Effects of epidural anesthesia on splanchnic capacitance.

Splanchnic veins play an important role in the active control of total body circulatory capacitance. The effects of epidural anesthesia on splanchnic venous capacitance have not previously been examined. A rabbit model using direct measures of mesenteric vein diameter and sympathetic efferent nerve activity was used to test the response to epidural lidocaine at three different doses and to intramuscular lidocaine at two doses. Epidural anesthesia produced hypotension, mesenteric venodilatation, and interruption of sympathetic activity. Maximal changes of these parameters were comparable in the three epidural dosage groups but were more prolonged with increasing dose. High-dose systemic lidocaine caused smaller changes in arterial pressure and sympathetic activity. Further experiments were done to investigate the mechanism of splanchnic venodilatation. Passive vein distension and effects of circulating lidocaine or catecholamines are not likely contributing factors. Blocks limited to thoracic segments, but including the origin of splanchnic preganglionic fibers, produce comparable mesenteric venodilatation and sympathetic interruption as extensive thoracolumbar blocks. Blocks limited to lumbar segments, however, showed mesenteric venoconstriction and increased splanchnic sympathetic activity. The variable responses in splanchnic capacitance with the onset of epidural anesthesia are the result of the competing influences of increased sympathetic activity from decreasing blood pressure and blockade of sympathetic fibers to the splanchnic veins.