High thoracic epidural anesthesia does not inhibit sympathetic nerve activity in the lower extremities

Background: Sympathetic nerve activity was recorded in the leg during high thoracic epidural anesthesia with a segmental sensory blockade of the upper thoracic dermatomes to test the hypothesis that the sympathetic blockade accompanying thoracic epidural anesthesia includes caudal parts of the sympathetic nervous system. Methods: Experiments were performed on 10 patients scheduled for thoracotomy. An epidural catheter was inserted at the T3-T4 or T4-T5 interspace, In the main protocol (seven patients), blood pressure, heart rate, and skin temperature (big toe, thumb) were continuously monitored, and multiunit postganglionic sympathetic nerve activity was recorded with a tungsten microelectrode in a muscle-innervating fascicle of the peroneal nerve, After baseline data collection, muscle sympathetic nerve activity was recorded for an additional 45-min period after epidural injection of 4-6 ml bupivacaine, 5 mg/ml. In an additional three patients, the effects of thoracic epidural anesthesia on skin-innervating sympathetic nerve activity were qualitatively assessed. Results: Activation of thoracic epidural anesthesia caused no significant changes in peroneal muscle sympathetic nerve activity (n = 7), blood pressure, or heart rate. Skin temperature increased significantly in the hand 15 min after activation of the blockade, from 32.7 +/- 2.4 degrees C to 34.4 +/- 1.5 degrees C (mean +/- SD), whereas no changes were observed in foot temperature. The sensory blockade extended from T1 (C4-T2) to T8 (T6-T11). Conclusions: A high thoracic epidural anesthesia with adequate sensory blockade of upper thoracic dermatomes may be achieved without blockade of caudal parts of the sympathetic nervous system. This finding differs from that of earlier studies that used indirect methods to evaluate changes in sympathetic nerve activity.