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Methemoglobinemia
Methemoglobinemia (or methaemoglobinaemia) is a disorder characterized by the presence of a higher than normal level of methemoglobin (metHb, i.e., ferric () rather than ferrous () haemoglobin) in the blood. Methemoglobin is a form of hemoglobin that contains ferric () iron and has a decreased ability to bind oxygen. However, the ferrous iron has an increased affinity for bound oxygen.〔(【引用サイトリンク】url=http://emedicine.medscape.com/article/204178-overview )〕 The binding of oxygen to methemoglobin results in an increased affinity of oxygen to the three other heme sites (that are still ferrous) within the same tetrameric hemoglobin unit. This leads to an overall reduced ability of the red blood cell to release oxygen to tissues, with the associated oxygen–hemoglobin dissociation curve therefore shifted to the left. When methemoglobin concentration is elevated in red blood cells, tissue hypoxia can occur.
==Signs and symptoms==
Signs and symptoms of methemoglobinemia (methemoglobin level above 1%) include shortness of breath, cyanosis, mental status changes (~50%), headache, fatigue, exercise intolerance, dizziness and loss of hairlines.
Patients with severe methemoglobinemia (methemoglobin level above 50%) may exhibit seizures, coma and death (>70%).〔 Healthy people may not have many symptoms with methemoglobin levels below 15%. However, patients with co-morbidities such as anemia, cardiovascular disease, lung disease, sepsis, or presence of other abnormal hemoglobin species (e.g. carboxyhemoglobin, sulfhemoglobin or sickle hemoglobin) may experience moderate to severe symptoms at much lower levels (as low as 5–8%).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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