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 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
CCC Toxicology Series
General
Approach to acute poisoning, ECGs in Tox, Evidenced-based Tox, Toxicology literature summaries, Does anti-venom work?
Toxins / Overdose
Amphetamines, Barbituates, Benzylpiperazine, Beta Blockers, Calcium Channel Blocker, Carbamazepine, Carbon Monoxide, Ciguatera, Citrate, Clenbuterol, Cocaine, Corrosive ingestion, Cyanide, Digoxin, Ethanol, Ethylene Glycol, Iron, Isoniazid, Lithium, Local anaesthetic, Methanol, Monoamine oxidase inhibitor (MAOI), Mushrooms (non-hallucinogenic), Opioids, Organophosphate, Paracetamol, Paraquat, Plants, Polonium, Salicylate, Scombroid, Sodium channel blockers, Sodium valproate, Theophylline, Toxic alcohols, Tricyclic antidepressants (TCA)
Envenomation
Marine, Snakebite, Spider, Tick paralysis
Syndromes
Alcohol withdrawal, Anticholinergic syndrome, Cholinergic syndrome, Drug withdrawals in ICU, Hyperthermia associated toxidromes, Malignant hyperthermia (MH), Neuroleptic malignant syndrome (NMS), Opioid withdrawal, Propofol Infusion Syndrome (PrIS) Sedative toxidrome, Serotonin syndrome, Sympatholytic toxidrome, Sympathomimetic toxidrome
Decontamination
Activated Charcoal, Gastric lavage, GI Decontamination
Enhanced Elimination
Enhanced elimination, Hyperbaric therapy for CO
Antidotes
Antidote summary, Digibind, Glucagon, Flumazenil, HIET – High dose euglycaemic therapy, Intralipid, Methylene Blue, N-Acetylcysteine (NAC), Naloxone
Miscellaneous
Cocaine chest pain, Digoxin and stone heart theory, Hyperbaric oxygen, Hypoxaemia in tox, Liver failure in tox, Liver transplant for paracetamol, Methaemoglobinaemia, Urine drug screen
- CCC – Methylene blue
- Tox Conundrum – Why so Blue?
- Tox Library – Methylene blue
Critical Care
Compendium
Chris is an Intensivist and ECMO specialist at The Alfred ICU, where he is Deputy Director (Education). He is a Clinical Adjunct Associate Professor at Monash University, the Lead for the Clinician Educator Incubator programme, and a CICM First Part Examiner.
He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives. He was one of the founders of the FOAM movement (Free Open-Access Medical education) has been recognised for his contributions to education with awards from ANZICS, ANZAHPE, and ACEM.
His one great achievement is being the father of three amazing children.
On Bluesky, he is @precordialthump.bsky.social and on the site that Elon has screwed up, he is @precordialthump.
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