Weight-Based Volume of Injection Influences Cranial to Caudal Spread of Local Anesthetic Solution in Ultrasound-Guided Transversus Abdominis Plane Blocks in Canine Cadavers

Objective To determine if the volume of injected local anesthetic solution affects cranial to caudal spread when performing ultrasound-guided transversus abdominis plane (TAP) blocks in dogs. Study Design Prospective experimental study. Animals Adult Beagle cadavers (n = 20) Methods Bilateral TAP blocks using ultrasound guidance was performed in 20 Beagle cadavers (mean +/- SD weight, 9.3 +/- 1.4 kg) using a 1:1 solution of methylene blue/bupivacaine injected at volumes of 0.25, 0.5, 0.75, and 1.0 mL/kg. Cadavers were dissected to determine injectate spread within the transversus abdominis fascial plane. Results The transversus abdominis fascial plane was adequately identified by ultrasonography, injected, and dissected in 38 beagle hemi-abdominal walls; injectate was not identified in 2 hemi-abdominal walls. Dermatomal spread (number of ventral nerve roots saturated by injected solution) was volume dependent (P = .026, Kruskal Wallis): 2.9 +/- 0.74 nerve roots for 0.25 mL/kg; 3.4 +/- 1.1 for 0.5 mL/kg; 4.0 +/- 0.67 for 0.75 mL/kg; and 4.2 +/- 1.2 for 1 mL/kg. Conclusion In Beagle cadavers, the volume of injected local anesthetic solution significantly affects cranial to caudal spread within the TAP during ultrasound-guided TAP blocks. The volume of local anesthetic injected could potentially be used to augment the spread of analgesic coverage for a given surgical procedure in dogs.