CO poisoning, P50, PO2, and O2 content - USMLE Forums
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Old 09-24-2010
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RBC CO poisoning, P50, PO2, and O2 content

If CO poisoning will shift the O2-Hb dissociation curve to the left, meaning that it will decrease P50 (the partial pressure of O2 needed to bind 50% of Hb), then won't it decrease PO2 (concentration of O2 dissolved)?

According to my notes, it will decrease P50, but will have no change on PO2. I don't understand this. Also, it says CO poisoning will decrease O2 content. Why is that?
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Old 09-24-2010
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O2 content of blood consist of PaO2 (dissolved O2) and Hemoglobin(%saturation), with hemoglobin contributing to almost all of O2 content. So in CO poisoning, the CO decreases the hemoglobin O2 saturation (much higher binding affinity than O2) so it decreases O2 content. PaO2 is responsible for Hg saturation, but Hg saturation is NOT responsible for determining PaO2 (ventilation is) so in CO poisoning, PaO2 will be unaltered because CO has no effect on ventilation.
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Old 09-24-2010
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Quote:
Originally Posted by Haisook View Post
If CO poisoning will shift the O2-Hb dissociation curve to the left, meaning that it will decrease P50 (the partial pressure of O2 needed to bind 50% of Hb), then won't it decrease PO2 (concentration of O2 dissolved)?

According to my notes, it will decrease P50, but will have no change on PO2. I don't understand this. Also, it says CO poisoning will decrease O2 content. Why is that?
I will add my two cents, please feel free to correct or ask for explanation if its not understandable.

Oxygen in blood is stored in two forms, O2 dissolved in blood and O2 attached to Hb which is also known as saturation. Since CO has higher affinity to Hb, the saturation of Hb will decrease but the O2 dissolved in blood won't because the pressure required to keep the oxygen dissolved in blood cannot be increased in human body beyond a limit. Now since O2 dissolved in the blood is same as before but the O2 attached to Hb is decreased, the O2 content has decreased leading to hypoxia.
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Old 09-24-2010
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Sorry if my above post is redundant but i was typing and i took ages which is why i posted my response late by which time their was an answer already posted.

By the way, the reason for decrease in P50 is because since the O2 saturation has decreased, whatever Hb binding sites are available can be occupied at a lower concentration than before. Its like lots of seats are available hence lots of candidates will be able to fill those seats but if the seats are lesser in number then less candidates will be able to fill those seats.
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Old 09-24-2010
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Yup, the PaO2 (amount dissolved in plasma) is unchanged in CO poisoning, SaO2 (amount bound to Hb) is changed due to the fact that Hb has a higher affinity to CO in place of O2, there was a good example above where in an overcrowded classroom if there is a seat that you always sit at has been taken you're still going to be in a classroom but just not in a seat anymore.

so the classroom in this case = plasma, and the seat = hemoglobin.

thus CO steals O2's spot on the Hb, but the amount of O2 in plasma will remain the same.

minor things to note about CO poisoning

***Cherry red color instead of cyanosis (even though there is Hypoxia), its because of the red pigment of CO, and that the most striking S&S is headche***

IMP sidenote "CO poisoning and CN poisoning can occur together in cases of housefires"
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Old 02-28-2015
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Hb has 4 sites which O2 can bind.
Site 4(97 percent saturation)= occupied when po2 is 100
Site 3(75 % saturation)= occupied when po2 is 40
Site 2(50 % sturation)= ooccupied when po2 is 26( which is called p50)n

Now CO has increased affinity for hb. Lets suppose it occupied position 3 and 4 and only position left for o2 to bind will be position 2 which if occupied by o2 NOW will give full saturation, was previously occupied at 26 mmhg and gave only 50 percent saturation. So hb will be fully saturated (with a lot of CO plus some O2) at this 26mmhg partial pressure. Moving down from this point lets suppose 15 mmhg oxygen comes off the site 2 and now only sites 3 and 4 remain occupied (by CO) so although not saturated by o2 the hb is still half saturated(p50) but the p50 now has decreased to 1the new value of 15mmhg.
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Other relatively simple way of understanding is that binding of CO to hb increases the Hb affinity to bind to oxygen. Leading to to a decreased p50, less partial pressure required to keep the O2 attatched to hb.
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Old 04-27-2016
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Thanks for the information.
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