Abstract: Objective: To investigate the relationship between state entropy (SE) and predicted effect-site propofol concentration and the accuracy for predicting different clinical endpoints. Methods: Twenty unpremedicated patients undergoing elective surgery with general anesthesia received a target-controlled infusion of propofol. The initial effect-site concentration of propofol was set at 1.0 μg/ml, and increased by 0.5 μg/ml at 1 min after having obtained a specific target concentration. The depth of sedationanesthesia was assessed by using the Observer′s Assessment of Alertness/Sedation (OAA/S) rating scale. The target sedation level of induction was defined as OAA/S score at 1. Results: With the increase of sedation, there was a progressive decrease of SE and BIS values. SE(r= 0.82, P＜0.01) and BIS(r=0.73, P＜0.01) positively correlated with OAA/S, while SE(r=-0.66, P＜0.01) and BIS(r=-0.70, P＜0.01 ) negatively correlated with the predicted effect-site propofol concentration. The predicted effect-site propofol concentration for 50% and 90% patients at loss of consciousness was 3.55（3.17，4.61）μg/ml and 4.83（4.07，7.00）μg/ml. Conclusion: Compared with BIS, SE has the same good correlation with the effect-site concentration and it is useful in predicting the different clinical endpoints.

Key words: Propofol, Bispectralindex, entropy, Electroencephalography, Drug delivery system, Anesthesia, intravenous