Evaluation of preoperative rectus sheath block with bupivacaine for analgesia in cats undergoing ovariohysterectomy: a cadaveric and randomized clinical study

Objective: Phase 1: to determine the feasibility of desensitizing ventral branches of spinal nerves within the rectus sheath using an ultrasound-guided rectus sheath block (USRSB). Phase 2: to determine the effect of preoperative USRSB on intraoperative responses to surgical stimulation and postoperative pain. Study design: Cadaveric study and prospective, randomized, blinded, parallel-arm clinical trial. Animals: A group of five cat cadavers and 37 shelter-owned cats undergoing ovariohysterectomy. Methods: Phase 1: anatomical dissection was performed on one uninjected cadaver. Abdominal walls were dissected in four cadavers (eight hemiabdomens) following bilateral USRSB using 1:1 new methylene blue and 0.5% bupivacaine (0.8 mL kg^–1 total). Phase 2: preoperative bilateral USRSB was performed with 0.8 mL kg^–1 of 0.25% bupivacaine (RSB) or equivalent volume of 0.9% saline (CONTROL). Intraoperative systolic arterial blood pressure (SAP), heart rate (HR), respiratory rate (f_R) and vaporizer setting (vap%) were recorded before skin incision, during celiotomy and abdominal wall closure. In recovery, cats were administered robenacoxib (2 mg kg^–1; CONTROL) or 0.9% saline (0.1 mL kg^–1; RSB) subcutaneously. Postoperative pain was evaluated for 6 hours using the Glasgow Composite Measure Pain Scale. Results: Phase 1: spinal nerves T9–L3 were identified within the rectus sheath, and stained in 0%, 40%, 63%, 75%, 100%, 88%, 50% and 13% of hemiabdomens, respectively. Phase 2: 37 cats were included (RSB, n = 17; CONTROL, n = 20). Intraoperatively, SAP, HR and f_R were not significantly different between groups. Vap% was significantly lower in RSB during celiotomy (p = 0.036) and closure (p = 0.044). Postoperatively, RSB cats were 5.3 times (95% CI 1.8–8.3) more likely to require rescue analgesia than CONTROL cats. Conclusions and clinical relevance: During surgery, USRSB with bupivacaine offered minor benefits and provided markedly less postoperative analgesia than robenacoxib, indicating that relying on USRSB provides insufficient postoperative analgesia for ovariohysterectomy in cats.