Objective:To compare the safety apnea time during endotracheal intubation in the patients from different altitudes using oxygen reserve index (ORI).Methods:Sixty American Society of Anesthesiologists physical status Ⅰor Ⅱ patients, aged 18-70 yr, undergoing elective surgery requiring tracheal intubation under general anesthesia and requiring catheterization via arterial puncture, were included.Among the patients, 30 cases who had long lived at an altitude of 1 500-3 000 m in Qinghai Province People′s Hospital (Xining, 2 200 m above sea level) served as middle-altitude group, and 30 Tibetan patients who had long lived at an altitude >3 000-meter area in Yushu People′s Hospital (Yushu, 3 600 m above sea level) served as high-altitude group.The patients were preoxygenated for 5 min before induction of anesthesia, and then endotracheal intubation was performed with a video laryngoscope.Before induction (T 0), at 3 min of pre-oxygenation (T 1), and at 5 min of pre-oxygenation (T 2), arterial blood was collected for blood gas analysis, and PaO 2 was recorded, ORI and SpO 2 were simultaneously recorded.The time from the beginning of intubation to the time when ORI was decreased to 0 and the time from the beginning of intubation to the time when SpO 2 was decreased to 98% were recorded. Results:Compared with middle-altitude group, the time from the beginning of intubation to the time when ORI was decreased to 0 and the time from the beginning of intubation to the time when SpO 2 was decreased to 98% were significantly prolonged ( P<0.05), and no significant change was found in SpO 2, ORI and PaO 2 at each time point in high-altitude group ( P>0.05). Conclusions:The safety apnea time during endotracheal intubation is longer in the patients at high altitudes (altitude > 3000 m) than those at the moderate altitudes (altitude 1500-3000 m).