Objective: To compare the effects of hydromorphone and butorphanol in horses undergoing arthroscopy and describe the pharmacokinetics of hydromorphone in anesthetized horses. Study design: Randomized controlled clinical trial. Animals: A total of 40 adult horses admitted for elective arthroscopy. Methods: Horses were randomly assigned to be administered intravenous hydromorphone (0.04 mg kg–1; group TxH; n = 19) or butorphanol (0.02 mg kg–1; group TxB; n = 21) prior to surgery as part of a standardized anesthetic protocol. Pain was scored by two observers unaware of group assignment using the Equine Utrecht University Scale for Facial Assessment of Pain (EQUUS-FAP) and a composite pain scale (CPS) prior to surgery (baseline), 2 hours (P2) and 4 hours (P4) following recovery from anesthesia. Blood samples were collected at various time points for determination of plasma hydromorphone concentration using liquid chromatography–tandem mass spectrometry. Data were analyzed with a mixed-effect model. Results: Median (range) baseline EQUUS-FAP was 1.2 (0.0–4.0) with no effect of group, time points or interaction. Baseline CPS was similar between groups. Group TxH baseline CPS was 2.5 (0.0–10.0), increased at P2 [4.5 (0–10.0); p = 0.046] and returned to baseline values at P4 [3.0 (0.0–11.0)]. Group TxB baseline CPS was 2.0 (0.0–8.0), increased at P2 [3.5 (0.0–11.0); p = 0.009] and P4 [5.0 (0.0–11.0); p < 0.001]. Pharmacokinetic terminal half-life was 774 ± 82.3 minutes, area under the curve was 1362 ± 314 ng minutes mL–1, clearance was 30.7 ± 7.23 mL minute–1 kg–1 and volume of distribution at steady state was 884 ± 740 mL kg–1. Conclusions: Hydromorphone, but not butorphanol, decreased CPS back to baseline at P4 after recovery. Clinical relevance: Hydromorphone may provide superior postoperative analgesia compared with butorphanol in horses undergoing arthroscopy.
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