IntroductionPostoperative cognitive decline, at least transiently, is considered a prevalent and significant complication in elderly patients undergoing surgery. If the mechanisms that underlie it impact brain function intraoperatively, an effective intraoperative marker of brain dysfunction would be useful. However, to date, there is no consensus marker for this aim. We have developed an EEG-based marker—the Brain Reactivity Index (BRI). To assess the efficacy of intraoperative BRI as a marker