Reviewed and revised 14 September 2014
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%
- cytochrome b5 reductase deficiency
- haemoglobin M disease
- aniline dyes
- benzene derivatives
- nitrites (nitroglycerin, NO, sodium nitroprusside)
- 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
- confirmation via ABG (co-oximetry +/- specific assay + history of exposure)
- high metHb
- high flow O2 (to ensure available Hb is saturated well)
— 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:
— 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 oxidizing agent
- sulfhaemoglobinemia (e.g. dapsone, sulfonamides)
- G6PD deficiency
- methaemoglobin reductase deficiency
- abnormal haemoglobin
- excessive methylene blue (paradoxical effect in high doses)
References and Links
Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.
After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.
He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE. He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.
His one great achievement is being the father of three amazing children.
On Twitter, he is @precordialthump.