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

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Abstract
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 studied. In 60 patients, anaesthesia was maintained by intravenous infusion of a propofol/alfentanil mixture. In 40, an isoflurane/nitrous oxide based technique was used. For each technique, patients were randomly allocated to receive either closed‐loop or manually controlled administration of the relevant agents (propofol/alfentanil or isoflurane), with an intra‐operative target bispectral index of 50 in all cases. Closed‐loop and manually controlled administration of anaesthesia resulted in similar intra‐operative conditions and initial recovery characteristics. During maintenance of anaesthesia, cardiovascular and electro‐encephalographic variables did not differ between closed‐loop and manual control groups and deviation of bispectral index from the target value was similar. Intra‐operative concentrations of propofol, alfentanil and isoflurane were within normal clinical ranges. Episodes of light anaesthesia were more common in the closed‐loop group for patients receiving propofol/alfentanil anaesthesia and in the manual group for patients receiving isoflurane/nitrous oxide anaesthesia. Convenience aside, the closed‐loop system showed no clinical advantage over conventional, manually adjusted techniques of anaesthetic administration.