Methemoglobinemia and transesophageal echo.

Methemoglobin (MHb) is the oxidized form of Hemoglobin (Hb) containing iron in its ferric (Fe3+) rather than ferrous (Fe2+) state. Under physiologic conditions, diaphorase II accounts for only a small percentage of the red blood cell reducing capacity but can be pharmacologically activated by exogenous cofactors. Methemoglobinemia is an abnormal elevation of MHb levels resulting in impaired oxygen delivery to tissues as well as a left shift of the oxygen-Hb dissociation curve.