Methaemoglobinaemia

Last Updated on by FRCEM Intermediate

Overview

  • Methaemoglobinaemia is the state of excessive methaemoglobin 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%

causes

Congenital

  • cytochrome b5 reductase deficiency
  • haemoglobin M disease

Acquired

  • Organic and inorganic nitrates and nitrites e.g. sodium nitroprusside, poppers
  • Chlorates
  • Topical and injected local anaesthetics
  • Dapsone
  • Phenytoin
  • Sulphonamides

clinical features

  • cyanosis
  • symptoms and signs of decreased oxygen delivery e.g. chest pain, dyspnea, altered metal state, end organ damage
  • SpO2 reading 85-90%
  • blood samples typically have a chocolate brown hue
  • Normal PaO2

Investigation

  • confirmation via ABG (co-oximetry +/- specific assay + history of exposure)
  • high metHb

Management

  • high flow O2 (to ensure available Hb is saturated well)
  • avoid / cessation of precipitants
  • Methylene blue – reduces the ferric iron of methaemoglobin back to the ferrous iron of Haemoglobin (contraindicated in patients with G6PD deficiency)- in high doses, methylthioninium chloride can itself cause methaemoglobinaemia.
  • IV hydration and bicarbonate for acidosis
    • alternatives to methylene blue:
    • ascorbic acid (if methylene blue contra-indicated, e.g. G6PD deficiency)
    • exchange transfusion
    • hyperbaric oxygen

REASONS FOR FAILURE OF METHYLENE BLUE

Consider the following if MetHb levels do not fall with methylene blue:

  • massive ongoing exposure to an oxidising agent
  • sulfhaemoglobinemia (e.g. dapson, sulfonamides)
  • G6PD deficiency
  • methaemoglobin reductase deficiency
  • abnormal haemoglobin
  • excessive methylene blue (paradoxical effect in high doses)

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