Suspect methemoglobinemia if you have a patient with persistent cyanosis, tachypnea, low pulse oximetry, and a lack of response to 100% oxygen therapy, or an elevated PaO2 on ABG and a low O2 sat on pulse ox.
Methemoglobin results from exposure to chemicals that oxidize the ferrous iron in hemoglobin to the ferric state, resulting in a functional anemia. Usually a low level is reduced back to hemoglobin by cytochrome b5, NAD, G6PD, and glutathione reduction enzyme systems.
Diagnose by confirmed by a methemoglobin level, although the most convenient and rapid test of choice is multiple wave co-oximetry. (not the standard 2 wave pulse ox).
Treat if they are symptomatic; use methylene blue, 1 mg/kg slow push. Patients should improve in one hour. Use with extreme caution in patients with G6PD disease (if at all).