Abstract

The effect of increasing interpleural injectate volume was determined in a chronic canine model. Changes in evoked potentials were used as a marker of nerve blockade. Electrodes were fastened to the right fifth, seventh and ninth ribs of adult dogs at distal (D), middle (M) and proximal (P) sites. Electrodes were also fastened to the ipsilateral laminae of the fifth (T5L), seventh (T7L) and ninth (T9L) thoracic vertebrae. Evoked potentials were recorded at each lamina after stimuli were applied to the corresponding distal, middle and proximal rib electrodes. Interpleural catheters were placed under direct vision during surgery. The effects of 10 ml and 20 ml of 0.5% bupivacaine were studied for each dog. Following 10 ml of 0.5% bupivacaine, intercostal nerve block was produced at T5 and T7, as evidenced by decreases in amplitude (range, 6-53% of control, p < 0.05) and increases in latency (range, 108-122% of control, p < 0.05) of evoked potentials recorded between laminae and distal or middle electrodes. No significant changes were seen in any potentials recorded over T9. After 20 ml of 0.5% bupivacaine, all T5 potentials were abolished. Significant decreases in amplitude (range, 22-36% of control, p < 0.05) and increases in latency (range, 117-126% of control, p < 0.05) were produced at all T7 rib sites. Except for the D-T7L comparison, all T5 and T7 changes produced by 20 ml of bupivacaine were greater (p < 0.05) than those produced by 10 ml. Twenty milliliters of bupivacaine also produced decreases in amplitude (range, 41-51% of control at all sites, p < 0.05) and increases in latency (115% at D-T9L, p < 0.05) at T9 sites. Twenty milliliters of 0.5% bupivacaine produces intercostal nerve block that is more pronounced and widespread than that produced by 10 ml of bupivacaine.