Late diagnosis of congenital methemoglobinemia in an elderly patient during cardiac surgery.

METHEMOGLOBIN (metHb), resulting from the oxidation of iron in hemoglobin from the ferrous state to ferric state, cannot bind oxygen and thereby leads to a displacement of the hemoglobin dissociation curve to the left. It can, therefore, impair tissue oxygen delivery. Methemoglobinemia, which is characterized by the presence of a higher-than-normal level of metHb, either can be congenital (due to Hb variants or deficiency of enzymes that convert metHb back to Hb) or acquired (caused by drugs or toxins). During the perioperative period, anesthesiologists should suspect methemoglobinemia when discrepancy is seen between arterial oxygen saturation and arterial partial pressure, with low SpO2 and normal PaO2. Diagnosis of methemoglobinemia requires measurements of metHb using a co-oximeter analyzer. The authors describe a case of congenital methemoglobinemia in a 78-year-old man, whose diagnosis was made during cardiopulmonary bypass (CPB) for cardiac surgery. Methemoglobinemia was undiagnosed 1 week before, despite the occurrence of low oxygen saturation and generalized seizures during a cervical block for carotid endarterectomy.