EPIDURAL-ANESTHESIA ENHANCES SYMPATHETIC-NERVE ACTIVITY IN THE UNANESTHETIZED SEGMENTS IN CATS

To evaluate compensatory sympathetic excitation during epidural anesth esia, we measured cardiac and renal sympathetic nerve activity during thoracic or lumbar epidural anesthesia in cats. Thirteen cats were div ided into three groups: five cats received thoracic epidural anesthesi a, five received lumbar epidural anesthesia, and three received lumbar epidural anesthesia after the carotid sinus and vagoaortic nerves wer e severed (denervated lumbar group). Heart rate (HR), mean arterial pr essure (MAP), and cardiac and renal sympathetic nerve activity were me asured repeatedly after administration of a single dose of 0.1 mL/kg o f 1% lidocaine via the epidural catheter. Epidural solution spread fro m 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 gr oup. During thoracic epidural anesthesia, HR, MAP, and cardiac sympath etic nerve activity decreased, while renal nerve activity increased. S imilarly, HR, MAP, and renal sympathetic nerve activity decreased duri ng 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 tho racic or lumbar epidural anesthesia in association with significant de creases in MAP and HR. After severance of the carotid sinus and vagoao rtic nerves, the absence of sympathetic excitation in the unanesthetiz ed segments during lumbar epidural anesthesia suggests that the compen satory response is produced by the baroreceptor reflex response to ane sthesia-induced hypotension.