Clinical assessment and plasma pharmacokinetics associated with intramuscular or extradural alfentanil

Patients with postoperative pain received alfentanil in doses of 15 micrograms kg-1 (n = 6) or 30 micrograms kg-1 (n = 6) via the extradural route or 15 micrograms kg-1 (n = 6) i.m. Effective analgesia was obtained in all patients in the extradural groups and in none in the i.m. group. Maximum pain relief developed within 10-15 min and the average duration of adequate analgesia was 78 min with the lower dose and 45 min with the higher dose, but this difference was not statistically significant. The 15-micrograms kg-1 extradural and i.m. groups showed the same pharmacokinetic pattern. Plasma concentrations were greater in the 30-micrograms kg-1 extradural group and this was associated with unwanted systemic effects. We conclude that the extradural administration of alfentanil can produce effective spinal analgesia. Its lipophilic property reduces the onset time, but does not modify its duration of action or impair absorption.