Results
orotracheal intubation, tooth, ORI
A total of 197 patients were included in the final analysis (Fig. 
Patient enrolment flowchart Among 218 patients scheduled for elective surgery under general anaesthesia who required orotracheal intubation, eight did not meet the inclusion criteria and 10 declined to participate. Three patients were excluded from the study due to technical problems regarding the recording of vital signs, including the oxygen reserve index. The final analysis included 197 patients
Characteristics of the patients in the two groupsAmerican Societyof Anaesthesiologists physical classData are presented as mean ± standard deviation, or the number of patients (percentage). Patients in the HFNO group reported more discomfort than those in the facemask oxygenation group (P < 0.001, Table 
Level of comfort and intubation characteristics in the two groupsData are presented as mean ± standard deviation or the number of patients (percentage).The ORI increased during preoxygenation in all patients (Fig. 
Changes in ORI and SpO Changes in the ORI and SpO
Changes in the oxygen reserve index during anaesthetic induction in the two groupsData are presented as mean ± standard deviation.
The ORI at 1 min of preoxygenation Violin plots show the distribution of the ORI at 1 min of preoxygenation for patients in the HFNO and facemask oxygenation groups. A boxplot comparing the ORI at 1 min of preoxygenation between the two groups, which was assessed using Student’s t-test (P < 0.01), is also shown. The horizontal bar represents the median, and the edges of the box represent the interquartile range, with the whiskers representing the lower quartile. Abbreviations: ORI, oxygen reserve index; HFNO, high-flow nasal oxygen; FMV, facemask ventilationThe baseline mean SpOAll patients underwent successful tracheal intubation. Two patients required more than one intubation attempt, including one patient with a loose tooth and another patient with a grade III airway who required a video laryngoscope (AceScope, Acemedical Co., Korea followed by Glidescope®, Saturn Biomedical System Inc., Burnaby, BC, Canada).