Pure mu-agonist opioids are a mainstay of analgesia and have been used extensively in horses.
Hydromorphone is currently the most affordable opioid in the US. However, the analgesic efficacy of hydromorphone in horses had only been investigated after epidural use until a recent publication (Reed et al. 2018) provided us with the pharmacokinetics and pharmacodynamics of IV hydromorphone.
Reed concluded hydromorphone at a single IV dose of 0.04mg/kg provided analgesia as measured by thermal threshold and was associated with minimal side effects.
Thermal threshold was significantly greater than baseline beginning 15 minutes after administration, lasting up to 12 hours. Certainly, elevated thermal threshold may not accurately reflect analgesia relevant to pain experiences by horses clinically. Studies evaluating efficacy of hydromorphone in clinically painful horses would be required. Excitement has been reported following IV opioids, particularly with high doses or when administered in non-painful horses. Some excitement behaviors experienced in Reed’s study of non-painful horses included pacing, vocalizing and occasional pawing. Interestingly, it has been shown in at least one study (and clinical experiences) that excitement may not be observed if horses are painful at time of administration. Additionally, opioids are known to decrease GI motility and an adverse effect reported in non-painful horses administered opioids. Reed reported decreased defecations, but no horses experienced clinical signs of colic. Again, studies of painful horses report no increased incidence of colic after general anesthesia associated with the use of morphine. My clinical experiences with a single dose of 0.04mg/kg IV hydromorphone in painful horses being sedated or anesthetized for imaging procedures, in addition to 0.04mg/kg IM administered every 12 hours in awake painful horses, have been successful and without negative side effects.
*Look forward to reading a study currently in press by the same author that reports hydromorphone administered IM has excellent bioavailability and provides a similar degree of analgesia compared to IV administration.
-Carrie A Davis, DVM, DACVAA