Methemoglobinemia occasionally causes cyanosis particularly in congenital methemoglobinemia. Avoidance of exposure to oxidizing agents is important for patients with congenital methemoglobinemia because of their deficient enzymatic pathways and decreased oxygen-carrying capacity. Here, we present a patient with preoperatively undiagnosed congenital methemoglobinemia who underwent uterine myomectomy under general anesthesia. The patient was a 35-year-old woman who displayed a low pulse oximetry reading of 91% prior to induction of anesthesia. Methemoglobinemia was first suspected intraoperatively because of a mismatch of SpO2 of finger pulse oximetry and SaO2 of arterial blood, and was later confirmed by multiple-wavelength CO-oximetry. The pathophysiology, etiology, clinical manifestations, anesthetic considerations, and treatment options of methemoglobinemia are discussed.
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