TABLE 93.4 SYMPTOMS ASSOCIATED WITH MHB LEVELS Methemoglobin % Total concentration hemoglobin (g/dL) Symptoms (assuming Hb 15 g/dL) 70 Death Reprinted with permission from Wright RO, Lewander WJ, Woolf AD Methemoglobinemia: etiology, pharmacology, and clinical management Ann Emerg Med 1999;34:646–656 Management/Diagnostic Testing The treatment of methemoglobinemia depends on the clinical severity In all cases, attempt to identify and remove the causative oxidant stress If symptoms are mild after oxidant exposure, therapy may be unnecessary RBCs with normal metabolism will reduce the MHb in several hours In general, treat patients with MHb level >20% with to mg/kg of methylene blue as a 1% solution in saline infused intravenously over minutes Administer a second dose if symptoms are still present hour later Patients with a significant concurrent medical condition, especially cardiopulmonary conditions, should be considered for treatment at MHb levels starting at 10% Methylene blue is an oxidant at high dosages, so total dosage should not exceed mg/kg to avoid paradoxical methemoglobinemia Use methylene blue with extreme caution in patients with G6PD due to the risk of hemolytic anemia The mechanism of action of methylene blue relies on NADPH These patients may not produce sufficient quantities of NADPH to respond to this therapy; however, some patients have partial enzyme activity Methylene blue at lower doses (0.3 to 0.5 mg/kg/dose) may lower MHb levels without causing significant hemolytic anemia The addition of ascorbic acid to mg/kg/day may benefit G6PD patients Consider exchange transfusion in patients who fail methylene blue treatment or have absent G6PD activity Clinical Indications for Discharge or Admission Even if treatment with methylene blue or ascorbic acid in the ED is successful, admit any child with symptomatic methemoglobinemia to the hospital for close observation, especially if the etiology is unknown Some oxidizing agents such as dapsone and