Sympathetic block during spinal anesthesia in volunteers using lidocaine, tetracaine, and bupivacaine

Background and Objectives. Spinal anesthesia to high thoracic dermatomes is alleged to result in almost complete block of all sympathetic efferent nerves. To examine the degree of sympathectomy during spinal anesthesia, the sympathetic response to a cold pressor test (CPT) applied to unblocked dermatomes before and during spinal anesthesia was measured with use of three different local anesthetics. Methods. Twelve healthy volunteers were studied in a randomized and double-blind fashion on three separate occasions. In random order, each volunteer received approximately equipotent spinal doses of tetracaine 15 mg, bupivacaine 15 mg, and lidocaine 100 mg in hyperbaric solutions. Prior to and 30 minutes after spinal injection of local anesthetic, a CPT was applied for 2 minutes, and changes from baseline resting conditions in five physiologic variables were measured. Results. The CPT 1 given before anesthetic administration resulted in an increase in heart rate, mean arterial pressure, cardiac index, and plasma concentrations of norepinephrine and epinephrine. Spinal anesthesia to a median level of T3 resulted in a decrease in mean arterial pressure by 10-12% but did not significantly decrease the other variables. Spinal anesthesia did not change the increase in heart rate or cardiac index in response to the second CPT, but the increase in mean arterial pressure was attenuated compared to the CPT before anesthesia. No increase in norepinephrine or epinephrine concentration was observed during the CPT given during spinal anesthesia. There was no significant relationship between level of analgesia and sympathetic response to stress. Conclusions. Spinal anesthesia with hyperbaric solutions of tetracaine 15 mg, bupivacaine 15 mg, and lidocaine 100 mg attenuated sympathetic function but did not produce complete sympathectomy. The effects were independent of the local anesthetic used.