REGION OF EPIDURAL BLOCKADE DETERMINES SYMPATHETIC AND MESENTERIC CAPACITANCE EFFECTS IN RABBITS

Background: The mechanisms producing hemodynamic changes during epidur al anesthesia are incompletely understood. The role of capacitance cha nges in the splanchnic venous bed can be clarified by comparing blocks of differing segmental distributions. Specifically, we speculated tha t blocks that include the innervation to the mesenteric circulation al ter hemodynamics, sympathetic activity, and venous capacitance to a gr eater extent than blocks without blockade of sympathetic nerves to thi s critical vascular bed. Methods: Rabbits were studied during alpha-ch loralose anesthesia and mechanical ventilation. Sympathetic efferent n erve activity to the mesenteric vessels was measured by surgically pla ced electrodes, and mesenteric vein diameter was measured by videomicr oscopy. Heart rate and mean arterial pressure were monitored by intraa rterial cannulation. Responses mere compared after administration of e pidural lidocaine using a dose and catheter level that limited anesthe tic to lumbar levels (lumbar group) or thoracic levels (thoracic group ). In addition, hemodynamic responses were recorded after thoracolumba r block in animals receiving alpha-chloralose but breathing spontaneou sly (spontaneous ventilation group) and in awake animals (awake group) . Results: Mean arterial pressure decreased 38.3 +/- 5.8% in the thora cic group but only 16.5 +/- 2.8 in the lumbar group. Sympathetic effer ent nerve activity decreased in the thoracic group but increased in th e lumbar group. An increase in vein diameter followed thoracic epidura l anesthesia, but venoconstriction was observed after lumbar epidural block. The addition of intravenous sedation with alpha-chloralose did not increase the hypotensive effect of epidural anesthesia in this mod el. Conclusions: Block of sympathetic fibers to the splanchnic circula tion with thoracic epidural lidocaine produces mesenteric venodilatati on that contributes to hypotension in rabbits, A lesser decrease in bl ood pressure follows blocks limited to lower segments, because barorec eptor stimulation produces increased splanchnic sympathetic activity a nd mesenteric venoconstriction. Responses in this model are comparable with and without general anesthesia and mechanical ventilation. To mi nimize hemodynamic consequences, epidural blockade should ideally be c onfined to the fewest necessary segments, avoiding splanchnic innervat ion if possible.