Methaemoglobin

Overview
Methemoglobin (pronounced MET-hemoglobin) 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.