Entropy of EEG during anaesthetic induction: a comparative study with propofol or nitrous oxide as sole agent

Background. The search continues for an anaesthetic monitor that can define the level of anaesthesia in an individual patient irrespective of anaesthetic agent(s) used. Studies of available monitors based on bispectral analysis or evoked auditory potentials show the complexity of the problem. We assessed a new monitor, based on the entropy of the EEG, during induction of anaesthesia with either propofol or nitrous oxide. Methods. In an open, randomized study (two groups; n=10) of day surgical patients, we induced loss of response with incremental boluses of propofol. The other group was given propofol 30 mg and then increasing concentrations of nitrous oxide until loss of response. We measured entropy with the M-Entropy Module S/5(TM) (Datex-Ohmeda) using forehead electrodes and recorded response entropy (RE; including frontal electromyogram) and state entropy (SE; only the cortical EEG). Values are median (range). Results. Baseline values were RE 98 (96-100), SE 89 (87-91) and RE 98 (96-99), SE 89 (87-91) for the propofol and nitrous oxide patients, respectively. During propofol induction, both entropy indices decreased with increasing sedation, with RE 40 (23-76) and SE 34 (17-70) at loss of response. Neither RE nor SE decreased during nitrous oxide inhalation, and at loss of response using nitrous oxide, RE and SE were unchanged at 98 (96-100) and 88 (85-91) respectively. Conclusions. The entropy monitor of anaesthetic depth shows a successive decrease with propofol but loss of consciousness with nitrous oxide is not associated with change in entropy indices.