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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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