CO Poisoning

That was a good case by Neil below.

A study in Annals showed that venous measurements of CO level accurately reflect arterial levels. Ann Emerg Med. 1995 Apr;25(4):481-3.

I will make the follow comments advocating arterial blood sample, though:

1. Few patients in the above study had high CO levels (>20%)
2. Pulse oximeter will give falsely elevated O2 sat values even in cases of extremely high COHb levels (Ann Emerg Med. 1994 Aug;24(2):252-5). In the setting of concomitant hypoxia (smoke inhalation), only an arterial sample will give an accurate measurement of arterial oxygen content when measured by lab CO-oxymetry Chest. 1998 Oct;114(4):1036-41.
3. ABG will allow evaluation for metabolic acidosis, which is very important, as several studies have shown that the degree of acidosis is a better predictor of severity than absolute CO level. J Accid Emerg Med. 1999 Mar;16(2):96-8 This is partially b/c CO also binds intracellular cytochromes needed for oxidative metabolism.
-Alex