BACKGROUND: Although the target plasma concentrations of propofol required for the induction of anesthesia have been studied extensively, the relationship between the duration of induction and target plasma concentration have not yet been described fully. We studied the target plasma concentration of propofol required for a rapid loss of responsiveness (LOR) during the induction of anesthesia, and the adverse effects and the incidence of induction failure, and compared this with the bolus administration of propofol.

METHODS: ASA physical status I, unpremedicated, 21-40 yr, 174 female patients scheduled for minor gynecologic surgery under general anesthesia were randomly assigned to eight different groups. In groups I and II, propofol 2.0 mg/kg or 2.3 mg/kg were administrated to induce anesthesia. In the other 6 groups, target plasma concentrations were set at 5.4, 6.1, 6.8, 7.4, 8.1 and 8.8 µg/ml. Every 5 seconds the patients were asked to open their eyes. The time to LOR was measured by a blinded investigator. The target plasma concentrations of propofol for LOR within 60, 90, 120, 180 and 300 seconds were calculated by regression and probit analysis.

RESULTS: The target plasma concentrations of propofol for LOR within 60, 90, 120, 180 and 300 seconds in 95% of patients were 9.5, 8.4, 8.2, 7.4 and 6.5 µg/ml, respectively. The incidence of hypotension and bradycardia were not different between groups, but the incidence of induction failure was significantly higher in Group I (P < 0.05).

CONCLUSIONS: We evaluated the use of the target-controlled infusion of propofol for LOR within 1 to 5 minutes in this prospective, randomized study and demonstrate the target plasma concentration of propofol required for the rapid LOR in 95% of healthy, unpremedicated female patients.