METHAEMOGLOBIN

'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.

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Common causes

Methemoglobinemia in infants

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Common causes

★ Reduced cellular defense mechanisms

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★ Children younger than 4 months exposed to various environmental agents

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★ Methemoglobin reductase deficiency

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★ G6PD deficiency

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★ Hemoglobin M disease

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★ Pyruvate kinase deficiency

★ Various pharmaceutical compounds

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★ Local anaesthetic agents, especially prilocaine as used in the Bier block

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★ Amyl nitrite, chloroquine, dapsone, nitrates, nitrites, nitroglycerin, nitroprusside, phenacetin, phenazopyridine, primaquine, quinones and sulfonamides

★ Environmental agents

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★ Aromatic amines

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★ Arsine

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★ Chlorobenzene

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★ Chromates

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★ 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.

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