Interaction between propofol and thiopental: Isobolographic analysis using dose, central compartment and effect compartment concentrations

Background: Giving a benzodiazepine or opioid with an intravenous anaesthetic agent generally result in synergism. However, few studies so far have detailed the relationship when two intravenous anaesthetics are given simultaneously. The aim of this study is to investigate the interaction between propofol and thiopental when given in fixed and variable dose ratios. Methods: Ninety patients were given propofol and / or thiopental for induction of anaesthesia. Twenty patients received an infusion of either drug while 30 patients received an infusion of an admixture of both drugs. Another 40 patients received a bolus of one drug followed by an infusion of the other. Isobolographic analysis was used to determine the interaction between the two drugs. Results: Both fixed and variable drug dose ratios resulted in an additive interaction between propofol and thiopental. The average dose of propofol at loss of the eyelash reflex was 1.14 mg kg-1. The corresponding average thiopental dose of 2.49 mg kg-1. Conclusions: Propofol and thiopental interact in an additive fashion when given at induction of anaesthesia.