The haem group at the centre of the haemoglobin molecule, has, at the centre of its own core, an iron atom. For oxygen carriage, the iron atom in the haem group must be maintained in the ferrous, i.e. Fe2+ or iron (II), form. Oxidation of iron into the ferric, i.e. Fe3+ or iron (III), gives rise to a haemoglobin variant called methaemoglobin. The change from iron (II) to (III) prevents oxygen binding and renders the haemoglobin molecule ineffective. Significant methaemoglobinaemia results in impairment of oxygen carriage, in effect, resulting in anaemia.
2 primary biochemical systems exist in the erythrocyte that resist oxidation in order to prevent methaemoglobin formation. The first is hexose monophosphate shunt pathway. The second is a NADH- and NADPH-dependent methaemoglobin reductase (also known as diaphorase I and II).
Chemicals and medications that favour oxidative stress can overwhelm these homeostatic mechanism leading to methaemoglobinaemia. Alternatively, defects in the (supporting) biochemical mechanisms, e.g. G6PDH deficiency can also contribute.
In methaemoglobinaemia (and some other dyshaemoglobinaemias) pulse oximetry gives falsely low indications.
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