College Answer

When there is a difference in the oxygen saturation between a pulse oximeter reading and a co- oximeter reading, seen with CO poisoning and other drugs which result in a methemoglobinemia.

Discussion

Though not a formally accepted term to describe this phenomenon, the "gap" is a well recognised feature of dyshemoglobinaemia. It develops when the pulse oximeter reads a certain saturation, and the ABG machine or CO-oximeter returns a different reading. The "gap" in saturation readings is vaguely representative of the concentration of the abnormal haemoglobin in the blood.