Equivalent analgesia and side effects during epidural and pharmacokinetically tailored intravenous infusion with matching plasma alfentanil concentration

Background Recently, several clinical studies comparing intravenous and epidural infusions of fentanyl and its derivatives suggested that epidural infusions act primarily by systemic absorption to produce supraspinal analgesia. To evaluate this hypothesis, the authors used pharmacokinetically tailored intravenous infusions to produce matching plasma alfentanil concentrations during epidural and intravenous administration. The analgesia and side effects achieved with each mode of administration were compared. Methods: Twelve volunteers participated in this placebo-controlled crossover study. The pain model was cutaneous electric stimulation of the finger and toe. The test battery included subjective rating of pain intensity; end-tidal carbon dioxide level; pupil size; ratings of alertness, nausea, and pruritus; and a plasma alfentanil assay. On one test day, the participants received epidural alfentanil (400 mu g bolus + a 400-mu g/h infusion for 2 h) and an intravenous saline infusion. The test battery was administered at regular intervals. On another test day, the participants received epidural saline and a computer-controlled intravenous infusion of alfentanil. The testing protocol was repeated as on the first test day. On the day the placebo was administered, the participants received epidural and intravenous saline infusions. The order of the placebo day was randomized. Results: Plasma alfentanil concentration-time profiles were identical during epidural and intravenous infusions. A nearly equivalent analgesic response was observed with epidural and intravenous alfentanil at the upper and lower extremities. There were no differences in side effects for epidural and intravenous administration. Conclusions: The systemic redistribution of alfentanil accounts for most of the analgesia and effects produced by epidural infusion.