Patients entered the operating room without premedication. Airway-related parameters (modified Mallampati score, interincisior gap, thyromental distance, sternomental distance, and retrognathia) were evaluated in the sitting position. After changing to the supine position, thyromental height was measured and patients were monitored with electrocardiography, pulse oximetry, non-invasive blood pressure measurement, and processed electroencephalography. Cotton towels were placed below the patient's head for horizontal alignment of the external auditory meatus and the suprasternal notch. After sufficient preoxygenation for \> 3 min, total intravenous anaesthesia was induced using a target-controlled infusion of propofol and remifentanil (effect site concentration: 4 µg/mL for propofol and 4 ng/mL for remifentanil). After confirming loss of consciousness, rocuronium (0.6-0.8 mg/kg) was administered to facilitate tracheal intubation, and mask ventilation was conducted. During mask ventilation, its difficulty was assessed and the use of the oropharyngeal airway was recorded. For invasive blood pressure measurement, a 20-gauge catheter was placed on the patient's radial artery. Thereafter, the patient's head was placed in the neutral position and an cervical collar was applied to minimise neck movement during intubation.

About 90 s after administrating rocuronium, tracheal intubation was attempted by one of five attending anaesthesiologists with a collective history of more than 30 successful videolaryngoscopic and fiberscopic intubations. A reinforced tracheal tube was used in both groups. Orotracheal intubation was performed using a videolaryngoscope with a disposable Macintosh-typed blade and a malleable stylet in the videolaryngoscope group, and using a flexible fiberscope with an eyepiece in the fiberscope group. A tracheal tube was mounted on the stylet and flexible fiberscope in the videolaryngoscope and fiberscope groups, respectively. The stylet was angulated about 60° at the proximal margin of the cuff. Tracheal intubation was initially attempted without any assistance, including airway manoeuvres and devices. If additional airway manoeuvres including external laryngeal manipulation (ELM), jaw thrust, and tongue traction were required during intubation attempts, they were performed by an assistant and noted. After confirming the glottis, the percentage of glottic opening (POGO) score was recorded just before inserting the tracheal tube into the trachea. The tracheal tube was inserted into the trachea after removing the stylet and confirming the carina in the videolaryngoscope and fiberscope groups, respectively. Successful intubation was confirmed by waveform capnography. The time from oral insertion of the intubation device to obtainment of the glottis view (glottis visualisation time), to placement of the tracheal tube (tube placement time), and to detection of the waveform capnograph (capnograph detection time) were also measured.

A maximum of three intubation attempts were allowed for the same anaesthesiologist, with a 180-s time limit for each attempt. If the peripheral oxygen saturation (SpO2) decreased to below 90% during the intubation attempt, the attempt was considered to have failed, and rescue mask ventilation was conducted until SpO2 reached 100%. Cases where tracheal intubation was unsuccessful on all three attempts were recorded as intubation failure.

After surgery, intubation-related airway complications were investigated. Airway injuries (blood in the oral cavity, blood staining on the tracheal tube, gum bleeding, and dental trauma) were assessed at the end of surgery. In addition, hoarseness and sore throat were evaluated at 1 h and 24 h after surgery. The severity of sore throat was measured using a numeric rating scale (NRS; 0: no pain, 10: worst pain imaginable).

Mean blood pressure (MBP), heart rate (HR), SpO2, and bispectral index (BIS) were recorded just before the intubation attempt and at 1 min after successful intubation. Other perioperative data (demographic, anaesthetic, and surgical data) were collected from electronic medical records.