She then became somnolent. Her face, ears, lips, and hands were noted to be cyanotic, but her chest was clear to auscultation. EKG was normal. She was extubated two hours later. The diagnosis of methemoglobinemia is supported by a normal p02, generalized cyanosis unresponsive to oxygen.
Diagnosis is confirmed by measurement of the methemoglobin concentration.
Pulse oximeters do not detect methemoglobin, and may give a misleading impression of normal oxygenation. Associated signs and symptoms include CNS depression, seizures, coma and cardiac arrhythmias. The mechanism is the conversion of the ferrous iron into the ferric form in the hemoglobin molecule destroying its oxygen-carrying capacity. At high concentration of methemoglobin, there may be a brown discoloration of the skin and blood. Methylene blue, a reducing agent, causes rapid reversal and corrects the left shift of the oxygen-hemoglobin dissociation curve.