This study aims to determine the optimal interval for periodic alveolar recruitment maneuvers (PARM) that can achieve effective lung re-expansion in 90% of patients undergoing intraoperative mechanical ventilation during abdominal laparoscopic surgery in the low Trendelenburg position. Identifying the most effective frequency of PARM is crucial for establishing lung-protective ventilation strategies, with the ultimate goal of reducing intraoperative atelectasis, decreasing postoperative pulmonary complications, and accelerating recovery. Patients are randomized using a biased-coin sequential design. PARM is initially applied every 0.5 hours, and the effectiveness of lung re-expansion is evaluated before the end of surgery. The primary outcome is the effectiveness of recruitment, assessed by a combination of lung ultrasound scores and shunt fraction. Secondary outcomes include mechanical power at the end of surgery, time-weighted average mechanical power during ventilation, postoperative P/F ratio, dead space fraction, air test results, intraoperative adverse events, incidence of respiratory failure in the PACU and postoperative period, and postoperative length of stay. This trial is expected to provide robust evidence for defining the optimal RM interval in protective ventilation protocols for patients undergoing laparoscopic abdominal surgery, thereby contributing to improved perioperative respiratory outcomes.

This study is designed to evaluate the optimal interval of periodic alveolar recruitment maneuvers (PARM) to achieve effective lung re-expansion (defined as LUS ≤1 and shunt fraction \<10%) in 90% of elderly patients undergoing laparoscopic anterior resection in low Trendelenburg position. This is a single-arm, biased-coin sequential design study. The initial PARM interval is 30 min, adjusted in 10 min increments based on response. The primary outcome is recruitment efficacy. Secondary outcomes include intraoperative mechanical power, P/F ratio, postoperative pulmonary complications, and length of hospital stay. This study aims to inform optimal RM intervals in protective ventilation protocols.