Is depth of anesthesia, as assessed by the bispectral index, related to postoperative cognitive dysfunction and recovery?

We randomized 74 patients to either a lower Bispectral Index (BIS) regimen (median BIS, 38.9) or a higher BIS regimen (mean BIS, 50.7) during the surgical procedure. Preoperatively and 4-6 wk after surgery, the patients' cognitive status was assessed with a cognitive test battery consisting of processing speed index, working memory index, and verbal memory index. Processing speed index was 113.7 +/- 1.5 (mean +/- SE) in the lower BIS group versus 107.9 +/- 1.4 in the higher BIS group (P = 0.006). No difference was observed in the other two test battery components. Somewhat deeper levels of anesthesia were therefore associated with better cognitive function 4-6 wk postoperatively, particularly with respect to the ability to process information.