[Effect of duration of infusion on elimination rate of alfentanil]

In the present study the disposition of alfentanil after cessation of a constant rate infusion was compared with intravenous bolus injections. In 11 patients undergoing intervertebral disk surgery general anaesthesia was induced with midazolam, alfentanil and vecuronium; anaesthesia was maintained with a constant rate infusion (1.5 to 2 micrograms/kg/min, 1.0 micrograms/kg/min at the end of the infusion period) of alfentanil. The parameters of bolus injections were obtained from 5 patients undergoing lithotripter therapy; an intravenous bolus of 15 micrograms/kg was given. After infusion times of 60 to 120 minutes the mean elimination half life of alfentanil was significantly increased (227 +/- 166 min) compared to the bolus injection (50.8 +/- 9.5 min). There were significant correlations of the elimination half life with the age (r = 0.53, p = 0.03) of the patients, the total dose (r = 0.607, p = 0.01) and the infusion time (r = 0.612, p = 0.01). The total clearance of alfentanil in the bolus group was 385 +/- 138 ml/min and in the infusion group 620 +/- 215 ml/min. There were no significant correlations between the total clearance of alfentanil and the total dose and between the duration of infusion and total clearance. The clinical implications of our study: After prolonged continuous infusions of alfentanil a significant reduction of the elimination rate has to be considered. The patients must be observed very carefully in the recovery room in order to prevent a possible respiratory depression.