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#1
09-03-2012
 USMLE Forums Scout Steps History: Step 1 Only Posts: 29 Threads: 17 Thanked 7 Times in 4 Posts Reputation: 17
CO and oxyhemoglobin curve

I know that CO reduces SaO2 and it also causes left shift to oxyhemoglobin dissociation curve (due to increased affinity).

Now a simple calculation is making me confused. In a normal person SaO2 is 97% in 100 mm PaO2 and 75% in 40 mm PaO2. If there is CO poisoning and 50% of his Hb is attached to CO, then the highest SaO2 he can get is 50% in 100 or 40 mm PaO2. This should put the curve BELOW the normal curve until PaO2 gets very low. Once the PaO2 is so low that SaO2 is less than 50% in the normal curve, only then we can appreciate the left shift.

What am I missing here??

#2
09-03-2012
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 89 Threads: 17 Thanked 51 Times in 26 Posts Reputation: 61

You aren't missing a thing (as far as I know!) ... this is exactly what happens in CO poisoning....the curve ascends from zero to 25 mmHg (which is the PO2 [50]) to the left of the normal curve, then totally flattens at that level

the curve is found in BRS physiology I don't know if you have it..
 The above post was thanked by: niloy (09-04-2012)

 Tags Physiology-

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