Closed loop control of anaesthesia: an assessment of the bispectral index as the target of control

We investigated the performance of a closed-loop system for administration of general anaesthesia, using the bispectral index as a target for control. One hundred patients undergoing gynaecological or general surgery were stu died. In 60 patients, anaesthesia was maintained by intravenous infusion of a propofol/alfentanil mixture. In 40, an isoflurane/nitrous oxide based te chnique was used. For each technique, patients were randomly allocated to r eceive either closed-loop or manually controlled administration of the rele vant agents (propofol/alfentanil or isoflurane), with an intra-operative ta rget bispectral index of 50 in all cases. Closed-loop and manually controll ed administration of anaesthesia resulted in similar intra-operative condit ions and initial recovery characteristics. During maintenance of anaesthesi a, cardiovascular and electro-encephalographic variables did not differ bet ween closed-loop and manual control groups and deviation of bispectral inde x from the target value was similar. Intra-operative concentrations of prop ofol, alfentanil and isoflurane were within normal clinical ranges. Episode s of light anaesthesia were more common in the closed-loop group for patien ts receiving propofol/alfentanil anaesthesia and in the manual group for pa tients receiving isoflurane/nitrous oxide anaesthesia. Convenience aside, t he closed-loop system showed no clinical advantage over conventional, manua lly adjusted techniques of anaesthetic administration.