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Suspected cyanide poisoning following enclosed-space smoke inhalation — MRCEM SBA MCQ

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HardToxicology and environmental emergenciesSuspected cyanide poisoning following enclosed-space smoke inhalationMRCEM SBA

A 42-year-old man is brought to the emergency department after being rescued from a fire in an enclosed workshop containing burning foam furnishings. He was confused at the scene. He is now alert and maintaining his airway, but has soot around his mouth. High-concentration oxygen has been started. After intravenous fluid resuscitation, his blood pressure is 118/74 mmHg. He has no significant cutaneous burns or other injuries. Arterial blood gas analysis shows pH 7.18 and lactate 12.6 mmol/L; co-oximetry shows carboxyhaemoglobin 5%. A blood cyanide assay has been requested but will not be available urgently. Which antidote strategy is most appropriate now?

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Correct answer: E — Give intravenous hydroxocobalamin now.

Enclosed-space smoke exposure, soot, altered mental status at the scene and marked lactic acidosis together make cyanide poisoning highly likely. In a smoke-inhalation victim, lactate ≥10 mmol/L is particularly suggestive; improvement in blood pressure does not remove that concern. The carboxyhaemoglobin result does not exclude concurrent cyanide poisoning. ([medicines.org.uk](https://www.medicines.org.uk/emc/product/4786/smpc)) Give hydroxocobalamin promptly alongside oxygen and supportive care. The initial adult dose is 5 g intravenously over 15 minutes. A cyanide concentration is not required to start treatment and must not delay it, making D incorrect. ([medicines.org.uk](https://www.medicines.org.uk/emc/product/4786/smpc)) A is tempting because dicobalt edetate is used for severe cyanide poisoning, but its established UK role is in cases with high suspicion of exposure such as cyanide salts; hydroxocobalamin is the appropriate choice for this smoke-inhalation presentation. B is another cyanide-antidote option, but sodium nitrite is not preferred here, where carbon monoxide may also impair oxygen delivery. C is inadequate as the sole immediate antidote: sodium thiosulphate is generally used as an adjunct. ([hey.nhs.uk](https://www.hey.nhs.uk/wp/wp-content/uploads/2016/04/antidoteGuidelineList.pdf))

Reference: Cyanokit 5 g powder for solution for infusion — Summary of Product Characteristics (24 March 2026) — https://www.medicines.org.uk/emc/product/4786/smpc Burn Inhalation Injury — Hydrogen Cyanide Intoxication (Date not stated) — https://www.lsebn.nhs.uk/_common/getdocument?id=385903 Health effects of explosions — Systemic toxins (8 July 2005) — https://www.gov.uk/government/publications/health-effects-of-explosions-and-fires/health-effects-of-explosions