[Use of morphinomimetics in regional anesthesia]

Pain relief is one of medicine's most important challenges and the first aim of anaesthesia. The most common technique of postoperative analgesia remains intramuscular or subcutaneous opiates. There has been a better understanding of the mechanisms of action of opiates over the last decade, and new techniques and methods of administration have been developed especially their regional application. In 1979, two reports acted as catalysts to promote further studies. Wang et al. reported on the efficacy of intrathecal morphine to relieve unbearable malignant pain in 8 patients whereas Behar et al reported on the efficacy of morphine by epidural route. More recently, several studies pointed out the usefulness of the peripheral perineural route for opiates. However, this remains controversial, as some discrepancies persist in the results. The classification of opiate receptors and their relationship to opiate analgesia, as well as the mechanisms of action of regionally administered opiates are analyzed. The dual pre- and postsynaptic actions of spinal opiates are then considered. The parts played by the different neurotransmitters and pathways are set out. The evidence for opiate receptors at peripheral nerve sites and the different hypotheses suggested to explain the effect of opiates given by the perineural route are discussed. The pharmacokinetics and pharmacodynamics of opiates given by the subarachnoid and epidural routes are considered, in particular with respect to the comparative pharmacology of the commonly used opiates. The adverse effects of spinal opiates are reviewed, with their potential risks, and their clinical and therapeutic implications. Opiates and local anaesthetics given by the spinal route are compared. The clinical applications of intrathecal and epidural opiates are discussed, especially in the fields of postoperative analgesia, treatment of chest trauma, and cancer pain. Lastly, the few controlled studies concerning the use of opiates in peripheral nerve blocks, especially brachial plexus blocks, and the prospects of this new technique of giving opiates regionally are discussed.