Suxamethonium (Succinylcholine)

Quick Answer

Suxamethonium (succinylcholine) is the only depolarizing neuromuscular blocker in clinical use, providing rapid onset (30-60 seconds) and ultra-short duration (5-10 minutes) ideal for rapid sequence intubation. Structure: Dimer of two acetylcholine (ACh) molecules linked by ester bond; quaternary ammonium compound (poor lipid solubility, does not cross blood-brain barrier). Mechanism: Depolarizing neuromuscular blockade—binds to nicotinic ACh receptors (nAChR) at motor end-plate causing persistent depolarization (phase I block); produces fasciculations (uncoordinated muscle contractions), then flaccid paralysis; not antagonized by anticholinesterases (neostigmine prolongs block by inhibiting acetylcholinesterase, increasing ACh which cannot repolarize end-plate). Metabolism: Hydrolysis by plasma cholinesterase (pseudocholinesterase, butyrylcholinesterase) to succinylmonocholine (weak NMB) then succinic acid + choline; rapid hydrolysis limits duration; 1 in 2500 patients have atypical plasma cholinesterase (dibucaine number <30) causing prolonged block (1-4 hours). Dosing: 1-1.5 mg/kg IV (intubating dose), 2-4 mg IM (if no IV access, onset 3-5 minutes), infusion 0.5-10 mg/min (rarely used). Side effects: Malignant hyperthermia (triggers MH in susceptible patients— MH occurs in 1:10,000-50,000 general anaesthetics, 50% triggered by suxamethonium, 50% by volatiles), hyperkalaemia (0.5-1.0 mmol/L increase from skeletal muscle depolarization, dangerous if pre-existing hyperkalaemia or denervation injuries, burns >24 hours old, massive trauma, neuromuscular disease—can cause cardiac arrest), bradycardia (vagal stimulation, especially with second dose in children, prevent with atropine 0.02 mg/kg), increased intraocular pressure (contraindicated in open eye injury—though controversial, many avoid), increased intragastric pressure (theoretically increases aspiration risk, clinically insignificant), fasciculations (myalgia post-op, 10-50% incidence, prevented by defasciculating dose of non-depolarizer 3 minutes before), phase II block (prolonged infusions or repeated doses cause desensitization block similar to non-depolarizers, responds to anticholinesterases). Contraindications: Malignant hyperthermia susceptibility, hyperkalaemia or risk of hyperkalaemia (renal failure, burns >24 hours, massive trauma, denervation), history of anaphylaxis to suxamethonium, atypical plasma cholinesterase (relative—know risk of prolonged block), increased intracranial pressure (relative), open eye injury (relative—avoid or use carefully). Clinical pearls: Gold standard for RSI when no contraindications (fastest onset, shortest duration, most reliable); rocuronium 1.2 mg/kg with sugammadex rescue acceptable alternative when suxamethonium contraindicated; suxamethonium apnea (prolonged block in atypical cholinesterase) treated with supportive care, mechanical ventilation until spontaneous recovery (hours). [1-10]