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Suxamethonium phase I block — FRCA Primary MCQ

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HardPharmacologySuxamethonium phase I blockFRCA Primary

After suxamethonium for rapid sequence induction, a patient with low plasma cholinesterase activity remains paralysed for 40 minutes. A peripheral nerve stimulator shows reduced twitch height without fade. Which block pattern is most consistent?

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Correct answer: C — Phase I depolarising block

A phase I suxamethonium block produces reduced twitch responses with little or no fade on train-of-four stimulation. A non-depolarising block usually shows fade and post-tetanic facilitation. Phase II block can resemble non-depolarising block but is more associated with repeated or prolonged suxamethonium exposure. Low plasma cholinesterase prolongs suxamethonium by slowing hydrolysis in plasma.

Reference: Peck, Hill & Williams Pharmacology for Anaesthesia and Intensive Care; Primary FRCA textbooks