Wednesday, 6 October 2010

MRCP revision battle 25.6: Methaemoglobinaemia

Methaemoglobin (MetHb) is formed when the iron in haemoglobin is oxidised from the ferrous state (Fe2+) to the ferric state (Fe3+) 

Methaemoglobinaemia means raised MetHb levels, and as MetHb has a far higher affinity for oxygen than normal haemoglobin this results in tissue hypoxia as the oxygen being carried in the blood is not given up to the tissues.

Clinical features depend on the % of MetHb:
  • 15-20% - cyanosis
  • 20-45% - impaired consciousness
  • >55% - seizures, arrhythmias, coma
  • >70% - lethal

Pulse oximetry will show low oxygen sats, as will the printout from blood gas analysis - but the PaO2 will be high.

Causes of methaemoglobinaemia:
  • congenital
    • pyruvate kinase deficiency
    • G6PD deficiency
    • abnormal haemoglobin - HbH or HbM
    • diaphorase 1 deficiency
  • acquired - accelerated rate of oxidised formation due to meds
    • antibiotics - sulphonamides, trimethoprim, dapsone
    • drugs used in anaesthetics - lidocaine, procaine
    • others -primaquine

  • 1% solution of methylene blue
  • hyperbaric oxygen
  • exchange transfusion
  • dialysis
  • asorbic acid in patients with G6PD deficiency

So to the final battle of the day - weeverfish!