Tricyclic antidepressant overdose — MCCQE Part 1 MCQ

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HardPoisoning/OverdoseTricyclic antidepressant overdoseMCCQE Part 1

A patient rescued from an enclosed-space house fire has soot around the mouth, altered consciousness, hypotension and lactate of 14 mmol/L despite oxygen and resuscitation. Cyanide toxicity is strongly suspected in addition to carbon-monoxide exposure. Which antidote should be prioritized with poison-centre consultation?

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Correct answer: B — Hydroxocobalamin

Enclosed-space smoke exposure, soot, neurologic depression, shock and severe lactic acidosis support suspected cyanide toxicity. Hydroxocobalamin is the Health Canada-approved antidote for known or suspected cyanide poisoning and should be given promptly with poison-centre support while airway management, high-concentration oxygen, burn care and resuscitation continue. Cyanide levels are not rapidly available enough to guide an unstable patient. Sodium nitrite induces methemoglobinemia and is contraindicated in smoke-inhalation victims whose oxygen carrying capacity may already be reduced by carboxyhemoglobin. Flumazenil reverses selected benzodiazepine effects, protamine reverses heparin, and methylene blue is not the standard cyanide antidote. Hydroxocobalamin can cause red discoloration and interfere with some laboratory assays and dialysis sensors.

Reference: Ontario Poison Centre, Hydroxocobalamin Guidance: https://www.ontariopoisoncentre.ca/Newsandevents/news-archive/2024/shortage-of-cyanokit/