Bispectral index, predicted and measured drug levels of target-controlled infusions of remifentanil and propofol during laparoscopic cholecystectomy and emergence

Background: Target-controlled infusions (TCI) have been launched as simple, accurate and reliable delivery systems of intravenous drugs. Bispectral index of EEG (BIS) seems promising:in measuring hypnotic effect of anaesthetic drugs. The aims of this study were to evaluate the accuracy of TCI systems in patients undergoing laparoscopic cholecystectomy and to correlate measured drug levels to BIS values. Data were analysed for possible gender differences during emergence. Methods: After written informed consent, 20 patients were enrolled in an open study. Remifentanil was set at 7.5 ng/ml as target throughout the whole procedure, and propofol at 5 mu g/ml at induction and 3 mu g/ml after intubation. Values in blood samples of remifentanil and propofol were correlated to the estimated values and to systolic blood pressure and BIS. BIS values and measured drug levels during emergence and emergence time were compared for the two sexes. Results: Measured drug values varied considerably from the set target with a prediction error of -22% for remifentanil and 49% for propofol. The anaesthesia level was regarded as quite deep with a mean BIS during stable surgery of 42+/-7, and at this level we found no correlation between measured values of either of the two drugs and BIS. The emergence time was significantly shorter for women (12.6+/-2.5 min) than for men (19.0+/-4.2 min) (P=0.001), with no significant differences in measured levels of propofol or remifentanil or BIS during the emergence period. Conclusion: Present systems for TCI of remifentanil and propofol result in large intra- and interindividual variations in measured drug levels, and measured levels differ from target. There may be possible interaction between the two anaesthetics at a pharmacokinetic level. Within the level of anaesthesia studied here, BIS was not an indicator of the actual drug levels. Women woke up significantly faster than men.