Experimental: Normobaric high-concentration oxygen (NBHO) group

After preoxygenating (FiO2=100%, 6 L/min) with a face mask for 3 min, patients will be sequentially administered intravenous sufentanil 0.2 µg/kg followed by propofol 2 mg/kg. Once the eyelash reflex was absent, all patients received 0.6 mg/kg rocuronium, with an endotracheal tube inserted approximately 90 seconds later. Mechanical ventilation is set to volume-controlled ventilation (VCV) mode, fresh gas flow 4 L/min, tidal volume (Vt) 6-8 ml/kg, respiratory rate (RR) 12-14 breaths/min, and positive end-expiratory pressure (PEEP) 5 cmH2O. End-tidal carbon dioxide (PetCO2) will be continuously monitored, maintaining it between 35-40 mmHg. Based on group allocation, the NBHO group will adjust the FiO2 at 80% throughout the surgery. Anesthesia will be maintained through total intravenous anesthesia, continuously infusing remifentanil 0.05-0.1 µg/kg/min and propofol 4-6 mg/kg/min, with intermittent 10 mg rocuronium as needed.

Experimental: Normobaric low-concentration oxygen (NBLO) group

After preoxygenating (FiO2=100%, 6 L/min) with a face mask for 3 min, patients will be sequentially administered intravenous sufentanil 0.2 µg/kg followed by propofol 2 mg/kg. Once the eyelash reflex was absent, all patients received 0.6 mg/kg rocuronium, with an endotracheal tube inserted approximately 90 seconds later. Mechanical ventilation is set to volume-controlled ventilation (VCV) mode, fresh gas flow 4 L/min, tidal volume (Vt) 6-8 ml/kg, respiratory rate (RR) 12-14 breaths/min, and positive end-expiratory pressure (PEEP) 5 cmH2O. End-tidal carbon dioxide (PetCO2) will be continuously monitored, maintaining it between 35-40 mmHg. Based on group allocation, the NBLO group will adjust the FiO2 at 30% throughout the surgery. Anesthesia will be maintained through total intravenous anesthesia, continuously infusing remifentanil 0.05-0.1 µg/kg/min and propofol 4-6 mg/kg/min, with intermittent 10 mg rocuronium as needed.