to do that one if you want to so here we flip things around a little bit so we're going to take our pf ratio of 333 multiply that by the fio2 of room air of 0.21 and this equals a theoretical pao2 of 70. so while this isn't necessarily great but if we actually look at our chart or our disassociation curve we can see that this still would give us a stat of like 93 or 94 percent which is actually okay all right let's go through one more example just to really cement this home for you guys uh patient number four they've got 100 fio2 their pao2 is also 100 so as a result we're going to see a sat of 97 as well what is the pf ratio of this patient so here we take our pao2 of 100 divided by our fio2 which we're going to express as 1.0 for 100 and this gives us a pf ratio of 100 as you can imagine this is not good so just for fun what would this patient's pao2 be on room air so here we're going to take our pf ratio of 100 multiply that by our fio2 of 0.21 so this gives us a whopping pao2 of 21. we're probably going to be coding this one if this person was on room air so the interesting thing to see with these four patients is every single one of them had the same pao2 of a hundred yet as you can see each of them has a completely different respiratory status so having this pf ratio we can now objectively assess each patient's hypoxemia and really have a quantitative value for the degree of respiratory failure that they have the other big value of this too is that we can also trend this value over time to determine if they are worsening if they're staying stable or if they're actually getting better so this gives us information at our disposal on like we talked about who needs that closer monitoring who we should be escalating therapy to try to avoid intubating or those that really we should be looking at intubating early so overall just a real quick easy tool to have a good understanding of your patient's respiratory failure and kind of what's going on with that as well as this is also the criteria that we use when we're evaluating the degree of ards that our patient has going on as well as some of the possible next interventions that we put in place which i am going to talk about in the next lesson 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 down below head on over there and check out some of the perks that you guys get for doing just that as well as check out some of the links to other nursing gear as well as some awesome 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