Mechanical Power-Integrated Oxygenation Indices for Predicting Postoperative Pulmonary Complications After Major Abdominal Surgery

The goal of this prospective observational study is to determine whether novel oxygenation and oxygen saturation indices that incorporate total or dynamic mechanical power can predict postoperative pulmonary complications (PPCs) in adults undergoing major abdominal surgery. The main questions it aims to answer are: * How well do OI-MPtot, OI-MPdyn, OSI-MPtot, and OSI-MPdyn predict PPCs within the first 7 days after surgery? * Do these novel indices predict PPCs better than mechanical power (MPtot and MPdyn), conventional oxygenation and saturation indices (OI and OSI), and the preoperative ARISCAT score? * What cutoff values and diagnostic accuracy of the novel indices can be used to identify patients at increased risk of PPCs? Participants will: * Undergo elective open major abdominal surgery under general anesthesia and mechanical ventilation. * Have intraoperative mechanical ventilation parameters recorded hourly during surgery. * Have mechanical power, oxygenation, and oxygen saturation indices calculated from the collected data. * Be followed during their hospital stay and for 7 days after discharge for the occurrence of PPCs, including respiratory infection, respiratory failure, pleural effusion, atelectasis, pneumothorax, bronchospasm, and aspiration pneumonitis. * Have their clinical, surgical, and ventilatory characteristics recorded and compared according to whether PPCs occur.