Study population

Patients were included if they met all the following criteria: (1) scheduled to undergo neurosurgery with an expected duration ≥ 2 h (the reason for choosing neurosurgery is that postoperative chest CT scans could be performed at the same time as routine brain CT scans in patients undergoing neurosurgery); (2) supine position during surgery; (3) age ≥ 18-years-old; (4) American Society of Anesthesiologists (ASA) of I-III; (5) oxygen saturation (SpO