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Methemoglobin is a form of the oxygen-carrying protein hemoglobin (British English: haemoglobin), in which the iron in the heme group is in the Fe3+ state, not the Fe2+ of normal hemoglobin. Methemoglobin is unable to carry oxygen. It is chocolate-brown in color. The NADH-dependent enzyme methemoglobin reductase (AKA diaphorase I) is responsible for converting methemoglobin back to hemoglobin.

Normally one to two percent of people's hemoglobin is methemoglobin; a higher percentage than this can be genetic or caused by exposure to various chemicals and depending on the level can cause health problems known as Methemoglobinemia. A higher level of methemoglobin will tend to cause a pulse oximeter to read closer to 85% regardless of the true level of oxygen saturation.

Common causes

  • Reduced cellular defense mechanisms
    • Children younger than 4 months exposed to various environmental agents
    • Methemoglobin reductase deficiency
    • G6PD deficiency
    • Hemoglobin M disease
    • Pyruvate kinase deficiency
  • Various pharmaceutical compounds
    • Local anaesthetic agents, especially prilocaine as used in the Bier block
    • Amyl nitrite, chloroquine, dapsone, nitrates, nitrites, nitroglycerin, nitroprusside, phenacetin, phenazopyridine, primaquine, quinones and sulfonamides
  • Environmental agents
    • Aromatic amines
    • Arsine
    • Chlorobenzene
    • Chromates
    • Nitrates/nitrites

Methemoglobinemia in infants

In children, this condition is known as blue baby syndrome, attributed primarily to excessive nitrate intake from drinking well water.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Methaemoglobin". A list of authors is available in Wikipedia.
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