Epidural anesthesia enhances sympathetic nerve activity in the unanesthetized segments in cats.

To evaluate compensatory sympathetic excitation during epidural anesthesia, we measured cardiac and renal sympathetic nerve activity during thoracic or lumbar epidural anesthesia in cats. Thirteen cats were divided into three groups: five cats received thoracic epidural anesthesia, five received lumbar epidural anesthesia, and three received lumbar epidural anesthesia after the carotid sinus and vagoaortic nerves were severed (denervated lumbar group). Heart rate (HR), mean arterial pressure (MAP), and cardiac and renal sympathetic nerve activity were measured repeatedly after administration of a single dose of 0.1 mL/kg of 1% lidocaine via the epidural catheter. Epidural solution spread from a median of C-8 to T-6 in the thoracic epidural group, T-8 to L-3 in the lumbar epidural group, and T-7 to L-3 in the denervated lumbar group. During thoracic epidural anesthesia, HR, MAP, and cardiac sympathetic nerve activity decreased, while renal nerve activity increased. Similarly, HR, MAP, and renal sympathetic nerve activity decreased during lumbar epidural anesthesia, and cardiac activity increased. In the denervated lumbar group, HR, MAP, and renal sympathetic nerve activity decreased but cardiac activity remained unchanged. Sympathetic nerve activity in corresponding unanesthetized segments increased during thoracic or lumbar epidural anesthesia in association with significant decreases in MAP and HR. After severance of the carotid sinus and vagoaortic nerves, the absence of sympathetic excitation in the unanesthetized segments during lumbar epidural anesthesia suggests that the compensatory response is produced by the baroreceptor reflex response to anesthesia-induced hypotension.