Methemoglobin
Methemoglobin (British English: methaemoglobin) (pronounced "met-hemoglobin") is a form of the oxygen-carrying metalloprotein hemoglobin, in which the iron in the heme group is in the Fe3+ (ferric) state, not the Fe2+ (ferrous) of normal hemoglobin. Methemoglobin cannot release bound oxygen, unlike oxyhemoglobin. 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]
Contents |
[edit] Common causes
- Reduced cellular defense mechanisms
- Children younger than 4 months exposed to various environmental agents
- Cytochrome b5 reductase deficiency
- G6PD deficiency
- Hemoglobin M disease
- Pyruvate kinase deficiency
- Various pharmaceutical compounds
- Local anesthetic 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 Paracetamol (i.e. acetaminophen, tylenol)
[edit] 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]
[edit] 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
[edit] See also
[edit] 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.
[edit] External links
|
||||||||||||||||||||||||||||||||
