Perioperative management
intravascular hypovolemia
The regimens were standardized in both groups. Patients were deprived of water for 2 h and fasted for 8 h before surgery. Based on the requirements of anesthesia and surgery, venous access (central vein of the upper limb) was established. As perioperative fasting and bowel preparation are believed to cause intravascular hypovolemia, preemptive intravenous fluid was infused with 10–15 ml/kg normal saline. Standard intraoperative monitoring, which included electrocardiogram (ECG), heart rate (HR), blood pressure (BP), respiratory rate (RR), and pulse oxygen saturation (SpO