Chronic methemoglobinemia: Improving hemoglobin saturation monitoring during anesthesia

Methemoglobin interferes with the accuracy of pulse oximetry data. Methemoglobinemia is caused by many factors, both congenital and acquired. However, the increasing usage of dapsone, which converts hemoglobin to methemoglobin, is increasing the number of patients with methemoglobinemia. We present the case of a patient with dapsone-induced methemoglobinemia who was successfully treated with methylene blue, which converts methemoglobin back to hemoglobin. Methemoglobin interferes with the accurate pulse oximetric monitoring of oxyhemoglobin saturation [1—5]. Generally, in the presence of methemoglobin levels greater than 1 g’dL^-1, pulse oximeter readings above 85% underestimate the saturation of functional hemoglobin (the hemoglobin functioning normally with respect to oxygen); readings below 85% overestimate the saturation [2]. These inaccuracies are exaggerated as methemoglobin levels increase or functional hemoglobin saturation decreases [2]. In addition to other causes of methemoglobinemia, the spread of the HIV epidemic and the increasing number of HIV patients receiving dapsone (diaminodiphenylsulfone) as prophylactic treatment forPneumocystis carinii will expand the population of patients with clinical methemoglobinemia who require anesthesia for surgery [6-8]. Virtually all patients treated with dapsone will have levels of methemoglobin sufficient to interfere with pulse oximetric monitoring [7]. We present a case that illustrates these problems and emphasizes the importance of treating methemoglobinemia before the induction of anesthesia to ensure accurate intraoperative monitoring of oxyhemoglobin saturation.