Transversus Abdominis Plane (TAP) Block in Rabbit Cadavers: Anatomical Description and Measurements of Injectate Spread Using One- and Two-Point Approaches

Simple Summary Anesthetic risk in rabbits is higher than in other species such as dogs and cats and, furthermore, the treatment of their perioperative pain is sometimes inadequate. Loco-regional anesthesia is seeing increasing application in veterinary medicine, as it allows a reduction in the systemic administration of drugs. The block of the transversus abdominis plane (TAP) is an anesthetic technique of great interest for locoregional anesthesia in veterinary practice. This technique is used to desensitize the abdominal wall and consists of injecting a volume of local anesthetic into the TAP with the aim of obtaining its distribution on the branches of the thoracolumbar nerve located in the fascial plane between the internal oblique and transversus abdominis muscles. In both human and veterinary medicine, this technique is considered promising for producing analgesia for surgical procedures performed on the abdominal wall (e.g., laparoscopies, mastectomies, abdominal hernias), representing a valid alternative to epidural analgesia. Different approaches to performing ultrasound-guided TAP block have been described. The preiliac approach is one of the most used; however, it seems to be useful in managing pain in the caudal abdomen, but the cranial diffusion of the local anesthetic does not seem to be sufficient to achieve complete desensitization of the abdominal wall. Alternative approaches, involving further retrocostal and/or subcostal infiltration, have been successfully studied in dogs, cats, calves and ponies to ensure effective desensitization of the cranial abdomen and, in part, also of the thorax; but these techniques do not have yet been described in rabbits.Abstract The aim of this study was to describe one-point (preiliac approach) and two-point (preiliac and retrocostal approach) blocks of the Transversus Abdominis Plane (TAP) on a cadaveric model. For this purpose, ultrasound-guided infiltration of the plane between the internal oblique and transversus abdominis muscles was performed and, after dissection of tissues, the areas and percentage of nerve fibers involved were analyzed. Injection into the TAP plexus of a 1 mL/kg solution of 2% lidocaine and 1% methylene blue (1:1) was performed in 30 rabbit cadavers. In fifteen rabbits (group S), the solution was inoculated at the preiliac level. In the other 15 rabbits (group D), the solution was divided into two inoculations (0.5 mL/kg at the retrocostal level and 0.5 mL/kg at the preiliac level). All cadavers were then dissected and stained spinal nerve branches were measured. Moreover, the percentage of length, height and the total area of the stained tissue were calculated. In the S group, T10, T11 and T12 nerve eminences were successfully stained in 18%, 52% and 75% of cases, respectively. Furthermore, L1, L2, L3 and L4 were stained in 95%, 100%, 60% and 40% of cases, respectively. In group D, the ventromedial eminence of T10, T11 and T12 were stained in 68.1%, 100% and 98% of cases, respectively, and L1, L2, L3 and L4 were stained in 88%, 100%, 62% and 31% of cases, respectively. In conclusion, a two-point TAP block is more effective in covering the nerve eminences of the cranial abdomen than the preiliac approach alone.