BackgroundHigh-flow nasal oxygenation and the oxygen reserve index (ORI), which is a non-invasive and innovative modality that reflects the arterial oxygen content, are used in general anaesthesia. This study compares the preoxygenation efficiency (measured by the ORI) of high-flow nasal oxygenation and facemask ventilation during the induction process.MethodsThis single-centre, two-group, randomised controlled trial included 197 patients aged ≥ 20 years who underwent orotracheal intubation for general anaesthesia for elective surgery. The patients were randomly allocated to receive preoxygenation via facemask ventilation or high-flow nasal oxygenation. The ORI was measured and compared between both groups.ResultsThe ORI increased during preoxygenation in all patients. At 1 min of preoxygenation, the ORI was significantly higher in the high-flow nasal oxygenation group (0.34 ± 0.33) than in the facemask ventilation group (0.21 ± 0.28; P = 0.003). The highest ORI was not significantly different between the two groups (0.68 ± 0.25 in the high-flow nasal oxygenation group vs. 0.70 ± 0.28 in the facemask ventilation group; P = 0.505).ConclusionsHigh-flow nasal oxygenation results in an oxygenation status similar to that provided by facemask ventilation during the induction process of general anaesthesia; therefore, high-flow nasal oxygenation is a feasible preoxygenation method.Trial RegistrationClinicaltrials.gov (NCT04291339).