Pharm 101: Thiopentone

Class

Barbiturate

Pharmacodynamics
  • Facilitate actions of GABA in CNS
  • Bind to specific GABA-A receptor subunits at CNS neuronal synapses
  • In contrast to benzodiazepines, increase duration of GABA-gated chloride channel openings
  • At high concentrations, may be GABA-mimetic, directly activating chloride channels
Organ system effects
  • CNS:
    • Dose-dependent depression ranging from sedation to general anaesthesia
    • Decreased CMR, blood flow and ICP
    • Decreased cerebral oxygen consumption
    • No analgesic effect, possible hyperalgesia
  • CVS:
    • Hypotension due to vasodilatation (less than that of propofol)
    • Negative inotrope
    • Compensatory tachycardia
  • Respiratory depression
Pharmacokinetics
  • Highly lipid soluble
  • 83% protein bound
  • Distribution to highly vascular tissue and rapidly crosses BBB, then rapid redistribution to body fat
  • Distribution half-life 2-4 mins
  • Elimination half-life 11 hours
  • Hepatic metabolism
  • < 1% excreted unchanged in urine
Clinical uses
  • Rapid Sequence Induction (RSI):
    • Induction dose 3-5 mg/kg
    • Onset within 30 seconds
    • Duration of action 5-10 mins
Adverse effects
  • Extension of organ system effects including hypotension, respiratory depression
  • Pain on injection
  • Hyperalgesia
  • Rarely, porphyric crisis due to induction of ALA synthase in liver
  • Severe tissue injury +/- gangrene in accidental intra-arterial injection
Precautions/contraindications
  • Acute porphyria
  • Hypovolaemia
  • Cardiovascular disease
  • Rapid administration of neuromuscular blocking drugs may result in precipitation of insoluble thiopentone acid
Further reading
References
  • Katzung BG. Basic & Clinical Pharmacology. 14th ed. United States of America: McGraw-Hill Education; 2018. 383-389, 450-454 p.
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MBBS CCPU (RCE, Biliary, DVT, E-FAST, AAA) Rob is an Emergency Medicine Advanced Trainee based in Melbourne, Australia. He has special interests in medical education, ECG interpretation, and the use of diagnostic and procedural ultrasound in the undifferentiated and unwell patient.

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