Last Updated on January 23, 2019 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)