Transoral Nasal Cannula Oxygen Therapy for Mouth-Breathing Patients in the Postanesthesia Care Unit

Inclusion Criteria: 1. Aged older than 18 years; 2. American Society of Anesthesiologists (ASA) physical status classification I-III; 3. Undergoing elective thoracic or abdominal surgery under general anesthesia with postoperative transfer to the postanesthesia care unit (PACU); 4. Anticipated PACU stay of at least 30 minutes to allow an adequate observation window for data collection; 5. Recovery of spontaneous breathing after tracheal extubation, and confirmed as persistent mouth breathing by the standardized three-step assessment procedure (visual screening, cotton-fiber airflow verification, and mirror confirmation); 6. Written informed consent signed by the patient or a family member. Exclusion Criteria: 1. Preoperative diagnosis of severe obstructive sleep apnea-hypopnea syndrome (apnea-hypopnea index \[AHI\] \>30) or requirement for continuous positive airway pressure therapy; 2. Severe hypoxemia requiring immediate escalation of oxygen therapy (SpO2 \<90% or PaO2 \<60 mmHg); 3. Requirement for reintubation or placement of an oropharyngeal or nasopharyngeal airway after surgery; 4. Hemodynamic instability requiring vasoactive agents to maintain mean arterial pressure (MAP \<65 mmHg).