Effect of Carotid Ultrasound-guided Fluid Management on Anesthetic-induced Hypotension in Elderly Patients Undergoing Gastrointestinal Surgery

The goal of this clinical trial is to learn about Whether carotid ultrasound guided fluid management can reduce the incidence of hypotension after general anesthesia induction in elderly patients undergoing gastrointestinal surgery. The main question\[s\] it aims to answer are: Whether fluid therapy reduces the incidence of general anaesthesia induced hypotension in elderly patients undergoing gastrointestinal surgery. Whether carotid ultrasound can guide fluid management in elderly patients undergoing gastrointestinal surgery. Participants will use carotid ultrasound to assess volume status. If volume is insufficient, fluid therapy will be selected until volume is sufficient, anesthesia will be then induced. If volume is sufficient, anesthesia will be directly induced. There is a comparison group: No carotid ultrasound will be used to assess the volume status of the patients, and anesthesia will be directly induced.