Transversus abdominis plane block in ponies: a preliminary anatomical study

Objective: To describe a single-site transversus abdominis plane (TAP) block technique in horses. Study design: Prospective, descriptive, experimental anatomical study. Animals: Four adult pony cadavers. Methods: Freshly euthanized ponies were positioned in dorsal recumbency. A 6–13 MHz linear ultrasonic probe was used to scan the abdominal wall bilaterally midway between the last rib and iliac crest in search of the TAP location. By modifying the technique to accommodate the equine anatomy, the TAP was successfully visualized with the transducer positioned in a transverse plane with its side indicator over the intercept of two lines, one connecting the most cranial aspect of the iliac crest and the most caudal extent of the last rib and another originating just caudal to the umbilicus and extending laterally. Each hemiabdomen was injected with 0.5 mL kg^–1 of a 1:1 solution of 1% methylene blue and 0.5% bupivacaine via a 21 gauge 10 cm stimulating needle inserted ventral–dorsally and in plane with the ultrasound beam. Approximately 3 hours after injection, the abdomen was dissected and nerves stained over 1 cm in length were identified. Results: Staining was evident from the fourteenth thoracic (T14) to the third lumbar (L3) nerves. The ventral branches of the fifteenth to the eighteenth thoracic nerves (T15–T18) and first and second lumbar nerves (L1 and L2) were stained in three, six, eight, eight, eight and seven of eight injections, respectively. Conclusions and clinical relevance: Nerves T16–L2 had over 75% success rate in staining, suggesting that this technique would block transmission from T16 to L2, assuming that staining indicates potential nerve block. Dorsal spread occurred in three of eight hemiabdomens. Further studies developing techniques for the cranial abdomen and adjusting volume and concentration of injectate are warranted.