Kaplan-Meier curve of the weaning time from MV. The curve shows the probability of successful weaning (P = 0.11). The difference was significant until the fifth day (P < 0.02). 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.