Evaluating the Accuracy of Patient State Index for Measuring Anaesthetic Depth in Patients Undergoing Propofol-Sevoflurane Anaesthesia

Processed electroencephalographic indices such as the Patient State Index (PSi) have been proposed as a method of monitoring the depth of general anaesthesia. However, in order to establish the validity of PSi, it is necessary to compare its accuracy to other techniques, such as the isolated forearm technique (IFT). 30 ASA (American Society of Anaesthesiologists classification) I–III patients were administered standard propofol-sevoflurane anaesthesia with PSi monitoring. Before each dose of muscle relaxant, the forearm contralateral to the intravenous cannula was isolated for 30 minutes. PSi, hand movements, and exhaled sevoflurane concentrations were recorded during the induction of anaesthesia, intubation, throughout the surgical procedure, prior to extubation, and patients’ responses to commands were classified as positive or negative. The Brice questionnaire was used to assess intraoperative awareness after the patient regained consciousness. All patients completed the study without memory of the surgery or IFT commands, 53.3% of patients showed a response to IFT commands during intubation. PSi values ranged around 26 (23.5; 30) in IFT non-responders and 33.5 (26; 42) in responders (p = 0.15). The best cut-off value for PSi to predict IFT response was 55, with a sensitivity of 60% and specificity of 85.2%, which led to the conclusion that PSi is not a fully reliable tool for monitoring the depth of anaesthesia.