Continuous TAP Blocks: Relative Effects of a Basal Infusion vs. Repeated Bolus Doses

Single-injection transversus abdominis plane (TAP) blocks have been used to treat postoperative pain, and are become very popular within the United States because of their high analgesic potency and relative ease of placement. Unfortunately, the longest local anesthetic available lasts only 8-12 hours. Continuous TAP blocks-also termed "perineural infusion"-involve bathing the multiple nerves of the abdominal wall in local anesthetic using a percutaneously-inserted perineural catheter inserted just anterior (ventral) and cephalad to the anterior superior iliac spine. For most catheter locations, an infusion is preferred to a single-injection nerve block because the duration of analgesia may be extended to better match the duration of surgical pain. But, unlike brachial plexus, femoral, and sciatic nerve perineural infusion, a continuous basal infusion of local anesthetic does not provide adequate analgesia for TAP catheters. This may be due to the fact that the TAP is a relatively tight space, so it might require a bolus of fluid to adequately spread the perineural local anesthetic to the multiple required nerves (as evidence of this, single-injection TAP blocks are very effective, although with a limited duration). The result is that while single-injection TAP blocks are widely used, TAP catheters have not been adopted. This scenario leaves surgical pain untreated following the resolution of the single-injection TAP block. The recent development of an infusion pump that can automatically deliver repeated bolus doses may allow the spread of local anesthetic to the multiple sensory nerves necessary to provide adequate analgesia, with a duration that better matches postoperative requirements. The investigators therefore propose a randomized, double-masked, controlled trial to determine if delivering local anesthetic as a repeated bolus dose results in improved local anesthetic spread/effects compared with a continuous basal infusion for TAP catheters.