Interpleural anesthetics in the dog: differential somatic neural blockade

Differential somatic neural effects of interpleural bupivacaine were determined in dogs. Alterations in evoked responses were used as a marker of neural blockade. Electrode pairs were fastened to the external surface of the right seventh ribs of five male mongrel dogs (25-30 kg) at distal (D), middle (M), and proximal (P) locations. Electrodes were similarly fastened to the ipsilateral laminae of the fifth (T5L), seventh (T7L), and ninth (T9L) thoracic vertebrae, and the contralateral cranium over the sensorimotor cortex (SMC). Pediatric feeding tubes were used as interpleural catheters. Following interpleural bupivacaine (10 ml, 0.5%) intercostal nerve block was produced, as manifested by decreases in amplitude (range 12-32% of control, P less than 0.05), and increases in latency (range 108-126% of control, P less than 0.05), of evoked potentials recorded between T7L and rib electrodes. The block was found to localize over dependent portions of the rib with changes in animal position, indicating a strong influence of gravity. No significant changes were seen in potentials recorded between T9L and T5L, and T9L and SMC, regardless of position. T9L-T5L and T9L-SMC potentials were abolished or severely attenuated following direct subarachnoid or epidural injection of bupivacaine at T7. Thus, there are no spinal, epidural, or gross CNS effects of interpleural bupivacaine.