Cerebral state index during anaesthetic induction: a comparative study with propofol or nitrous oxide

Background: Confidently predicting the depth of anaesthesia for the individual patient and independently of drug(s) type using EEG-based monitors has proven difficult. This open, randomized, explorative study of day surgical patients evaluates the ability of the Cerebral State Monitor (TM) (Danmeter AB, Odense, Denmark) of anaesthetic depth to identify loss of response (LOR) using either propofol or N2O for induction. Methods: In this open, randomized study, day surgical patients (n = 10 in each group) were studied using the Cerebral State Index Monitor (TM). After baseline measurements, induction to LOR was achieved with either repeated 30-mg boluses of propofol every second minute or with N2O (after premedication 5 min before with 30 mg propofol) increased every other minute in 15% increments (max. 75%). Sedation level was evaluated every other minute using the Observer's Assessment of Alertness/Sedation scale Results: Baseline values were 91 (82-98) and 94 (82-100) for N2O and propofol patients, respectively. During induction CSI (TM) decreased with increasing sedation in patients given propofol (P < 0.001) but not in patients given nitrous oxide. Median value at LOR was 56 (40-76) and 95 (87-100) for the propofol and nitrous oxide group of patients, respectively. Conclusion: The Cerebral State Index (TM) behaves as other depth of anaesthesia monitors with a progressive decrease during propofol induction but loss of consciousness with N2O results in no change in CSI (TM).