The Impact of Multimodal Brain Monitoring on Postoperative Outcomes in Elderly Patients Undergoing Major Abdominal Surgery

The purpose of this clinical trial is to investigate the effect of multimodal brain monitoring under general anesthesia on postoperative outcomes in elderly patients undergoing abdominal surgery.The research hypothesis is that multimodal brain monitoring under general anesthesia can reduce the incidence of postoperative composite complications and improve postoperative outcomes in elderly patients undergoing major abdominal surgery. Eligible patients will be randomly assigned in a 1:1 ratio to the Multimodal Brain Monitoring under General Anesthesia Group (MBMGA group) and the Bispectral Index Monitoring under General Anesthesia Group (BIS group). The primary outcome is the postoperative composite complication rate . Secondary outcomes include intraoperative hyperglycemia, adverse events during anesthesia induction and maintanance, changes in blood pressure and heart rate, intraoperative hypotension, the dosage of perioperative anesthetics and vasoactive drugs, changes in serum cortisol, cardiac troponin I, blood creatinine (Cr) level, extubation time and awake time, the rate of transferring to ICU or AICU, The time to first use analgesia pump and the cumulative dose of postoperative oxycodone, systemic complications, hospitalization time, first exhaust time , first oral intake time, rate of second surgery and mortality within 30 days postoperatively. Participants will: 1. Undergo routine preoperative fasting for 6-8 hours; 2. Prior to anesthesia induction, cooperate with the establishment of intravenous access, arterial catheterization, monitoring of physiological parameters, blood sample collection, and receive fluid preloading; 3. During surgery, for patients in the MBMGA group , propofol and remifentanyl will be adjusted according to multimodal brain monitoring indices (WLi and PTi), while for patients in the BIS group, only propofol will be adjusted based on bispectral index and remifentanyl will be adjusted according to anesthesiologists' experience; simult