Resumen de Methaemoglobinaemia
Methaemoglobin is formed when ferrous haemoglobin iron (II) is oxidized to ferric iron (III), which cannot participate in oxygen transport. Methaemoglobin-forming chemical groups of particular importance in poisoning are organic nitrites (e.g. amyl nitrite, isobutyl nitrite) and amino- or nitro-derivatives of benzene (e.g. aniline, dapsone, lidocaine). An asymptomatic, apparent ‘cyanosis’ is the earliest clinical feature, occurring when approximately 15% of the total haemoglobin is replaced by methaemoglobin. Progressive manifestations of tissue hypoxia ensue at increasing methaemoglobin concentrations, and concentrations approaching 80% may be fatal. The antidote, methylthioninium chloride (methylene blue), has to be administered intravenously. It works by donating electrons to ferric haem to restore ferrous haem.
