Methaemoglobinaemia

Methaemoglobinaemia

• Definition: Excessive methaemoglobin in blood
  • Altered state of Hb where ferrous (Fe2+) is oxidised to ferric state (Fe3+) and unable to bind O2
  • Normal level <1.5%

Causes

• Congenital
  • Cytochrome b5 reductase deficiency
  • Haemoglobin M disease
• Acquired
  • Aniline dyes
  • Benzene derivatives
  • Chloroquine
  • Dapsone
  • Prilocaine and lignocaine (not dose-dependent and more common in children)
  • Metoclopramide
  • Nitrites (nitroglycerine, NO, sodium nitroprusside, amyl nitrate, alkyl nitrates)
  • Sulphonamides
  • Rifampicin
  • Sodium chlorite/chlorate

Methaemoglobinaemia

• Clinical presentation
  • Cyanosis with grey-blue skin discolouration
  • Reduced oxygen delivery e.g. chest pain, SOB, altered mental state
  • SpO2 85-90%
    • Methaemoglobin absorbs light similarly to HbO and sats nadir at 85%
  • Chocolate brown blood that remains brown on exposure to air
  • Normal PaO2 (as measures dissolved)
  • Co-oximetry measures true HbO2 and metHb in order to confirm
• Ix
  • ABG confirmation (co-oximetry +/- specific assay + history of exposure)
  • High metHb

Methaemoglobinaemia

• Management
  • High flow O2 (to maximise functional Hb saturation)
  • Cessation/avoidance of precipitants
  • Methylene blue
    • 1-2mg/kg over 5 minutes
    • Artificial electron acceptor to reduce MetHb via NADPH-dependent pathway
    • Indications: Symptomatic, >20% MetHb or >10% and hx of anaemia/IHD
    • Repeat at 30-60min as required
  • Alternatives
    • Ascorbic acid (e.g. used in G6PD deficiency when methylene blue contraindicated)
    • Exchange transfusion
    • Hyperbaric oxygen

Methaemoglobinaemia

• What if methylene blue fails?
  • Massive ongoing exposure to oxidising agent
  • Sulfhaemoglobinaemia (e.g. dapsone, sulfonamides)
  • G6PD deficiency
  • Methaemoglobin reductase deficiency
  • Abnormal Hb
  • Excessive methylene blue (paradoxical effect in high doses)

Last Updated on October 14, 2020 by Andrew Crofton