"Many surgical and medical procedures can be accomplished in the standing horse if appropriate combinations of physical and chemical restraint are employed," began John Hubbell, DVM, MS, professor of veterinary clinical sciences at The Ohio State University. He discussed medication choices and usage for restraining/tranquilizing horses for standing procedures at the 2009 American Association of Equine Practitioners convention, held Dec. 5-9 in Las Vegas, Nev.

It’s preferable to perform procedures with the horse standing rather than under general anesthesia where possible because horses have a greater risk of complications from general anesthesia than other species, he noted.

"The goals of standing chemical restraint are to produce a horse that’s quiet, cooperative, immobile, and unreactive to stimuli or manipulation," he explained. "More goals are to gain a variable, controlled duration of these effects and minimize pain if it’s present or anticipated."

Hubbell reported that while there are nine medications approved in the United States for restraining horses, only five are currently on the market: Acepromazine, butorphanol, detomidine, romifidine, and xylazine. Other medications such as morphine and fentanyl are frequently used off-label.

"No single drug produces ‘ideal’ standing chemical restraint in every horse," he noted. Thus, veterinarians often mix smaller doses of multiple tranquilizers to gain the desired combination of onset speed/smoothness, quality of restraint, and duration of action.

Hubbell discussed the classification, modes of action, and common usages of the available medications, summarizing as follows:

Phenothiazines (such as acepromazine) calm horses, but do not provide analgesia (pain relief). However, they can enhance the analgesic effects of some other medications. They can be given orally, intramuscularly, or intravenously. They also reduce blood pressure, which may be a concern for their usage in excitable or dehydrated horses, or those with low blood volume from extensive bleeding. Acepromazine in particular yields sedation without significant ataxia (incoordination), but a known issue is rare persistent penile prolapse/paralysis .

Alpha-2 agonists (including xylazine, detomidine, and romifidine) provide sedation, muscle relaxation, ataxia, and analgesia when given sublingually (under the tongue), intravenously, or intramuscularly. Sedation is more powerful than that provided by phenothiazines. Xylazine is one of the cheapest and most commonly used drugs in this class, but Hubbell cautions that unprovoked aggression can occur in horses dosed with it (and detomidine). He suggests having someone always keep a hand on the horse’s halter to avoid surprises.

Detomidine is 100 times more powerful than xylazine and lasts at least twice as long, but it is more expensive. Hubbell notes that in needle-shy horses, detomidine placed under the tongue can give you good sedation after 30-45 minutes. Romifidine’s effects are similar to those of detomidine, although the horse’s head tends to droop less and the analgesic effect wears off before the sedation.

Side effects of alpha-2 agonists can include decreased heart rate and cardiac output, heartbeat irregularities, hypertension (increased blood pressure), decreased respiratory rate, and reduced salivation/swallowing/gastrointestinal motility. Continuous rate infusions (IV drip) of alpha-2 agonists are becoming more commonly used. Lastly, these medications can act synergistically with each other and with opioids (more on these in a moment); they can increase the potency of opioids by 10 to 100-fold.

Opioids (such as butorphanol, morphine, and fentanyl) primarily provide analgesia, but they also enhance the effects of sedatives/tranquilizers. They will not keep the horse from feeling pain at surgical sites, however, so local anesthesia is still required for potentially painful procedures. One issue when giving them to horses is that they can cause nervousness and excitability when given to non-painful animals; another is the possibility of reduced gastrointestinal motility (risk of colic) with repeated administration.

Butorphanol causes a lesser degree of excitability than morphine or fentanyl, but it can cause significant ataxia at higher doses. Thus, it is often combined with xylazine to attenuate the ataxia while maintaining analgesia. Horses will tend to lean forward on this combination, so head elevation and possibly a twitch is advised. Naloxone can be used to antagonize morphine if needed to reduce excitability, but naloxone clears the system more quickly than morphine. Thus, a "booster" sedative of acepromazine might be needed after 4-6 hours if the horse is becoming excited.

Phenothiazine (acepromazine) + alpha-2 agonist combination is "widely practiced and appropriate" to gain long-term sedation with fewer cardiopulmonary and ataxic side effects than with alpha-2 agonists alone.

Continuous ketamine infusions have shown some promise for analgesia, particularly for burn patients. Hubbell noted their effectiveness seems to vary with the type of pain being treated, and that further research is warranted. He also added that he has not been impressed with the consistency of single doses of ketamine.

Lidocaine infusions are used to augment inhalant anesthetics (they reduce the amount of inhaled anesthetic needed) and to control postoperative pain. Single large doses (bolus) can cause hypotension (low blood pressure).

"I’ve always been a dose (by weight) guy, but now I think dose is dependent in part on the setting, intended procedure, available facilities, assistance available, horse age, size, and temperament," Hubbell commented. "Regarding the setting, if the horse is in a quiet environment in his home barn, you can often halve the recommended dose. But in a busy hospital with people banging things around, cows mooing, etc., you might need a lot more."

He also recommends using local anesthesia for potentially painful procedures along with sedatives. "You can extend the effects of sedatives/tranquilizers (up to 1.5-2 hours) if you have good anesthetic techniques on board," he commented.