Suxamethonium ED95 — FRCA Primary MCQ

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HardPharmacology - General PrinciplesSuxamethonium ED95FRCA Primary

In a dose-response study of intravenous suxamethonium in healthy adults, neuromuscular block is measured as depression of the adductor pollicis single-twitch response to ulnar nerve stimulation. Which dose most closely approximates the ED95?

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Correct answer: D — 0.3 mg/kg

Explanation lettering: E = shown as A · A = shown as E

D is correct. The ED95 is the dose producing 95% depression of the monitored evoked twitch, and for suxamethonium in adults it is just under 0.3 mg/kg. It is a measure of pharmacodynamic potency, not the dose required to guarantee rapid, reliable intubating conditions. The usual adult intubating dose of about 1 mg/kg is roughly 3.5–4 times the ED95, because larger multiples speed onset and ensure profound block at the laryngeal and diaphragmatic muscles, which are relatively resistant compared with adductor pollicis. Options B and E are therefore clinical intubating doses, not ED95 values. C (0.5 mg/kg) exceeds the ED95 and may give acceptable conditions in some patients. A (2 mg/kg) is a high dose used in specific circumstances and markedly prolongs paralysis.

Reference: Ezzat A, Fathi E, Zarour A, et al. The optimal succinylcholine dose for intubating emergency patients: retrospective comparative study. Libyan Journal of Medicine. 2011;6:7041. https://pubmed.ncbi.nlm.nih.gov/21772925/