Methaemoglobin

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.

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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.