EFFECTS OF EPIDURAL AND SYSTEMIC LIDOCAINE ON SYMPATHETIC ACTIVITY AND MESENTERIC CIRCULATION IN RABBITS

Background: The mechanisms producing hemodynamic changes during epidur al anesthesia are incompletely understood. This study examines the sym pathetic block and splanchnic venodilatation that result from extensiv e thoracolumbar epidural anesthesia in rabbits using direct measuremen ts of sympathetic efferent nerve activity (SENA) and mesenteric vein d iameter (VD). Methods: Epidural catheters were inserted in rabbits ane sthetized with a-chloralose, paralyzed with vecuronium, and receiving mechanical ventilation. Arterial pressure was monitored with a femoral cannula, heart rate was determined from the pressure signal, SENA was measured from a postganglionic splanchnic nerve, and VD was measured from segments of ileum externalized in situ. Epidural anesthesia was i nduced with 0.4 ml/kg lidocaine, using concentrations of either 0.5, 1 , or 1.5%. Control animals received intramuscular lidocaine in a dose of either 6 or 15 mg/kg. After recovery from epidural anesthesia, comp lete sympathetic blockade was induced by systemic administration of th e ganglionic blocker hexamethonium (HX). Individual groups included fr om five to eight animals. Results. A mild decrease in arterial pressur e and SENA followed the larger dose of intramuscular lidocaine, but no changes occurred in VD in the control animals exposed to systemic lid ocaine at levels comparable to that in the epidural groups (0.96-3.58 mug/ml). Epidural injectate extended from T2 to L5. All concentrations of epidural lidocaine produced comparable degrees of hypotension (-53 .5 to -61.4%), decreased SENA (-82.6 to -95.5%), and increased VD (7.5 to 10.2%). The duration of the changes was greater with more concentr ated lidocaine. Hexamethonium produced changes in arterial pressure an d VD comparable to those evoked by epidural anesthesia. Conclusions. E pidural anesthesia increases splanchnic venous capacitance by markedly decreasing splanchnic sympathetic nerve activity.