Lightning Learning: Methaemoglobinaemia

Lightning Learning: Methaemoglobinaemia

“Death will occur when Methaemoglobinaemia (MetHb) = 70%. Early recognition is essential.”
— advice is based on local guidelines & procedures


Methaemoglobinaemia is a rare but life-threatening emergency that can be fully reversed. It is characterised by an increase in the amount of MetHb in the blood.

Methaemoglobin is an altered state of Hb where ferrous ions (Fe2+) of haem are oxidised to the ferric state (Fe3+) and rendered unable to bind O2: normal level is < 1.5%.


  • Innate

  • Acquired toxins (nitrites, benzene derivatives, chloroquine, metoclopramide, dapsone, prilocaine and aniline dyes), Diet (contamination), Oxidative stress

  • Iatrogenic (such as local anaesthetic)


Symptoms include: cyanosis, anxiety, headache, tachycardia developing to coma, seizures arrhythmias, respiratory depression and death.

A patient with MetHb level of 5% or a patient with a MetHb level of 40% would have approximately an O2 sats (~85%). Therefore O2 sats do not confer to the degree of MetHb!

Degree of cyanosis ≠ MetHb level. Early recognition is essential!


Treatment is supportive unless MetHb is greater than 30% or symptomatic, then methylene blue may be indicated.

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