Effects of segmental thoracic extradural analgesia on sympathetic block in conscious dogs

To test the hypothesis that segmental thoracic extradural block causes sympathetic denervation caudally beyond dermatomes rendered analgesic, we have measured regional skin temperatures in six conscious dogs after upper thoracic, mid thoracic, and lumbar extradural injection of 0.5% bupivacaine 0.5, 1 and 2 ml cumulatively (total dose: 3.5 ml) given at 45-min intervals. Dogs were studied at constant ambient and rectal temperatures. Upper thoracic extradural injections resulted in a significant increase in skin temperatures on both the front (+1.4 (SEM 0.2) degrees C) and hind paw (+1.4 (0.3) degrees C), while the area of analgesia was confined to the upper trunk. With lumbar extradural injection, skin temperatures increased significantly (+2.0 (0.5) degrees C) on the lower extremities only. Mid thoracic injection significantly increased both front (+2.4 (0.9) degrees C) and hind paw (+2.2 (0.6) degrees C) skin temperatures, but decreased temperatures on the thorax (-0.9 (0.2) degrees C) and abdomen (-1.0 (0.2) degrees C), reversing the normal temperature gradient along the body axis. Irrespective of the injection site, skin temperatures on the trunk failed to increase or even decreased significantly. These data suggest that small doses of local anaesthetics applied to the extradural space of conscious dogs cause increased lower extremity skin temperatures caudal to areas unresponsive to pinprick stimulation when injected at a high thoracic level, and decreased trunk skin temperature even in analgesic areas, so that skin temperature measurements are unlikely to reflect purely sympathetic efferent activity on the trunk. Upper thoracic segmental extradural analgesia induced a decrease in sympathetic tone distal to the area of analgesia.