Unexpected difficult intubation was defined as the presence of at least 2 following conditions which were laryngoscopic view 3 or 4, attempted intubation > 3 times, total intubation time > 300 second or presence of desaturation (SpO2 < 95%). The intubation time was defined as the time starting from applying laryngoscope to presence of positive capnography wave form. In case of fail intubation, the intubation time started from applying laryngoscope to remove laryngoscope from children's mouth. The total intubation time was the combined timing of each intubation attempts