Behaviour of entropy/complexity measures of the electroencephalogram during propofol-induced sedation - Dose-dependent effects of remifentanil

Background: Several new measures based on the regularity of the electroencephalogram signal for the assessment of depth of anesthesia/sedation have been proposed recently. in this study we analyze the influence of remifentanil and electroencephalogram frequency content of the performance of a set of such measures. Methods: Forty-five patients with American Society of Anesthesiologists physical status I were randomly allocated to one of three groups according to the received dose of predicted effect compartment-controlled remifentanil (0, 2, and 4 ng/ml). All 45 patients received stepwise increased effect site concentration-controlled dose of propofol. At every step of propofol increase, the Observer's Assessment of Alertness/Sedation score was assessed. The following measures were calulated from the electroencephalographic signal: spectral entropy, approximate entropy, Higuchi fractal dimension, Lempel-Ziv complexity, relative beta ratio, and SyncFastSlow measure. Results: The behavior of the electroencephalogram-based measures is highly sensitive to the frequency content of the signal and the dose of remifentanil. The prediction probability with respect to the Observer's Assessment of Alertness/Sedation score of the most discriminative measure, the Higuchi fractal dimension, dropped from 0.90 (electroencephalographic frequency band 6-47 Hz, no remifentanil) to 0.55 when the frequency band was changed to 0.5-19 Hz and to 0.83 when remifentanil concentration was increased to 4 ng/ml. The coeffect of remifentanil on electroencephalographic regularity is bimodal depending on the frequency band of the signal. Conclusions: Cutting off high frequencies from the electroencephalogram and increased remifentanil concentration deteriorate the performance of the electroencephalogram-based entropy/complexity measures as indicators of the depth of propofol sedation.