Depolarising NMBs

Succinylcholine binds to the nicotinic ACh receptor causing depolarisation. It cannot be hydrolysed by acetylcholinesterase in the NMJ, and so remains bound to the receptor. This:

  • Produces a sustained depolarisation which keeps voltage-gated sodium channels in their inactive state
  • Prevents the post-junctional membrane from responding to further ACh release
Property Succinylcholine
Class Depolarising muscle relaxant.
Uses Facilitate tracheal intubation.
Presentation Colourless solution of pH 3, at Structually, it is two ACh groups joined at the acetyl groups.
Route of Administration IV, IM.
Dosing IV, IM up to 150mg.
Onset and Duration IV onset in 30s to 1 minute, lasting 2-3 minutes, with offset typically within 10 minutes. Offset occurs due to dissociation of drug out of NMJ into plasma, as a concentration gradient is established by drug breakdown in plasma. Prolonged duration in patients with pseudocholinesterase deficiency. IM onset in 2-3 minues.
Distribution 30% protein bound. Nil distribution due to rapid metabolism - VD Crosses placenta in very small amounts.
Metabolism Rapid hydrolysis by plasma cholinesterases such that only 20% of administered dose reaches the NMJ.
Elimination Minimal renal elimination due to rapid metabolism.
Resp Apnoea, and suxamethonium apnoea. May cause masseter spasm. ↑ Salivation due to muscarinic effects.
CVS Arrhythmia due to SA node stimulation, as well as secondary to hyperkalaemia. Bradycardia (due to muscarinic effects with second/large doses, or in children).
CNS ICP (due to contraction), ↑ IOP (by 10mmHg - this is significant) such that it is contraindicated in globe perforation.
Metabolic Malignant Hyperthermia.
MSK Myalgias post depolarisation, particularly in young females. Prolonged blockade with pseudocholinesterase deficiency.
Renal and Electrolyte Hyperkalaemia (K+ ↑ by ~0.5mmol.L-1) due to depolarisation causing K+ efflux, ↑ in burns (>10%), paraplegia (first 6 months) and neuromuscular disorders including muscular dystrophy and myopathies (including critical illness myopathy).
GIT Intragastric pressure ↑ by 10cmH2O, matched by ↑ in LoS pressure.
Immunological Anaphylaxis - highest risk of all NMBs.

Adverse Effects

The adverse effects of suxamethonium can be remembered as three major, three minor, and three pressures:

  • Major
    • Anaphylaxis
    • Suxamethonium Apnoea
    • Malignant hyperthermia
  • Minor
    • Hyperkalaemia
    • Myalgias
    • Bradycardia
  • Pressure
    • IOP
    • ICP
    • Intragastric pressure

Phase I and Phase II Blockade

Initial blockade is termed Phase I, which is a partial depolarising block. Sustained use of suxamethonium may causes a Phase II block which:

  • Appears similar to a non-depolarising block
  • May be due to:
    • Presynaptic inhibition of ACh synthesis and release
    • Desensitisation of the postjunctional receptor

Key differences include:

Property Phase I Block Phase II Block
Block Amplitude Reduced Reduced
Train-of-four ratio >0.7 < 0.7
Post-tetanic potentiation No Yes
Effect of anticholinesterases Block augmented Block inhibited

Malignant Hyperthermia

  • Rare autosomal dominant genetic condition
  • Triggered by suxamethonium and volatile anaesthetic agents
  • Mutation of the ryanodine receptor causes excessive amounts of calcium to leave the sarcoplasmic reticulum, causing continual muscle contraction
    • Results in greatly increased carbon dioxide, lactate, and heat production
    • Cell lysis with myoglobulinaemia and hyperkalaemia results

Suxamethonium Apnoea

  • A deficiency of butylcholinesterase causes suxamethonium to not be metabolsied
  • May be congenital (genetic) or acquired (hepatic failure)
  • Can be treated with fresh frozen plasma


  1. Peck TE, Hill SA. Pharmacology for Anaesthesia and Intensive Care. 4th Ed. Cambridge University Press. 2014.
  2. Suxamethonium Chloride Injection BP PRODUCT INFORMATION
  3. Appiah-Ankam J, Hunter JM. Pharmacology of neuromuscular blocking drugs. Continuing Education in Anaesthesia Critical Care & Pain, Volume 4, Issue 1, 1 February 2004, Pages 2–7.
Last updated 2017-10-08

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