Closed-loop control of propofol anaesthesia

We describe the use of a closed-loop system to control depth of propofol an aesthesia automatically. We used the auditory evoked potential index (AEP(i ndex)) as the input signal of this system to validate it as a true measure of depth of anaesthesia. Auditory evoked potentials were acquired and proce ssed in real time to provide the AEP(index). The AEP(index) was used in a p roportional integral (PI) controller to determine the target blood concentr ation of propofol required to induce and maintain general anaesthesia autom atically. We studied 100 spontaneously breathing patients. The mean AEP(ind ex) before induction of anaesthesia was 73.5 (SD 17.6), during surgical ana esthesia 37.8 (4.5) and at recovery of consciousness 89.7 (17.9). Twenty-tw o patients required assisted ventilation before incision. After incision, v entilation was assisted in four of these 22 patients for more than 5 min. T here was no incidence of intraoperative awareness and all patients were pre pared to have the same anaesthetic in future. Movement interfering with sur gery was minimal. Cardiovascular stability and overall control of anaesthes ia were satisfactory.