Methemoglobinemia diagnosed as a consequence of cardiopulmonary bypass.

Clinically significant methemoglobinemia is rare and difficult to diagnose when other causes of cyanosis are likely. We report a patient in whom unstable angina pectoris and chronic obstructive pulmonary disease were assumed to be responsible for preoperative cyanosis. The use of cardiopulmonary bypass for aortocoronary grafting enabled the clinical diagnosis of methemoglobinemia to be made.