Methaemoglobinaemia

Reviewed and revised 14 September 2014

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 (toxin/drugs)

• aniline dyes
• benzene derivatives
• chloroquine
• dapsone
• prilocaine
• metoclopramide
• nitrites (nitroglycerin, NO, sodium nitroprusside)
• 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

INVESTIGATIONS

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

MANAGEMENT

Resuscitation

• high flow O2 (to ensure available Hb is saturated well)

Specific therapy

• congenital
  — avoid precipitants
• cessation of precipitants
• methylene blue (1-2mg/kg over 5 minutes) provides an artificial electron acceptor to facilitate the reduction of MetHb via the NADPH-dependent pathway; give if:
  — symptomatic
  — consider if asymptomatic with >20% MetHb, or >10% if risk factors such as anaemia or ischemic heart disease
• repeat methylene blue at 30-60 min if inadequate response
• alternatives to methylene blue:
  —ascorbic acid (if methylene blue contra-indicated, e.g. G6PD deficiency)
  — exchange transfusion
  — hyperbaric oxygen

Supportive care and monitoring

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)

References and Links

• CCC — Methylene blue