Khursheed R. Mama, Erin K. Contino
Postoperative Pain Control
The negative consequences of pain are far reaching and have been well described. In the perioperative period, pain may be associated with the presenting lesion, may result from the surgical procedure, or may be a result of both. Appropriate treatment of these acute pain scenarios lessens the likelihood of development of chronic and debilitating pain states and will be the focus of this chapter.
Nonsteroidal Antiinflammatory Drugs
Nonsteroidal antiinflammatory drugs (NSAIDs) are the mainstay of analgesic therapy in the horse and are used to reduce inflammation commonly observed at a site of injury or as a result of surgery. These drugs are often given intravenously before or during anesthesia so that they will be effective in the perioperative period. Phenylbutazone and flunixin meglumine have a long history of use, but use of ketoprofen, carprofen, and, more recently, firocoxib, is on the rise. The interested reader is referred to additional sources for in-depth information pertaining to the efficacy and toxicity of individual drugs and their actions at cyclooxygenase isoforms. Dosages are summarized (Table 15-1).
TABLE 15-1
Commonly Used Dosages of Systemically Administered Nonsteroidal Antiinflammatory Drugs for Treatment of Perioperative Pain in Horses
| Drug | Route | Dosage |
| Phenylbutazone | Oral or intravenous | 2.2-4.4 mg/kg twice daily on day 1, then 2.2 mg/kg twice daily for 2-4 days |
| Flunixin meglumine | Oral or intravenous | 1.1 mg/kg once daily for up to 5 days |
| Firocoxib | Oral or intravenous | 0.3 mg/kg on day 1 and 0.1 mg/kg subsequently |
| Carprofen | Oral or intravenous | 0.7 mg/kg IV or 1.4 mg/kg orally once daily for 7 days |
| Ketoprofen | Intravenous | 2.2 mg/kg once daily for up to 5 days |
It appears that NSAID-associated toxicosis in the horse is primarily associated with the gastrointestinal tract, but renal papillary necrosis and prolonged clotting times may also be observed. Use of multiple NSAIDs in combination appears to increase toxicosis and is not recommended. In one comparative study, flunixin and ketoprofen had less toxicity than phenylbutazone, but additional experimental work suggests that when phenylbutazone is used at a dose less than or equal to 2.2 mg/kg given twice daily, toxicity is minimized. Given that its efficacy in treating musculoskeletal pain is well documented, phenylbutazone remains the preferred drug in adult horses for treating orthopedic pain, whereas flunixin meglumine is frequently used to manage visceral pain. Firocoxib is being used with increasing frequency for both musculoskeletal and visceral pain because it appears to have fewer adverse gastrointestinal effects. All three of these drugs may be given by the oral and intravenous routes.
In addition to systemically used NSAIDs, one might consider the perioperative application of the liposomal suspension diclofenac, which is efficacious in reducing inflammation associated with osteoarthritis when an approximately 5-inch ribbon is applied topically over the affected site. If applied postoperatively, caution must be used to prevent the medication from contacting the incision site or sutures directly, and the veterinarian should be aware that, anecdotally, some horses have developed a mild skin reaction when diclofenac is applied under bandages.
Although NSAIDs form the basis of perioperative pain treatment, many of the drugs commonly used for anesthetic management of the horse also have analgesic benefits and so will be reviewed in this chapter.
α2-Adrenergic Agonists
α2-Adrenergic agonists are commonly used to provide sedation before induction of anesthesia and, in addition, to mediate analgesia through the central inhibitory pain pathways. They can be used to provide perioperative analgesia for invasive procedures done both in conscious (standing) horses and those undergoing general anesthesia. Detomidine (given as a constant-rate infusion at 0.02 to 0.04 mg/kg/hour) is commonly used to provide sedation and analgesia in the standing horse and also results in an up to 55% reduction in the dose requirement for inhaled anesthetics. Medetomidine, an α2-adrenergic agonist, is similarly used, at 3.5 µg/kg per hour. Dexmedetomidine (consisting only of the dextroisomer) is available in the United States. A dose of 1 to 2 µg/kg is commonly used. For those who prefer a simpler approach, a single dose of xylazine (0.5 to 1 mg/kg, as is typically used for premedication) provides analgesia and reduces the requirement for inhaled anesthetics by 25% to 35%.
Adverse effects, including sedation, cardiovascular depression, increased urine production, and decreased gastrointestinal motility, limit the long-term systemic administration of these drugs. Alternative administration routes, such as the epidural or intraarticular routes, may, however, be used to provide pain relief while limiting these side effects. Xylazine given epidurally (0.17 mg/kg, injected at the sacrococcygeal site) provides about 2.5 hours of perineal analgesia with minimal ataxia and no systemic effects; detomidine (20 to 40 µg/kg), being more lipophilic, is absorbed to a greater extent from the sacrococcygeal site and, although regional analgesia is observed, systemic side effects are also increased. For perineal analgesia in an average-sized adult horse, either drug may be given with saline added to a total volume of 5 to 7 mL. To extend analgesia to the level of the flank or hind limbs, a total volume of 20 mL can be used. This approach is recommended only with detomidine, because cranial spread of xylazine can result in significant ataxia and even recumbency as a result of its local anesthetic properties.
