Assessment of a ropivacaine ultrasound-guided Transversus Abdominis Plane (TAP) block for peri-operative analgesia in the dog undergoing ovariohysterectomy surgery

© 2019 Emma Kate JohnsonAdvances in veterinary medicine have led to the development of further techniques to provide analgesia for our patients. Ovariohysterectomy is a commonly performed surgery in female dogs which causes significant pain. Multimodal analgesia often combining opioids, non-steroidal anti-inflammatories and local anaesthetic techniques is implemented to treat this type of pain. The transversus abdominis plane (TAP) block is a local anaesthetic technique that provides analgesia to the afferent nerves supplying the parietal peritoneum, muscles and skin of the anterior abdominal wall. The TAP block is utilized in human anaesthesia and has been shown to reduce post-operative pain scores and opioid consumption. The TAP block has potential to provide local anaesthesia to the mid-caudal abdomen for dogs undergoing ovariohysterectomy. The initial pilot study was a cadaveric study carried out in 5 dogs with an ultrasound guided two-point TAP injection performed in each hemiabdomen. The study identified that a two-point TAP injection delivered consistent dye dispersion to adequately stain branches of thoracic nerve T13 and lumbar nerves L1, L2 and L3. It concluded that this technique should be assessed in vivo to evaluate the analgesic efficacy in mid to caudal abdominal surgeries. A subsequent randomized controlled trial assessed the two-point ultrasound-guided ropivacaine TAP block for dogs undergoing ovariohysterectomy, with the hypothesis that dogs receiving the ropivacaine TAP block would have a lower minimum alveolar concentration (MAC) isoflurane at skin incision and a lower opioid rescue analgesic requirement post-operatively. The results concluded that the estimated population MACiso for the treatment group was lower than the control group but that this was not significant. There was also no significant difference in pain scores between the groups as measured by the Glasgow composite pain scale short form and mechanical nociceptive threshold testing. The proportion of subjects receiving rescue analgesia in the treatment group was lower than the control group, but this was not significant. The study concluded that the two-point TAP block did not provide significant MAC reduction or additional intra-operative or post-operative analgesia compared with the control for dogs undergoing ovariohysterectomy. Multiple confounding factors such as the administration of morphine as premedication and the low concentration of local anaesthetic used, ropivacaine 0.2%, could have contributed to these findings. Although the cadaver study identified that a two-point TAP injection delivered consistent dye dispersion to thoracic nerve T13 and lumbar nerves L1, L2 and L3, the technique when performed with 0.2% ropivacaine in dogs undergoing ovariohysterectomy did not provide significant additional MACiso reduction or intra-operative or post-operative analgesia compared with the control for dogs. Further assessment of the toxic dose of ropivacaine in dogs and the minimum effective concentration of ropivacaine for TAP blocks in dogs needs to be evaluated in order to further develop and assess this technique