Methemoglobinemia is a life-threatening condition that can be congenital or acquired. It is characterized by the inability of hemoglobin to carry oxygen because the ferrous part of the heme molecule has been oxidized to a ferric state. Acquired methemoglobinemia is due to medications or chemicals that cause the rate of methemoglobin formation to exceed its rate of reduction.1–,9 These chemicals and drugs include nitrites, aniline, dapsone, phenazopyridine, and topical anesthetics such as benzocaine and lidocaine.2–,9 

The case study (shaded box) describes the development of acquired methemoglobinemia in a 73-year-old woman after trans-esophageal echocardiography in which a topical anesthetic containing benzocaine was used.

Hemoglobin consists of 4 heme groups, each containing an iron atom. Each atom is capable of binding with oxygen only if the iron is in the reduced or ferrous state (Fe2+). Removal of an electron (oxidation) from a reduced iron...

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