This study intends to evaluate the effect of different inhaled oxygen concentrations on the effect of lung recruitment in patients undergoing abdominal surgery. During perioperative mechanical ventilation, higher FiO2 is usually given to prevent the occurrence of hypoxemia, but there are no conclusions about the setting of optimal FiO2 during surgery.

Dynamic pulmonary compliance enables real-time monitoring of alveolar changes and is used to assess pulmonary function in intraoperative patients. The value of compliance represents the number of air-containing alveoli; the more alveoli are trapped, the less air-containing tissue there is and the lower the compliance. Lung recruitment, as part of a lung-protective ventilation strategy, is effective in opening alveoli and improving oxygenation. This single-center, randomized controlled trial will explore the optimal inspired oxygen concentration that allows patients to benefit from lung recruitment and the duration of improvement in lung compliance after lung recruitment.

Inclusion Criteria:

• ASA physical status classification: I or II.
• Adults aged 18 to 64 years, regardless of gender.
• BMI between 18 kg/m² and 25 kg/m².
• Patients undergoing elective abdominal surgery under general anesthesia.
• Patients with estimated operating time ≥1.5h and intraoperative bleeding ≤500ml.
• Patients who have provided written informed consent after understanding the study purpose, procedures, and potential risks.

Exclusion Criteria:

• Previous thoracic surgery.
• Chest X-ray or CT suggestive of pneumothorax or alveoli.
• Lung disease: chronic bronchitis, asthma, moderate to severe obstructive ventilatory dysfunction.
• Preoperative pulse oxygen saturation (SpO2) <90% on breathing air or SpO2 <95% on oxygen.
• Contraindications to lung reanimation: high intracranial pressure, hypovolemic shock, right heart failure.
• Severe heart disease (New York Heart Association, NYHA class III or IV.
• Acute coronary syndrome or sustained ventricular tachyarrhythmia).
• Participation in other interventional studies or refusal of enrollment.

De-identified individual participant data (IPD) will be shared. Data will include demographic characteristics, procedural details, and outcome measures. Access will be granted to qualified researchers upon request via a secure data repository.

De-identified individual participant data (IPD) will be available starting 6 months after the publication of the primary study results. Data will remain available for 3 years.

Qualified researchers affiliated with academic, governmental, or non-profit institutions may request access to the IPD. Requests must include a detailed research proposal, institutional review board (IRB) approval (if applicable), and a signed data-sharing agreement. Access will be provided to de-identified data relevant to the study objectives, including demographic information, procedural details, and primary/secondary outcome measures. Data will be shared via a secure repository upon approval of the request.