Potency of propofol, thiopentone and ketamine at various endpoints in New Zealand White rabbits

Effective plasma concentrations of propofol, thiopentone and ketamine were determined at different endpoints in a study with randomized, crossover des ign in nine New Zealand White rabbits. A continuous infusion was used (30 m l/h) with concentrations of 10 mg/ml for propofol, 25 mg/ml for thiopentone and 20 mg/ml for ketamine. The endpoints were loss of the righting reflex, loss of purposeful reactions to tail clamping (as an example of a peripher al pain stimulus) or to intranostril insufflation of ammonia vapour (as an example of a central reflex stimulus), and the recovery of these reflexes a nd reactions. According to the ED50 values the potency ratios of propofol, thiopentone and ketamine were at the loss of righting reflex 1:1.8:1.2, at the loss of reaction to ammonia vapour 1:1.5:1.6, and at the loss of reacti on to tail clamping 1:1.5:3.9, respectively. Recovery was significantly fas ter after propofol than after thiopentone and ketamine. Measuring the effec tive plasma concentrations of intravenous anaesthetics provides a method of relating dose to effect, but there still remains a variable gap between pl asma concentration and effect.