Q: All of the following blood tests are vital on a patient during first few hours of presentation in suspected cyanide poisoning except?
A) Anion gap
C) Arterial blood gas (ABG)
D) Venous blood gas (VBG)
E) Blood cyanide level
Two major teaching objectives for the above question are:
First, the turn around time for blood cyanide level is long and even if available the results are not reliable due to possible improper storage conditions.
Second, besides history, physical exam and other lab tests, simultaneous ABG and VBG can be very helpful. A narrowing of the venous-arterial PO2 gradient due to venous hyperoxia is one of the features of cyanide-poisoning. Cyanide inhibits cellular oxidative phosphorylation. This causes a marked decrease in peripheral tissue oxygen extraction from the blood, resulting in venous hyperoxia.
Johnson RP, Mellors JW. Arteriolization of venous blood gases: a clue to the diagnosis of cyanide poisoning. J Emerg Med 1988; 6:401.