The patient was laid in a supine position, with the head laid on a 7cm-high pillow for the sake of stability. The peripheral vein was accessed routinely and connected to a standard monitor for electrocardiography (ECG), SpO2 and noninvasive blood pressure (BP). Sufficient oxygen inhalation was initiated for > 3min before induction of general anesthesia with midazolam, propofol, fentanyl and rocuronium bromide. The anesthesiologist was not allowed to give any instruction and suggestion or assistance to the operator during the whole process. The coaching anesthesiologist supervised the whole TI process closely. Should any hidden risk occur or the TI exceed the required time of the experiment, the TI attempt of the operator would be terminated and replaced by the anesthesiologist in order to ensure patients' safety: TI exceeded 120s; Attempt of TI exceeded 2 times; SpO2 decreasing to below 95%; appearance of unstable hemodynamic situations (mean BP or HR fluctuating by more than 25% of the baseline value); or occurrence of airway injury as evidenced by stained blood on the Macintosh Blade. Any patient with C&L grade III~IV as discovered by the anesthesiologist during TI would be excluded from the present study and replaced by anther patient to re-start the experiment. A 7.0- or 7.5-mm-ID endotracheal tube was used as the guide according to the patient's condition.