During sedated gastroscopy, the insertion of the fiberscope and gastric distension required to perform the examination may induce respiratory depression, airway obstruction, and decreased chest wall compliance. Patients with obesity, especially visceral fat, have poor lung and chest wall compliance, lower lung capacity and functional residual capacity, and an unbalanced ventilation-to-perfusion ratio. Thus, obese patients are at a high risk of hypoxemia. Increasing evidence supports the use of High-flow nasal cannula (HFNC) oxygenation in obese patients during sedated gastrointestinal endoscopy. Obesity, especially visceral obesity, is an established risk factor associated with all-cause mortality. Body roundness index (BRI) is a newer anthropometric measure associated with identification of high-risk individuals. Owing to the limited evidence, we designed this unblinded randomized controlled trial to assess whether HFNC, compared to standard mask oxygenation, improves oxygenation at the end of the procedure (primary endpoint) in patients with visceral obesity.