The mechanisms producing hemodynamic changes during epidural anesthesia are incompletely understood. This study examines the sympathetic block and splanchnic venodilatation that result from extensive thoracolumbar epidural anesthesia in rabbits using direct measurements of sympathetic efferent nerve activity (SENA) and mesenteric vein diameter (VD).Methods
Epidural catheters were inserted in rabbits anesthetized with α-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 induced 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, complete sympathetic blockade was induced by systemic administration of the ganglionic blocker hexamethonium (HX). Individual groups included from five to eight animals.Results
A mild decrease in arterial pressure and SENA followed the larger dose of intramuscular lidocaine, but no changes occurred in VD in the control animals exposed to systemic lidocaine at levels comparable to that in the epidural groups (0.96–3.58 μg/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 concentrated lidocaine. Hexamethonium produced changes in arterial pressure and VD comparable to those evoked by epidural anesthesia.Conclusions
Epidural anesthesia increases splanchnic venous capacitance by markedly decreasing splanchnic sympathetic nerve activity.