The Narcotrend 'depth of anaesthesia' monitor cannot reliably detect consciousness during general anaesthesia: an investigation using the isolated forearm technique

Background. During general anaesthesia in the presence of neuromuscular blocking drugs clinical criteria cannot detect the presence of consciousness. Various 'depth of anaesthesia' monitors are available which claim to prevent consciousness and/or reduce anaesthetic drug use. This study uses the Narcotrend anaesthesia brain monitor to guide anaesthetic administration but at the same time checks for the presence of intra-operative consciousness by using the 'isolated forearm' technique throughout the whole surgical/anaesthetic procedure. Methods. Twelve women presenting for major gynaecological surgery under general anaesthesia, which included the use of neuromuscular blocking drugs, had a target controlled infusion of propofol adjusted according to the anaesthetic 'stage' indicated by a Narcotrend 'Depth of Anaesthesia Monitor'. Throughout surgery the isolated forearm technique was used to detect for the presence of consciousness at 1 min intervals. Results. Isolated forearm responses to commands occurred in all 12 patients at some time during surgery, frequently in the absence of any significant changes in the usually monitored clinical variables. Overall, the 12 patients responded a total of 92 times during surgery. Only 41 ( 45%) responses were associated with an increase in the Narcotrend stage to a level suggesting consciousness ( above stage C-0). For the remaining responses, either there was no significant increase in the Narcotrend stage ( above C-0) or there was no change at all in the Narcotrend stage before, during, or after the patient responded to the taped command. Conclusions. The Narcotrend was unable to differentiate reliably between conscious and unconscious patients during general anaesthesia when neuromuscular blocking agents were used.