Epidural 0.125% ropivacaine is safe but insufficient for induced tibiotarsal joint pain.

BACKGROUND: Epidural injection of a low concentration of the local anaesthetic ropivacaine produces analgesia without neurological deficits or ileus in people. OBJECTIVE: To determine if epidural 0.125% ropivacaine mitigates induced hindlimb lameness in horses while preserving neurological function and gastrointestinal (GI) motility. STUDY DESIGN: Randomised, blinded, controlled, crossover study. METHODS: Eight healthy, sound horses underwent two treatments in random order, separated by at least 3 weeks: epidural saline or epidural ropivacaine. For each treatment, an epidural catheter was inserted at the first intercoccygeal space and advanced 5 cm cranially. Correct catheter placement was confirmed radiographically after injecting contrast through it. The tibiotarsal joint was injected with 75 ng of recombinant equine interleukin-1β to induce synovitis. Twelve hours later, baseline variables were recorded, including heart rate, respiratory rate, borborygmi, lameness score and neurological examination score. Thirty minutes thereafter [time = 0 h (t0)], 0.045 mL/kg of preservative-free 0.9% saline or 0.125% ropivacaine was administered through the catheter over 10 min. Data were collected at t0.5, 1, 1.5, 2.5, 3.5, 4.5, 5.5, and 6.5. Faecal piles passed the 24 h before and after t0 were counted. Continuous and categorical data were analysed using a linear mixed effects model and Fisher's exact test, respectively, with significance set at p < 0.05. RESULTS: All horses developed moderate-to-severe lameness that did not improve after ropivacaine injection. No horses developed hindlimb neurological deficits. However, from t0.5-2.5, more horses in the ropivacaine (7/8) than saline (0/7) treatment had decreased to absent anus and tail tone (p = 0.014). There were no other significant differences between treatments. MAIN LIMITATIONS: A single volume, concentration, and injection rate were tested. CONCLUSIONS: A single dose of 0.125% ropivacaine did not mitigate hindlimb lameness. Anus and tail tone decreased, but hindlimb neurological function and GI motility were preserved.