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Author: Nick Hatch, M.D. Educational Pearls: Methemoglobinemia is when the iron in hemoglobin is in the Fe3+ (ferric) state rather than the normal Fe2+ (ferrous) state. Methemoglobin cannot release oxygen at the tissues. Symptoms include cyanosis, headache, tachycardia, dyspnea, and lethargy. Suspect in setting of hypoxia that does not improve with oxygenation, and clinical cyanosis with a normal PaO2 on ABG. Treatment is methylene blue which reduces the iron back to the ferrous state. Causes can be Dapsone, Lidocaine, Benzocaine. References: Agarwal N, Nagel RL, Prchal JT. Dyshemoglobinemias. In: Disorders of Hemoglobin: Genetics, Pathophysiology, and Clinical Management, 2nd ed, Steinberg M (Ed), 2009. P.607 Cortazzo JA, Lichtman AD. (2014). Methemoglobinemia: a review and recommendations for management. Journal of Cardiothoracic and Vascular Anesthesia. 28:1043. Darling R, Roughton F. (1942). The effect of methemoglobin on the equilibrium between oxygen and hemoglobin. American Journal of Physiology . 137:56.