A question about CO poisoning - gmn05

[ArchivalUser]

Here is a question form Kaplan Qbank:
A patient's arterial blood gas sample showed that oxygen saturation was reduced despite normal PaO2 level. Which of the following most likely caused this shift of the patient's oxygen-hemoglobin dissociation curve?
A. CO poisoning
B. Decreased PCO2
C. Decreased pH
D. Decreased temperature
E. Decreased 2,3-BPG
I know that C is right, but why not A? Could anyone explain why choice A is not correct?

[ArchivalUser]

CO >> increase affinity of already binding O2 to Hb ( curve shifts to the left ) >> decrease unloading O2 in the tissue BUT still CO has more affinity to bind Hb than O2 (200 times ) .

[ArchivalUser]

I know the affinity of O2 during CO poisoning is increased, but how is the O2 saturation?

[ArchivalUser]

oxygen saturation will be 100%!!!!! this is very important to know!

[ArchivalUser]

@chapeau You mean ABG that calculates O2 saturation from PaO2 will show a false 100% ？

[ArchivalUser]

First, think logically: All of the options ---CO poisoning, decreased PCO2 (Respiratory alkalosis), Decreased temperature, Decreased 2,3 BPG causes left shift of oxygen dissociation curve except option C which is decreased ph (acidosis). Acidosis or increased H+ causes right shift of oxygen dissociation curve.

I think the question will have some clue about CO poisoning in the stem. The above question is weird in this sense. Decreased oxygen saturation with normal PaO2 level is seen in CO poisoning and cyanide poisoning but right shift of oxygen dissociation is seen only in decreased pH.

[ArchivalUser]

good explaination drforever1891

[ArchivalUser]

@drforever1891 Thanks!!!

[ArchivalUser]

@drforever,your explanation is right but in cyanide poisoning,oxygen saturation is normal.However,it's decreased in methemoglobinemia.

[ArchivalUser]

http://www.pulseox.info/pulseox/limits5.htm