Methaemoglobin (MetHb) is an oxidised form of haemoglobin that cannot carry oxygen, caused by genetic mutations, enzyme deficiencies, or exposure to oxidising agents like dapsone and cocaine-derived anaesthetics. Symptoms emerge at MetHb concentrations above 10% and include cyanosis, confusion, arrhythmias, and potentially death, though arterial blood gas readings appear misleadingly normal. Intravenous methylene blue is the first-line treatment for symptomatic cases, with alternative therapies including high-dose vitamin C and exchange transfusion, though evidence for optimal use of alternatives remains limited.
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IV methylene blue is first-line for symptomatic methaemoglobinaemia; recognising misleadingly normal ABGs is key to timely intervention.