Effect of Different Oxygen Concentrations Before Extubation After General Anesthesia on Hypoxemia After Extubation in Post-anesthesia Care Unit

Inclusion Criteria: * Patients aged 18-65 years old, scheduled for elective abdominal surgery in general anesthesia with endotracheal intubation; ASA I-III grade; cardiac function 1-2 grade; 18 kg/m2 \< BMI \< 28 kg/m2; Preoperative SpO2 ≥ 94% without oxygen supplementation at rest; pre-anesthesia assessment shows no difficult airway, no difficulty with mask ventilation, no difficulty in intubation during tracheal insertion, and no difficulty in extubation as expected. Exclusion Criteria: * Respiratory infection recently, or atelectasis, inflammation, fibrosis, or pleural effusion by chest CT preoperatively. History thoracic surgery and fractures of the sternum or ribs, chest deformity, difficulty in raising both upper limbs, or scoliosis. High risk of reflux aspiration. Severe hepatic or renal dysfunction (e.g., Child-Pugh class C liver disease, or requiring dialysis). Limb movement disorders. Mask ventilation or intubation difficulty during anesthesia induction. Occurrence of severe allergy, massive bleeding, suspected pulmonary embolism, pulmonary edema, myocardial injury, or cardiopulmonary arrest during surgery. Currently participating in other clinical studies, which may have an impact on this study. Inability to cooperate well for mental disorder, or hypophrenia.