Evaluation of injection volumes for the transversus abdominis plane block in dog cadavers: a preliminary trial

Objective To evaluate staining of nerve branches after the injection of different volumes during ultrasound-guided transversus abdominis plane (TAP) block in dog cadavers. Study design Prospective randomized study. Animals A total of 15 frozen/thawed adult dog cadavers. Methods Hemiabdomens were randomly allocated to one of four groups. In groups G0.3, G0.6 and G1.0, ropivacaine-methylene blue solution (0.3, 0.6 and 1.0 mL kg(-1) in seven, eight and eight hemiabdomens, respectively) was injected at the midpoint between the iliac crest and the last rib at the height of the shoulder. In group G0.3 x 2 (seven hemiabdomens), two injections (0.3 mL kg(-1)) were performed, caudal to the last rib and cranial to the iliac crest at the same height. Total time for injection was recorded; after 30 minutes, cadavers were dissected and spread of dye was evaluated. Results Accuracy of injection site was 80% and injection time was 71 (48-120) seconds for all groups together. Craniocaudal spread was 6.4 +/- 1.6, 9.1 +/- 2.6, 11.4 +/- 2.3 and 11.2 +/- 3.8 cm for G0.3, G0.6, G1.0 and G0.3 x2, respectively [G0.3 to G0.3 x 2 (p = 0.044) and G1.0 (p = 0.034)]. There was no difference in dorsoventral spread among groups. Number of ventral nerve branches stained was 3 (2-4), 3 (2-4), 3 (3-4) and 3 (2-4) for G0.3, G0.6, G1.0 and G0.3 x 2, respectively, including nerve branches from twelfth thoracic to third lumbar (L3) in different proportions among groups. Conclusions and clinical relevance Results suggest that a single-injection TAP block, using 0.3 mL kg(-1), stains comparable number of nerve branches as higher volumes or two-point injection. Despite the volume or technique, consistent staining of the innervation of the caudal abdomen (L1-L3) was observed. Additional cadaveric studies are necessary to identify the optimal technique for complete abdominal wall staining.