Closed-loop control of propofol anaesthesia using bispectral index™:: performance assessment in patients receiving computer-controlled propofol and manually controlled remifentanil infusions for minor surgery

Background. In a previous study we used the bispectral index (BIS)(TM) for automatic control of propofol anaesthesia, using a proportional-integral-differential control algorithm. As control was less than optimal in some patients, we revised the constants of the control algorithm. The aim of the current study was to measure the performance of the revised system in patients undergoing minor surgery under propofol and remifentanil anaesthesia. Methods. Twenty adult patients scheduled for body surface surgery were enrolled. Anaesthesia was manually induced with target-controlled infusions (TCI) of propofol and remifentanil. After the start of surgery, when anaesthesia was clinically adequate, automatic control of the propofol TCI was commenced using the revised closed-loop system. For patients 11-20, effect-site steering was also incorporated into the closed-loop control algorithm. Adequacy of anaesthesia during closed-loop control was assessed clinically, and by calculating the median performance error (MDPE), the median absolute performance error (MDAPE) and the mean offset of the control variable. Results. The system provided adequate operating conditions and stable cardiovascular values in all patients during closed-loop control. The mean MDPE and MDAPE were -0.42% and 5.63%, respectively. Mean offset of the BIS(TM) from setpoint was -0.2. No patients reported awareness or recall of intraoperative events. Conclusions. The system was able to provide clinically adequate anaesthesia in all patients, with better accuracy of control than in the previous study. There was a tendency for more accurate control in those patients in whom the control algorithm incorporated effect-site steering.