Before extubation during the anesthesia recovery period, 100% oxygen is routinely inhaled to increase the oxygen reserves, maximizing the time window for anesthesiologists to adjust strategies when they encounter hypoxemia after extubation.

However, even inhaling a short period of pure oxygen can cause absorptive atelectasis, and may even impair the effectiveness of intraoperative protective ventilation measures continuing to post-operative period. The purpose of this study is to determine whether 30% oxygen before extubation after abdominal surgery could reduce hypoxemia incidence after extubation during the recovery period or not, compared to 100% oxygen. 590 patients scheduled to abdominal surgeries, will be randomly assigned to receive 30% or 100% oxygen concentration from the end of surgery to extubation after general anesthesia in the post-anesthesia care unit. The incidence of hypoxemia (SpO2 < 90%) from extubation to leaving the post-anesthesia care unit (PACU) is the primary outcome.

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.