Ultrasound-Guided Rectus Abdominis Sheath Block in Cats Undergoing Ovariectomy: A Prospective, Randomized, Investigator-Blinded, Placebo-Controlled Clinical Trial

Objective: To evaluate the analgesic effects of an ultrasound-guided rectus abdominis sheath block performed with lidocaine and bupivacaine, in cats undergoing midline ovariectomy. Study design: Randomized, prospective, placebo-controlled, investigator-blinded clinical trial. Animals: Forty-one client-owned female cats scheduled for ovariectomy were enrolled in the study and randomly assigned to one of the treatment groups. Methods: Both treatment and control groups received general anesthesia and a bilateral rectus abdominis sheath block (RSB) under ultrasound guidance. The block was performed with 0.05 mL kg-1 of 2% lidocaine and 0.15 mL kg-1 of 0.5% bupivacaine mixed together in each rectus abdominis sheath (group A) or the equivalent volume of saline (group B). Intraoperative pain assessment was based on changes in physiological parameters compared to the baseline values and rescue analgesic requirement. Postoperative pain assessment was performed using the 4A-Vet pain scale and the need for rescue analgesics at 1, 2, 4 and 6 hours after extubation. The sedation was evaluated and graded at the same times. The owners were asked to fill a questionnaire at three time points: the night the cat was back home, the next morning and ten days after the surgery. Results: There were two significant differences between group A and B: less isoflurane was needed in group A during the suturing phases (p = 0.04 during peritoneal muscular sutures and 0.02 during skin suture) and cats from group A were less sedated 6 hours after extubation (p = 0.03338). Conclusions and clinical relevance: In summary, our study concludes that ultrasound-guided RSB decreases the amount of isoflurane used during surgical closing time and the sedation score 6 hours after extubation. The block was easily and successfully performed. Further studies are needed to adjust the local anesthetic volume and prove its long-term efficacy in cats.