Inclusion Criteria:  ASA grade Ⅰ-Ⅲ. patients aged between 20 and 70, signed informed consent form, and are willing to participate in the study. BMI ≤ 30kg/m2. elective major abdominal surgery for the first time (radical resection of ovarian cancer, radical resection of cervical cancer, radical resection of gastric cancer, radical resection of colorectal cancer, radical resection of bladder tumor and radical resection of prostate tumor). NYHA cardiac function grade Ⅰ-Ⅲ. the function of liver and kidney is normal before operation. the blood coagulation function was normal before operation. hemoglobin > 70g/L. sinus rhythm was detected by electrocardiogram and there were no other malignant arrhythmias.  Exclusion Criteria:  Emergency surgery. liver and kidney surgery. complicated with chronic respiratory diseases and FEV1/FVC < 70%. the operation time is less than 2 hours. patients with mental illness or cognitive impairment. uncontrolled hypertension with blood pressure higher than 180 × 100mmHg. patients with hypothyroidism. pregnant or lactating patients. -