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

Abstract

Abstract 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.