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Morphine Fentanyl Equipotency — FRCA Primary MCQ

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HardPharmacology - AnalgesicsMorphine Fentanyl EquipotencyFRCA Primary

Immediately after a rapid high-dose fentanyl bolus, ventilation becomes impossible despite an apparently patent tracheal tube. Peak pressure rises, the chest wall is rigid and there is no wheeze or urticaria. Which diagnosis best unifies the findings?

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Correct answer: E — Acute opioid-induced truncal and respiratory-muscle rigidity

The best answer is “Acute opioid-induced truncal and respiratory-muscle rigidity”. Rapid intravenous fentanyl can cause marked skeletal-muscle and chest-wall rigidity, particularly with large doses, making manual or mechanical ventilation difficult despite a patent airway. The absence of wheeze, rash and circulatory collapse argues against anaphylaxis; aspiration and pneumothorax do not characteristically cause abrupt generalized truncal rigidity, and pseudocholinesterase deficiency causes paralysis rather than rigidity.

Reference: Fentanyl 50 micrograms/mL solution for injection, UK SmPC: https://www.medicines.org.uk/emc/product/3387/smpc; Royal College of Anaesthetists, Primary FRCA syllabus v2.2: https://www.rcoa.ac.uk/sites/default/files/documents/2026-01/Primary-FRCA-Syllabus.pdf