Continuous thoracic epidural anesthesia induces segmental sympathetic block in the awake rat.

Thoracic epidural anesthesia (TEA) is used increasingly in critical care, especially for cardiac and intestinal sympathetic block. In this study we evaluated cardiorespiratory function and sympathetic activity in a new model of continuous TEA in awake rats. Thirteen rats received epidural saline control (CON) or bupivacaine 0.5% epidural infusion (EPI) at 15 microl/h for 2 h on day 1 and day 3. Mean arterial blood pressure, heart rate, respiration rate, arterial PCO2, and motor score were recorded at baseline and after 30, 60, 90, and 120 min. Skin temperature was measured at front paws, high-thoracic, mid-thoracic, and low-thoracic, hind paws, and the proximal and distal tail. Changes in sympathetic activity were assessed by skin temperature changes from baseline (DeltaT). In the EPI group, hemodynamics and respiration remained unchanged and only mild motor deficits occurred. DeltaT in thoracic segments was higher in the EPI than in the CON group (P <0.001 at all times at high-thoracic, mid-thoracic, and low-thoracic segments). Skin temperature decreased in the distal tail in the EPI group, e.g., after 90 min DeltaT=-0.86 +/- 0.25 degrees C (EPI) versus 0.4 +/- 0.12 degrees C (CON) (P <0.05 at 60, 90, and 120 min). DeltaT on day 3 was comparable to day 1. TEA induced stable segmental sympathetic block without cardiorespiratory and motor side effects in awake rats. This new technique may be applied in prolonged models of critical illness.