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Background: A deep level of anaesthesia measured by the bispectral index has been found to improve processing speed as one aspect of cognitive function after surgery. The purpose of the present study was to assess the possible effect of the level of anaesthesia on post-operative cognitive dysfunction (POCD) 1 week after surgery, as assessed by a neuropsychological test battery.

Methods: We included 70 patients >60 years of age scheduled for elective non-cardiac surgery with general anaesthesia. The depth of anaesthesia was monitored using the cerebral state monitor, which provided a cerebral state index (CSI) value. Cognitive function was assessed by the ISPOCD neuropsychological test battery before and at 1 week (or hospital discharge) after surgery and POCD was defined as a Z score above 1.96.

Results: Five patients were not assessed after surgery. The mean CSI was 40 and 43 in patients with (N=9) and without POCD (N=56), respectively (P=0.41). The cumulated time of both deep anaesthesia (CSI<40) and light anaesthesia (CSI>60) did not differ significantly, and no significant correlation was found between the mean CSI and the Z score.

Conclusion: We were unable to detect a significant association between the depth of anaesthesia and the presence of POCD 1 week after the surgery.