Kaplan-Meier curve of the weaning time from MV among the 54 patients who were successfully weaned. The curve shows the probability of successful weaning from MV (P < 0.004). Dotted line: CG (parenteral fentanyl); continuous line: MG. The numbers on the perpendicular dotted lines are the median durations of the weaning time in each group. CG, control group; MG, methadone group; MV, mechanical ventilation.