Major abdominal surgery is associated with a substantial risk of early postoperative pulmonary complications, including atelectasis, hypoxemia, pneumonia, and increased need for respiratory support. High-flow nasal cannula (HFNC) provides warmed and humidified oxygen at high flow rates and may improve oxygenation, reduce respiratory workload, and support lung volume during the immediate postoperative period. However, whether routine prophylactic HFNC after major abdominal surgery reduces clinically significant postoperative pulmonary complications remains uncertain.

This prospective, single-center, randomized controlled trial will evaluate whether prophylactic HFNC applied for 24 hours immediately after extubation reduces postoperative pulmonary complications compared with standard postoperative respiratory care in adult patients undergoing major abdominal surgery under general anesthesia with an actual anesthetic duration of 3 hours or longer.

Postoperative pulmonary complications (PPCs) remain an important cause of delayed recovery after major abdominal surgery. Early postoperative respiratory impairment, including atelectasis, impaired oxygenation, secretion retention, and increased respiratory effort, may lead to prolonged hospitalization, escalation of respiratory support, and increased postoperative morbidity. High-flow nasal cannula (HFNC) provides several physiological benefits, including delivery of warmed and humidified oxygen, improved mucociliary clearance, reduction of anatomical dead space, decreased upper airway resistance, and mild positive airway pressure that may help preserve functional residual capacity. Despite these physiological advantages, evidence supporting the routine preventive use of HFNC after major abdominal surgery remains inconclusive.

This study is a prospective, single-center, randomized, controlled, open-label, parallel-group clinical trial designed to evaluate the clinical effectiveness of prophylactic HFNC in adult patients undergoing major abdominal surgery under general anesthesia. Eligible participants are adults aged 19 years or older who undergo protocol-defined major abdominal surgery with an actual anesthetic duration of at least 3 hours and are successfully extubated immediately after surgery. Final eligibility is confirmed after surgery based on the actual surgical procedure performed and the actual anesthetic duration.

After arrival in the post-anesthesia care unit or intensive care unit following extubation, participants will be randomized in a 1:1 ratio to either the HFNC group or the standard postoperative respiratory care group using a centralized web-based randomization system. Randomization will use block randomization with stratification according to surgical approach and surgical category. The HFNC group will receive warmed and humidified high-flow nasal cannula therapy initiated within 30 minutes after arrival and continued for 24 hours in addition to standard postoperative respiratory care. The standard care group will receive conventional postoperative respiratory management, including low-flow oxygen as clinically indicated, deep-breathing and coughing instruction, pain control, early mobilization, respiratory physiotherapy, and secretion clearance support as needed.

The primary outcome is the occurrence of clinically significant postoperative pulmonary complications within 7 postoperative days, assessed using standardized consensus-based definitions. Secondary outcomes include individual respiratory complications, escalation of respiratory support, reintubation, intensive care unit and hospital length of stay, 30-day readmission, 30-day all-cause mortality, and selected postoperative non-pulmonary complications.

This trial is intended to provide high-quality evidence regarding whether short-term prophylactic HFNC during the immediate postoperative period can reduce clinically meaningful pulmonary complications in a clearly defined high-risk population after major abdominal surgery.

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.

Individual participant data (IPD) will not be shared because no formal data-sharing mechanism has been established for this investigator-initiated single-center study, and data access may be limited by institutional and privacy regulations.