Methaemoglobinaemia can be caused either by a genetic
defect in red cell metabolism or haemoglobin
structure, or acquired by a variety of drugs and toxins. Common drugs dapsone,
nitrates, prilocaine, antimalarials,
sulphonamides and dyes. Domestic
causes of acquired methaemoglobinaemia include ingestion of food and water
high in nitrites and
nitrates exposure to aniline dyes in dyed blankets, laundry markings,
freshly dyed shoes, red wax crayons
and cleaning solution.
Standard pulse oximeters give spuriously low readings in the
presence of excess methaemoglobin. Methylene blue is indicated
in any patient with symptoms
and/or signs of hypoxia (mental changes,
tachycardia, dyspnoea, chest pain). It is contraindicated in G6PD deficiency.
High flow oxygen should be administered.
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