High-Flow Nasal Cannula After Major Abdominal Surgery to Prevent Postoperative Pulmonary Complications

Inclusion Criteria: * Adults aged 19 years or older. * Undergoing major abdominal surgery under general anesthesia. * Major abdominal surgery is defined as one of the following: 1. Upper gastrointestinal surgery: total gastrectomy, distal gastrectomy, proximal gastrectomy, or esophagogastric junction surgery. 2. Lower gastrointestinal surgery: right or left hemicolectomy, low anterior resection, anterior resection, total colectomy, abdominoperineal resection, or segmental small bowel resection with anastomosis. 3. Hepatobiliary and pancreatic surgery: hepatic resection (≥1 segment), right or left hepatectomy, central hepatectomy, distal pancreatectomy, pancreaticoduodenectomy (Whipple procedure), or bile duct resection with reconstruction. 4. Other moderate-to-major intra-abdominal procedures involving major organ resection or reconstruction, as determined by the investigator (e.g., complex cholecystectomy with extensive adhesiolysis, intra-abdominal tumor resection). * Actual anesthetic duration ≥ 3 hours, confirmed at the end of surgery. * Able to undergo immediate extubation at the end of surgery and be transferred to the post-anesthesia care unit or intensive care unit with spontaneous breathing. * Provided written informed consent before surgery. Final enrollment will occur only if the actual surgical procedure and anesthetic duration meet all eligibility criteria after surgery. Exclusion Criteria: * Requirement for postoperative invasive mechanical ventilation. Failed extubation immediately after surgery. * Severe intraoperative instability preventing routine postoperative recovery (e.g., major hemodynamic instability, massive bleeding, or cardiopulmonary resuscitation). * Requirement for continuous oxygen therapy, noninvasive ventilation, or invasive mechanical ventilation before surgery. * Thoracic surgery within 4 weeks before the index surgery. Inability to receive HFNC because of upper airway or nasal anatomical limitations. * Planned reoperation at the completion of the index surgery. * Inability to complete study participation or follow-up because of impaired decision-making capacity or practical inability to complete follow-up. * Any condition that, in the investigator's judgment, makes study participation inappropriate.