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Preserved Memory Function during Bispectral Index–guided Anesthesia with Sevoflurane for Major Orthopedic Surgery

Background:Memory function under anesthesia is undesired but may arise from light hypnosis as well as stress-enhanced learning during surgery. The bispectral index (BIS, Aspect Medical Systems, Norwood, MA) is a monitor of hypnotic state that can help to avoid light hypnosis (i.e., BIS above 60). This study tested the hypothesis that BIS-guided anesthesia maintaining BIS 50–60 reduces the likelihood of memory function under anesthesia. Methods:After obtaining informed consent, 128 patients scheduled for joint replacement surgery under general anesthesia with sevoflurane were randomly assigned to BIS-guided anesthesia, titrating drugs to BIS 50–60 (BIS group), or a standard practice group in which BIS was recorded but did not guide drug administration. After induction, all patients were repeatedly played a list of 15 words. After recovery from anesthesia, all were interviewed about recall and completed a recognition memory test containing the presented words (targets, 33.3%) and new words (distractors). Preoperatively, patients filled out a stress questionnaire (Spielberger State-Trait Anxiety Inventory). Results:BIS values above 60 were recorded in both groups, but no patient recalled the presented words postoperatively. Only patients in the BIS group selected targets more often than distractors (37.1% vs. 31.5% hit rate, P = 0.001). Target hit rates correlated poorly to stress scores (P > 0.9), but were associated with the amount of fentanyl received preoperatively (P = 0.01) and pain management technique (P < 0.01). Conclusions:BIS titration to BIS 50–60 does not necessarily curb memory function under anesthesia when BIS values higher than 60 occur. Preoperative analgesia attenuated the likelihood of memory function under anesthesia.