A derivative of HAEMOGLOBIN in which the iron has been oxidised from ferrous to ferric form. It does not combine with oxygen and therefore plays no part in oxygen transport. Normal concentration of methaemoglobin in red blood cells is less than 1 per cent of the total haemoglobin. When a large concentration of the haemoglobin is in the form of methaemoglobin, the patient will suffer from HYPOXIA and will be cyanosed (see CYANOSIS). Most cases of METHAEMOGLOBINAEMIA are due to chemical agents.
n. a substance formed when the iron atoms of the blood pigment *haemoglobin have been oxidized from the ferrous to the ferric form (compare oxyhaemoglobin). The methaemoglobin cannot bind molecular oxygen and therefore it cannot transport oxygen round the body. The presence of methaemoglobin in the blood (methaemoglobinaemia) may result from the ingestion of oxidizing drugs or from an inherited abnormality of the haemoglobin molecule. Symptoms include fatigue, headache, dizziness, and *cyanosis.
Methaemoglobinaemia is a condition due to the presence in the blood of METHAEMOGLOBIN. It is characterised by CYANOSIS which turns the skin and lips a blue colour, shortness of breath, headache, fatigue and sickness. There are two main forms: a hereditary form and a toxic form. The latter is caused by certain drugs, including acetanilide, phenacetin, the sulphonamides and benzocaine. The treatment of the toxic form is the withdrawal of the causative drug. In the more severe cases the administration of methylene blue or ascorbic acid may also be needed, and these are the drugs used in the hereditary form.... methaemoglobinaemia
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