Regional epidural analgesia: kinetics of pethidine

[Use of epidural and intrathecal opiates in obstetrics]

The discovery of opiate receptors and naturally occurring opiate-like substances in the central nervous system started a new era in pain control. Epidural and spinal opiates have been increasingly used since 1979. However, applying these analgesic techniques in obstetrics has been criticized because of possible side-effects on the mother and foetus. In this literature survey, their advantages and disadvantages are analyzed. Maternal side-effects include pruritus, nausea, urinary retention, and, most of all, respiratory depression. As a general rule, these side-effects are greater with the intrathecal route, high doses, and the use of morphine. The effects on the course of labour are small, and neonatal status is not altered. Spinal and epidural opiates are efficient analgesic techniques for labour and caesarean section. They provide a dose-related, but not surgical, analgesia. Currently, there is a great deal of interest in mixtures of a diluted local anaesthetic agent and a lipophilic drug for use during labour or caesarean section. An opiate alone may not consistently provide satisfactory analgesia during labour, and it cannot be recommended for routine use, except for patients in whom the cardiovascular effects of routine regional anaesthesia are to be avoided. The choice of a lipid-soluble opiate like fentanyl is safe. However, when considering new drugs, great care must be taken to avoid unforeseen problems. A good knowledge of the problem and a cautious approach combined with careful monitoring of the respiratory rate and adequacy of ventilation are the keys to the safe use of spinal and epidural opiates.

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.