Entropy Versus Bispectral Index in Evaluation of Anesthetic Depth in Adults

Objective: To compare the sensitivity and specificity of the entropy and Bispectral Index (BIS) values with respect to predicting loss of consciousness and emergence from general anesthesia and to compare the effects of electrosurgical unit on the displayed index values of both monitors during operation. Methods: Thirty consenting patients, aged 18-65 yr, ASA physical status I or II scheduled for elective abdominal, gynecologic, urologic, or orthopedic surgery expected to last at least 1h, were studied. The simultaneous state entropy (SE), response entropy (RE) and BIS values were recorded at six time points which coincided with anesthetic and surgical maneuvers: Pre induction (baseline value), Loss of consciousness, Preincision value, 5 minutes after surgery started, Preawakening, 1 minute post awakening. Results: The BIS, RE, and SE values decreased progressively from preinduction (baseline) values of 96±3, 95±4 and 88±3 to preincision values of 37±13, 35±14 and 30±13, respectively Although the indices were comparable during the induction period, the SE values were significantly less than the RE and BIS values during the emergence period this is to be expected as SE is scored out of 91 while RE and BIS are scored out of 100. RE and SE are compared with BIS in Bland Altman plots demonstrated a good agreement especially between RE and BIS [average mean difference (95%CI)=-1.1 (-17.5 – 15.4)]. Logistic regression analysis demonstrated that the BIS, RE and SE were all significant predictors of unconsciousness (p 0.01), with area under the Receiver operating characteristics (ROC) curve values of 0.976±0.058, 0.965±0.075 and 0.950±0.115 for the BIS, RE and SE, respectively. Finally, the entropy indices were less interfered with by the electrocautery unit during the operation (11% versus 63% for the BIS monitor). Conclusions: The changes in SE and RE values followed a similar pattern to the BIS values during anesthesia. Use of entropy module is associated with less frequent intraoperative interference (by electromyographic activity and the electrocautery) during general anesthesia.