Detomidine for Equine Surgical Procedures

Long ago, veterinarians discovered that there are many surgical and other medical procedures which can be carried out in standing horses if the appropriate combinations of physical and chemical restraint are employed. The ability to perform procedures in the standing position is more important in the horse than in other species because of the greater risk of complications associated with anesthesia in the horse.¹

The goal of most standing restraint is to produce a quiet, calm horse that is immobile and does not react to stimuli or manipulation. Analgesia should be provided if the intended procedure is expected to be painful.

“There are nine drugs approved for use in horses for standing chemical restraint: acepromazine, butorphanol, chloropent, detomidine, pentazocine, promazine, romifidine, triflupromazine and xylazine. Of these, only five drugs (acepromazine, butorphanol, detomidine, romifidine and xylazine) are commercially marketed by pharmaceutical houses. Other drugs, such as morphine and fentanyl, are used with some frequency for standing chemical restraint but are not labeled for use in the horse. No single drug produces “ideal” standing chemical restraint in every horse.”²

Detomidine is a more recent and more potent agonist than xylazine.³ Although the majority of sedative and analgesic drugs used for restraint are labeled for use as “sole” agents, the majority of equine veterinarians use them in combination with the goal of optimizing the onset, quality, and duration of the alteration in mental state while minimizing potentially deleterious side effects. Many combinations are recommended in the literature, but relatively few have been rigorously studied scientifically.

Drugs Used in Standing Procedures

Phenothiazine tranquilizers are used to produce calming.⁴ They are not believed to produce analgesia, but they do enhance the analgesic activity of other drugs such as α2- agonists and opioids. Phenothiazines activate when given orally, intramuscularly, and intravenously by blocking the action of neurotransmitters (dopamine) centrally and peripherally and causing an α adrenergic blockade that can lead to arterial hypotension.⁵ This is of particular concern in excitable horses, horses that have hemorrhaged, or dehydrated.

α-2 agonists such as detomidine produce sedation with muscle relaxation, ataxia, and analgesia when given orally, intravenously, or intramuscularly. The α-2 agonists produce a number of cardiorespiratory and other side effects, the significance of which are dose dependent in the clinical dose range. Heart rate decreases, and sinus arrhythmia, first degree atrioventricular blockade, and second degree atrioventricular blockade are common.²

With α-2 agonists, arterial blood pressure is initially increased because of drug-induced increases in peripheral vascular resistance.⁶ Hypertension may be sustained (20 – 60 min), particularly when detomidine and romifidine are used.^6,7 Decreases in heart rate and increases in peripheral vascular resistance produce significant decreases in cardiac output, of-ten to levels 50% of the pre-drug values. Respiratory rate is usually decreased, but tidal volume increases to compensate for the drop.⁸

The administration of an α-2 agonist decreases salivation, gastric secretions, and gastrointestinal motility, and it also increases urine volume.^9,10 Swallowing is depressed, and thus, passage of nasogastric tubes maybe more difficult. Other incidental effects of α-2 administration include increases in intrauterine pressure, hyperglycemia, and hypoinsulinemia.^11,12 The level of sedation produced by administration of an α-2 agonist is more pronounced than that produced by phenothiazine administration. Depending on the dose and drug administered, horses assume a “head-down” or “saw horse” stance and frequently shift their weight from side to side. Xylazine is useful for restraint for procedures on the head, neck, and forequarters, but some researchers have expressed concern about its suitability when used as the sole agent for procedures on the hindquarters because of the potential for rapid arousal (startling) and kicking.¹³

Opioid agents, used primarily for their analgesic properties in veterinary practices, are of limited use in equine practice because of the possibility of central nervous system (CNS) excitation.² However, opioids are often used with α-2 agonists, the combination of which increases clinical effects and decreases side effects, compared to the individual use of the drugs. Opioids used in horses include morphine, butorphanol, and buprenorphine.^2,5

Other opioids, including morphine and pentazocine, have also been used to produce standing chemical restraint.² The combination of xylazine and detomidine produces a profoundly dissociated horse. Detomidine administration should precede morphine to prevent opioid-induced excitement.⁴ Horses sedated with detomidine and morphine, especially those undergoing prolonged procedures,will also receive a crystalloid solution such as lactated Ringer’s solution (2–5 mL/kg per hour) to maintain fluid balance and replace losses caused bypolyuria.¹⁴

Detomidine administered by infusion provides prolonged periods of chemical restraint in standing horses.³ Supplemental sedatives or analgesics may be needed in horses undergoing surgery. Detomidine provides an effective method that provides prolonged periods of chemical restraint in standing horses. Detomidine alone may not provide sufficient analgesia for surgery and the horse is likely to require supplemental analgesics.