Abstract: Objective:To explore the preventive effects of preoxygenation on hyoxemia in obese patients treated with painless colonoscopy.Methods:A total of 128 patients with body mass index≥30 kg/m² treated with painless colonoscopy were equally divided into the preoxygenation group and control group.The preoxygenation group were treated with high flux inhalating oxygen for 3 to 5 mins before anesthesia,while the control group took natural breathing.The levels of blood oxygen saturation(SpO₂),mean arterial pressure and heart rate between two groups were compared in different time-points.The cases of SpO₂ decline range during examination,examination time and total quantity of propofol administration between two groups were compared.Results:The levels of SpO₂ before anesthesia,after 1 and 3 mins of colonoscopy,and at the end of examination in preoxygenation group were obviously higher than that in control group(P<0.01).The proportion of SpO₂<95% and examination time in preoxygenation group were obviously less than those in control group(P<0.05).Conclusions:The preoxygenation in obese patients before painless colonoscopy can effectively improve the reserving oxygen capacity,reduce the risk of hypoxemia,and shorten the time of colonoscopy.