Before intubation, the patient's shoulder is supported by a 5cm pillow to make neck flexion with extension of atlanto-occipital joint, confirming that the external auditory meatus and sternal notch plane are horizontal and aligned. Then, nasotracheal intubation is performed with a reinforced tube (Mallinckrodt Medical, Dublin, Ireland) using video-laryngoscope (AceScope, AceMedical, Seoul, Korea).

Before intubation, the patient's head is placed without a pillow on the bed. Then, nasotracheal intubation is performed with a reinforced tube (Mallinckrodt Medical, Dublin, Ireland) using video-laryngoscope (AceScope, AceMedical, Seoul, Korea).

Before intubation, the patient's neck is flexed with pads until the chin touches the chest. Then, nasotracheal intubation is performed with a reinforced tube (Mallinckrodt Medical, Dublin, Ireland) using video-laryngoscope (AceScope, AceMedical, Seoul, Korea).

Before nasotracheal intubation, patient's head is positioned to one of three positions according to assigned group: sniffing, neutral or flexed position.

The time between entry of the nasotracheal tube into a nostril and appearance of a carbon dioxide wave on the monitor.

The sum of score from seven variables including intubation attempts, supplementary operators, the use of alternative techniques, glottic exposure, the lifting force applied during laryngoscopy, the necessity of applied external laryngeal pressure, and position of vocal cords.

From 0 (easiest) to 8 (most difficult)