Epidural versus intramuscular fentanyl. Analgesia and pharmacokinetics in labour.

In a randomised double blind trial, 36 patients in the first stage of labour received either epidural or intramuscular fentanyl at the same time as the epidural test dose of bupivacaine. Analgesia was more rapid in onset and more complete in the epidural fentanyl group. Supplementary doses of bupivacaine were required within the first hour in 62% of the intramuscular fentanyl group compared with only 16% in the epidural group. Plasma fentanyl concentrations showed wide interindividual variation, but after epidural fentanyl the peak occurred earlier. There was no correlation between analgesia and plasma fentanyl concentration, and epidural fentanyl produced superior analgesia but a systemic contribution to this effect cannot be ruled out.