Sensory and Sympathetic Block During Interpleural Analgesia

Background and Objectives Interpleural analgesia is an effective method for pain relief after upper abdominal surgery. To examine whether the analgesic effect is obtained by block of the intercostal nerves, we assessed the analgesic efficacy of the block, the skin sensitivity, and indices of sympathetic outflow over the trunk. Methods Interpleural analgesia was instituted at the end of open cholecystectomy in 20 patients 24-81 years of age (mean, 42 years). After a washout period of 8 hours, the analgesic effect was tested 5-12 times during the postoperative follow-up period by using a visual analogue scale before and 20 minutes after injection of 20 mL of bupivacaine 0.25%. Temperature and pain sensations were tested on the day after surgery, and in nine of the patients, the cutaneous blood flow over the trunk was studied by an electronic thermometer, laser Doppler flowmetry, and fluorescein flowmetry. In addition, the conduction velocity in the phrenic nerve was studied in four of the patients. Results Interpleural analgesia significantly reduced the median visual analogue score from of 5.7 (range 2-10) to 1.1 (range, 0-4). Although the analgesic effect was very good in all patients, half of them still showed skin sensitivity to pain and temperature. Cutaneous blood flow did not change, which showed that block of the intercostal nerves was incomplete. The phrenic nerve was not affected. Conclusion The incomplete cutaneous sensory and sympathetic block indicates that the analgesic effect of interpleural analgesia cannot be explained by retrograde diffusion of the local anesthetic solution into the intercostal nerves alone.