Anaes SAQs · Intravenous induction agents

Thiopental — pharmacology, obstetric RSI and neuroprotection

Final SAQ — thiopental: sodium thiopentone, a thiobarbiturate (sulfur at position 2 increases lipophilicity); reconstituted to a 2.5 percent solution at alkaline pH 10 to 11 (no pain on injection, incompatible with suxamethonium, caustic if extravasated or injected intra-arterially); GABA-A receptor action INCREASING the DURATION of chloride channel opening (versus benzodiazepines increasing FREQUENCY), with direct channel opening at high doses and AMPA/kainate inhibition; onset 10 to 30 seconds and recovery 5 to 10 minutes from ultra-high lipophilicity and rapid redistribution, hepatic low-extraction metabolism with a steeply rising context-sensitive half-time (about 20 min at 1 hour to about 3 hours at 8 hours) so NOT suitable for maintenance infusion; 10 to 25 percent blood pressure drop from myocardial depression and venodilation with reflex tachycardia, safe in pregnancy; barbiturate coma to burst suppression reduces CMRO2 by about 50 percent for refractory intracranial hypertension and status epilepticus; obstetric caesarean RSI induction 4 to 5 mg/kg (less neonatal depression than propofol); absolute contraindication porphyria (induces ALA synthase).