Methemoglobinemia

By Ilene B. Anderson, PharmD; and Susan Y. Kim, PharmD

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Objectives

1. Understand the pathophysiology of methemoglobinemia.
2. Identify the typical clinical features of methemoglobinemia.
3. Clarify the diagnosis of methemoglobinemia in light of laboratory and bedside tests, exposure to causative agents, and clinical symptoms.
4. Recognize the limitations of the pulse oximeter and standard blood gas measurement when evaluating oxygen saturation in the presence of methemoglobin.
5. Understand the rationale for, and pitfalls in, methylene blue therapy.

Key words

cyanosis; co-oximeter; hemolysis; methemoglobinemia; methylene blue; poisoning

Abbreviations

G6PD = glucose-6-phosphate dehydrogenase; Mhgb = methemoglobinemia; NADH = nicotinamide-adenine dinucleotide; NADPH = the reduced form of nicotinamide-adenine dinucleotide phosphate

Methemoglobin is a type of dyshemoglobin in which iron molecules in the hemoglobin of circulating erythrocytes are in the oxidized ferric (Fe³⁺) state rather than the normally reduced ferrous (Fe²⁺) state. Methemoglobin is incapable of carrying oxygen. In healthy adults, about 1% of total hemoglobin is normally present in the methemoglobin form. Methemoglobinemia (Mhgb) is defined as an increase in methemoglobin formation above this normally low level. Mhgb is an unusual but potentially life-threatening condition that can present a significant challenge to the treating clinician. A thorough understanding of the pathophysiology of Mhgb, together with its diagnostic and therapeutic pitfalls, is essential for prompt recognition and appropriate treatment of this disorder.

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