SPECIFIC ENHANCEMENT BY FENTANYL OF THE EFFECTS OF INTRATHECAL BUPIVACAINE ON NOCICEPTIVE AFFERENT BUT NOT ON SYMPATHETIC EFFERENT PATHWAYSIN DOGS

Background. Bupivacaine alone, or in combination with opioids, has bee n shown to provide adequate pain relief without motor paralysis. This study examined the effects of bupivacaine administered intrathecally o n sympathetic efferent and Adelta- and C-fiber-mediated afferent pathw ays in dogs and the interactions with intrathecal fentanyl. Methods: S pontaneous activity in renal sympathetic nerves was observed, as were reflex somatosympathetic responses mediated by Adelta and C fibers evo ked by supramaximal electrical stimulation of the tibial and radial ne rve. Bupivacaine was administered intrathecally in doses of 0.5, 1, 2, and 3.5 mg, each in 0.5 ml, and 7 mg in 1 ml with or without pretreat ment with 5.4 mg intrathecal fentanyl (ED25 for depression of tibial r eflexes) in each of five preparations.Results: Bupivacaine caused a do se-dependent inhibition of both Adelta- and C-fiber-mediated somatosym pathetic responses evoked by tibial nerve stimulation. The depression of radial and tibial nerve reflexes and spontaneous renal sympathetic activity was similar. Pretreatment with fentanyl (5.4 mug, intrathecal ly) depressed tibial C-fiber reflexes by only 23.8% without any signif icant effect on either tibial Adelta or radial Adelta and C fiber resp onses. Fentanyl markedly enhanced the effect of subsequent doses of bu pivacaine on tibial Adelta and C reflexes without any additional effec t on either spontaneous sympathetic activity or radial responses. Conc lusions: Intrathecal bupivacaine has no selectivity for the afferent a nd efferent pathways, and intrathecal fentanyl acts synergistically to enhance the effect of bupivacaine on the afferent pathway without a m easurable effect on sympathetic outflow.