[Epidural and intrathecal administration of alpha 2-adreno-ceptor agonists for postoperative pain relief]

Spinal clonidine interacts with pre- and postsynaptic alpha(2)-adrenoceptors on afferent neurons in the superficial dorsal horn of the spinal cord: it causes analgesia by inhibition of the synaptic and electrotonic neurotransmission of nociceptive impulses. Epidural doses higher than 4 microg/kg have an analgesic onset time of less than 30 min, reduce pain by more than 70 %; these effects last for 4-5 h. Epidural clonidine analgesia is accompanied by a reduction in heart rate, cardiac output and blood pressure of approximately 20 % compared with baseline. The haemodynamic side effects mean close supervision is needed during the first hour after epidural application and limit the use of epidural clonidine to patients who are refractory to the analgesic effects of epidural opioid or local anaesthetics. In these patients excellent results can be achieved either with clonidine alone or with a combination of clonidine and an opioid or a local anaesthetic to exploit the additive or supra-additive interactions of these drugs.