After the end of surgery, as soon as the patient was awake, the pain intensity was assessed using the 4-points verbal rating scale (VRS) score (0: no pain, 1: mild pain, 2: moderate pain, and 3: intense pain), at 5-min interval, while staying at PACU. The pain was assessed at rest (VRS-R). If the patient reported VRS-R ≥ 3, an anesthetist not involved in the study administered a rescue dose of IV fentanyl 1 µg/kg. Then, pain was assessed every 5 min, and fentanyl given until VRS-R < 3. If the score did not fall below 3 despite 5 times of bolus fentanyl administration (30 min after arrival on PACU), the patient was dropped out of the study.