Smoke inhalation with suspected cyanide poisoning — MRCEM SBA MCQ

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HardRespiratory emergenciesSmoke inhalation with suspected cyanide poisoningMRCEM SBA

A 42-year-old man is brought to the emergency department after being rescued from a room filled with smoke from burning upholstered furniture. He is coughing and newly disorientated. There is soot around his mouth, but no facial burns, hoarseness, stridor or oropharyngeal swelling; he is protecting his airway. His blood pressure is 128/76 mmHg. High-flow oxygen has been started through a reservoir mask. Arterial blood gas analysis shows pH 7.24, PaCO₂ 3.4 kPa and lactate 12.1 mmol/L; co-oximetry shows carboxyhaemoglobin 4%. He has no substantial burns or other injuries. Which intravenous treatment is the most appropriate next intervention?

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Correct answer: E — Administer hydroxocobalamin.

Enclosed-space smoke exposure, soot, new confusion and a lactate above 10 mmol/L strongly suggest cyanide toxicity. Neither major burns nor hypotension provides an alternative explanation for this degree of lactic acidosis. A low carboxyhaemoglobin concentration does not exclude concurrent cyanide poisoning: the two toxicities need not track together. Continue high-flow oxygen and airway surveillance, but give hydroxocobalamin promptly rather than waiting for a cyanide assay. The adult initial dose is 5 g intravenously over 15 minutes. He requires monitored admission and reassessment for evolving inhalation injury. ([gov.uk](https://www.gov.uk/government/publications/health-effects-of-explosions-and-fires/health-effects-of-explosions)) A treats an acid–base consequence without addressing the suspected toxin. B is an alternative cyanide antidote, but inducing methaemoglobinaemia is unattractive when smoke inhalation may also impair oxygen carriage. C supplies sulphur for cyanide detoxification but is generally an adjunct rather than the preferred immediate treatment for this presentation. D may be considered when severe cyanide poisoning is strongly attributable to a cyanide salt; it is a poorer choice when smoke exposure creates diagnostic uncertainty. Hydroxocobalamin is specifically supported for smoke-exposed patients with significant lactic acidosis. ([hey.nhs.uk](https://www.hey.nhs.uk/wp/wp-content/uploads/2016/04/antidoteGuidelineList.pdf?utm_source=openai))

Reference: London and South East Burns Network: Burn Inhalation Injury (Approved 20 January 2026) — https://www.lsebn.nhs.uk/_common/getdocument?id=385903 Cyanokit 5 g powder for solution for infusion: Summary of Product Characteristics (24 March 2026) — https://www.medicines.org.uk/emc/product/4786/smpc Health effects of explosions: acute effects of smoke inhalation (8 July 2005) — https://www.gov.uk/government/publications/health-effects-of-explosions-and-fires/health-effects-of-explosions