High Thoracic Segmental Epidural Anesthesia Diminishes Sympathetic Outflow to the Legs, Despite Restriction of Sensory Blockade to the Upper Thorax

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Abstract
To evaluate whether, after high thoracic segmental epidural anesthesia, sympathetic blockade spreads caudally beyond sensory blockade, we assessed regional skin temperatures by infrared telethermometry in 53 nonpremedicated patients at constant ambient temperature. Either bupivacaine (4.2 ml. 0.75, n = 10) or an equal volume of saline (placebo, n = 10) was injected at the C7-T2 epidural space in a randomized double-blind fashion. Results were contrasted to those observed after midthoracic (T6-T9, n = 13) and lumbar (L2-T12, n = 10) epidural injection of an identical dose of bupivacaine or saline (n=10). Despite restriction of sensory block to the upper throax with high thoracic epidural anesthesia, skin temperatures increased signicantly (P < 0.05 vs. saline) on the foot (great toe: +1.2.degree.C .+-. 2.9 SD; little toe: +09.degree.C .+-. 2.6) and hand (thumb: +2.0.degree.C .+-. +4.0, digit 5: 2.9.degree.C .+-. 4.2) but decreased after saline. Midthoracic injection also increased significantly skin temperature on the foot (great toe: 4.0.degree.C .+-. 4.9; little toe: 3.6.degree.C .+-. 4.8) but not on the hand. In contrast, with lumbar epidural anesthesia, skin temperature increased significantly on the foot (great toe: 8.5.degree.C .+-. 2.5; little toe +8.6.degree.C .+-. 2.8) but decreased significantly on the hand (thumb: -3.1.degree.C .+-. 1.2; digit 5: 2.8.degree.C .+-. 2.5). Whereas the increase in foot skin temperature was greater than lumbar than after high (P < 0.003) or midthoracic (P < 0.03) segmental epidural anesthesia, there was no difference (P = 0.6) in foot temperature change between high and midthoracic injection. On the trunk, skin temperature either did not change or decreased significantly even within analgesic dermatomes. Thus, a substantial albeit submaximal, increase in foot skin temperature was observed with thoracic epidural anesthesia. Assuming that increased foot skin temperatures reflect diminished sympathetic outflow, we conclude that mid- and even high thoracic segmental epidural anesthesia involving only a few dermatomes can result in a widespread sympathetic block that includes the most caudal parts of the sympathetic nervous system.