Study design

Investigator-blind, controlled, randomized, prospective clinical trial.

Animals

A group of 20 client-owned dogs undergoing 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 intramuscular acepromazine. General anaesthesia was induced with propofol and maintained with isoflurane 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 buprenorphine. Groups were compared using one-way repeated-measure analysis of variance followed by the Holm–Sidak method for multiple comparison or nonparametric tests when appropriate.

Results

The two treatment groups did not differ (p > 0.05) with respect to intraoperative physiological variables, rescue analgesia, postoperative pain scores (Sammarco q = 1.00; Glasgow q = 3.10; VAS q = 0.50) and duration of the motor block (Tarlov’s q = 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 orthopaedic surgery.