Problems, Pearls and Pitfalls

1) The diagnosis of methaemoglobinaemia is often delayed. Top tips that should make you think MetHb:

• cyanosis or hypoxia on pulse oximetry which does not respond to supplemental O2
• a patient who is tolerating their hypoxia better than they should be
• chocolate brown coloured blood on venepuncture
• a history of exposure to substances with oxidative capacity
• saturation gap. The SpO2 and SaO2 are different by > 5%

2) The agent which has caused methaemoglobinaemia may also have other toxic effects which require specific treatment. Consult Toxbase or the NPIS for further advice

3) Consider the whole patient (presentation, co-morbidities, extent of exposure) not just their MetHb level when deciding on active treatment

4) Pulse oximetry is not accurate in methaemoglobinaemia and paradoxically worsens in treatment with Methylene Blue. Do not be led by the SpO2

5) Methylene Blue does not work in NADPH-methaemoglobin-reductase deficiency, G6PD deficiency, the presence of haemoglobin M or Sulfhaemoglobin

6) Infants presenting with cyanosis may be having their first presentation of innate methaemoglobinaemia