Pharm 101: Suxamethonium

Class

Paralytic agent


Pharmacodynamics
  • Depolarising neuromuscular blocker that causes rapid neuromuscular blockade at motor endplate nicotinic receptors
  • Two phases of action: Phase 1 (depolarising) and Phase 2 (desensitising)
  • Phase 1:
    • Binding of drug to nicotinic receptors causes depolarisation of motor endplate
    • Fasciculations (transient contractions of muscle motor units) occur due to spread of impulse to adjacent membranes
    • Depolarised membranes remain depolarised and unresponsive to subsequent impulses, causing flaccid paralysis
  • Phase 2:
    • With prolonged exposure, initial end plate depolarisation decreases and membranes become repolarised
    • Membrane is desensitised and cannot easily be depolarised again

Pharmacokinetics
  • Rapid onset
  • Short duration of action 5-10 minutes
  • Rapid hydrolysis by:
    • Pseudocholinesterase in plasma (main pathway)
    • Butyrylcholinesterase in liver
  • Because plasma cholinesterase has an enormous capacity to hydrolyse suxamethonium, only a small percentage of the original intravenous dose ever reaches the neuromuscular junction

Clinical uses
  • Rapid Sequence Induction (RSI)
    • Dose 0.75-1.50 mg/kg
  • Clinical effects
    • Transient muscle fasciculations occur over chest and abdomen within 30 seconds following administration
    • As paralysis develops rapidly (< 90 seconds), the arm, neck and leg muscles are initially relaxed followed by respiratory muscles
    • Duration of blockade ~5-10 minutes

Adverse effects
  • Cardiac arrhythmias:
    • Stimulates nicotinic receptors at both sympathetic and parasympathetic ganglia and muscarinic receptors in the heart (e.g. sinus node)
    • Negative inotropic and chronotropic responses
  • Hyperkalaemia:
    • Fasciculations following administration of suxamethonium cause release of potassium from muscles
    • Patients with burns, nerve damage or neuromuscular disease, closed head injury and other trauma may develop proliferation of extrajunctional acetylcholine receptors
    • If proliferation is great enough, hyperkalaemia causing cardiac arrest may ensue
  • Raised intraocular pressure
  • Raised intragastric pressure
    • Increases of 5-40 cm H2O in heavily muscled patients due to fasciculations
  • Post-operative myalgia
    • Common in patients who receive large doses
    • Incidence 1-20%
  • Prolongation of neuromuscular blockade in patients with plasma cholinesterase deficiency

Further Reading

Pharm 101 700

Pharmacology 101

Top 200 drugs

MBBS (UWA) CCPU Emergency Medicine Trainee with interests in medical education, ECG interpretation, and the use of point-of-care ultrasound in the undifferentiated patient. Co-author of the LITFL ECG Library | Twitter

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.