Methemoglobin (British English: Methaemoglobin) (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 cannot carry oxygen. It is a bluish chocolate-brown in color. The NADH-dependent enzyme methemoglobin reductase (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.[1]
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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 (e.g. p-nitroaniline , patient case)
- Arsine
- Chlorobenzene
- Chromates
- Nitrates/nitrites
- In cats
- Ingestion of acetaminophen (tylenol)
Therapeutic Uses
Amyl nitrite is administered to treat cyanide poisoning. It works by converting hemoglobin to methemoglobin, which allows for the binding of cyanide and the formation of non-toxic cyanomethemoglobin. [2]
Methemoglobin Saturation
Methemoglobin saturation is expressed as the percentage of hemoglobin in the methemoglobin state; That is MetHb as a proportion of Hb.
- 1-2% Normal
- Less than 10% metHb - No symptoms
- 10-20% metHb - Skin discoloration only (most notably on mucus membranes)
- 20-30% metHb - Anxiety, headache, dyspnea on exertion
- 30-50% metHb - Fatigue, confusion, dizziness, tachypnea, palpitations
- 50-70% metHb - Coma, seizures, arrhythmias, acidosis
- Greater than 70% metHb - Death
References
- ^ Denshaw-Burke, Mary (2006-11-07). "Methemoglobinema". http://www.emedicine.com/med/TOPIC1466.HTM. Retrieved 2008-03-31.
- ^ Vale, J. A. (2001). "Cyanide Antidotes: from Amyl Nitrite to Hydroxocobalamin - Which Antidote is Best?". Toxicology 168 (1): 37–38.
See also
External links
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