Combination of magnesium sulphate and ropivacaine epidural analgesia for hip arthroplasty in dogs

Objective The aim of this study was to determine whether lumbosacral epidural administration of magnesium sulphate added to ropivacaine pro- longs and improves perioperative analgesia, without adverse effects on motor block duration or hind limb neurological function, in dogs undergo- ing hip arthroplasty. Study design Investigator-blind, controlled, ran- domized, prospective clinical trial. Animals A group of 20 client-owned dogs un- dergoing hip arthroplasty were allocated randomly to either group C (control, 1 mg kg 1epidural ropivacaine) or group M (magnesium, epidural injection of 1 mg kg 1 ropivacaine and 2 mg kg 1magnesium sulphate). Methods All dogs were premedicated with intra- muscular acepromazine. General anaesthesia was induced with propofol and maintained with iso- flurane in oxygen. Intraoperatively, nociception was assessed based on changes in heart rate, respiratory rate and mean arterial pressure above baseline values. Postoperatively, pain was evaluated with a Sammarco pain score, a Glasgow pain scale and a visual analogue scale (VAS). Tarlov’s scale was used to quantify motor block. All dogs were evaluated at recovery and then 1, 2, 3, 4, 5 and 24 hours after that. Rescue analgesia was provided during surgery with fentanyl and, postoperatively, with buprenor- phine. Groups were compared using one-way repeated-measure analysis of variance followed by the HolmeSidak method for multiple comparison or nonparametric tests when appropriate. Results The two treatment groups did not differ (p > 0.05) with respect to intraoperative physiolog- ical variables, rescue analgesia, postoperative pain scores (Sammarco q 1⁄4 1.00; Glasgow q 1⁄4 3.10; VAS q 1⁄4 0.50) and duration of the motor block (Tarlov’s q 1⁄4 2.40). Conclusions and clinical relevance The addition of epidural magnesium to ropivacaine did not improve or prolong the analgesia provided by ropivacaine alone. Further studies are needed to determine whether an epidural magnesium dose of >2 mg kg 1 would exert better analgesia, without causing adverse effects, in dogs undergoing or- thopaedic surgery.