RESUMO
Intraoperative cyanosis is an utmost emergency for anesthesiologist. If the patient has adequate control ventilation, and normal cardiac pulmonary physiology, then methemoglobinemia must be considered. Reported here is a normal female with dark color lip on the second day after her second parturition and was undergoing tubal ligation. Twenty minutes after induction of general anesthesia and endotracheal intubation, dark blood at the incision site was noted by the operator. After emergent check up of the anesthesia machine, tubings, breathing sound and arterial blood gas, there was only one suspicion left. Methemoglobinemia was confirmed by the hematological examination. Methemoglobinemia is a product from the oxidation of the iron atom in the heme ring when oxygen dissociates from it. This process exists in nature, but can also be induced by nitrate or nitrite-containing drugs or foods or benzene-like organic compounds. Methemoglobinemia can be differentiated from normal hemoglobin by mass spectrometry. If acute illness develops, patients should be treated with methylene blue. Otherwise ascorbic acid will do. This case is reported to remind all anesthesia personnel about one of the rare but serious hemoglobinopathy.