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.
1Johnston GM, Eastment JK, Wood JLN, et al. The confidential enquiry into perioperative equine fatalities (CEPEF):mortality results of Phases 1 and 2. Vet Anaesth Analg2002; 29:159 –170.
2Hubbell, J., DVM. Practical Standing Chemical Restraint of the Horse. Proceedings: Knowledge of the pharmacology of sedatives, tranquilizers, and analgesics allows equine veterinarians to provide safe restraint for standing procedures, Ohio State University, 2009.
3Parentoni, R., et. al. Detomidine and xylazine, at different doses, in donkeys. Brazilian Journal of Veterinary Medicine, 42, e100820. DOI: 10.29374/2527-2179.bjvm100820.
4Muir WW. Anxiolytics, nonopioid sedative-analgesics, and opioid analgesics. In: Muir WW, Hubbell JAE, eds. Equine anesthesia: monitoring and emergency therapy, 2nd ed. St. Louis, MO: Saunders Elsevier, 2009; 186 –190.
5Muir WW, Skarda RT, Sheehan WC. Hemodynamic and respiratory effects of a xylazine-acetylpromazine drug combination in horses. Am J Vet Res1979; 40:1518 –1522.
6Wagner AE, Muir WW, Hinchcliff KW. Cardiovascular effects of xylazine and detomidine in horses. Am J Vet Res1991; 52:651– 657.
7Figueiredo JP, Muir WW, Smith J, et al. Sedative and analgesic effects of romifidine in horses. Int J Appl Res VetMed2005; 3:249 –258.
8Lavoie JP, Pascoe JR, Kurpershoek CJ. Effect of head and neck position on respiratory mechanics in horses sedated with xylazine. Am J Vet Res1992; 53:1652–1657.
9Trim CM, Hanson RR. Effects of xylazine on renal function and plasma glucose in ponies. Vet Rec1986; 118:65– 67.
10Rutkowski JA, Ross MW, Cullen K. Effects of xylazine and/or butorphanol or neostigmine on myoelectrical activity of the cecum and right ventral colon in female ponies. Am JVet Res1989; 50:1096 –1101.
11Schatzmann U, Josseck H, Stauffer JL, et al. Effects of α-2agonists on intrauterine pressure and sedation in horses: comparison between detomidine, romifidine, and xylazine. J Vet Med1994; 41:523–529.
12Luukkanen L, Katila T, Koskinen E. Some effects of multiple administrations of detomidine during the last trimester of equine pregnancy. Equine Vet J1997; 29:400 – 403.
13Hoffman PE. Clinical evaluation of xylazine as a chemical restraining agent, sedative, and analgesic in horses. JAmVet Med Assoc1974; 164:42– 45.
14Guedes, A., DVM. How to Maximize Standing Chemical Restraint, How-To Session: Field Anesthesia And Pain Management, J. University of California, Davis, March 2012.
About NexGen Pharmaceuticals
NexGen Pharmaceuticals is an industry-leading veterinary compounding pharmacy, offering sterile and non-sterile compounding services nationwide. Unlike other veterinary compounding pharmacies, NexGen focuses on drugs that are difficult to find or are no longer available due to manufacturer discontinuance or have yet to be offered commercially for veterinary applications, but which still serve a critical need for our customers. We also specialize in wildlife pharmaceuticals, including sedatives and their antagonists, offering many unique options to serve a wide array of zoo animal and wildlife immobilization and anesthesia requirements.
Our pharmacists are also encouraged to develop strong working relationships with our veterinarians in order to better care for veterinary patients. Such relationships foster an ever-increasing knowledge base upon which pharmacists and veterinarians can draw, making both significantly more effective in their professional roles.
Disclaimer
The information contained in this blog post is general in nature and is intended for use as an informational aid. It does not cover all possible uses, actions, precautions, side effects, or interactions of the medications shown, nor is the information intended as medical advice or diagnosis for individual health problems or for making an evaluation as to the risks and benefits of using a particular medication. You should consult your veterinarian about diagnosis and treatment of any health problems. Information and statements have not been evaluated by the Food and Drug Administration ("FDA"), nor has the FDA approved the medications to diagnose, cure or prevent disease. Medications compounded by NexGen Pharmaceuticals are prepared at the direction of a veterinarian. NexGen Pharmaceuticals compounded veterinary preparations are not intended for use in food and food-producing animals.
NexGen Pharmaceuticals, LLC does not recommend, endorse or make any representation about the efficacy, appropriateness or suitability of any specific dosing, products, procedures, treatments, services, opinions, veterinary care providers or other information that may be contained in this blog post. NEXGEN PHARMACEUTICALS, LLC IS NOT RESPONSIBLE NOR LIABLE FOR ANY ADVICE, COURSE OF TREATMENT, DIAGNOSIS OR ANY OTHER INFORMATION, SERVICES OR PRODUCTS THAT YOU OBTAIN THROUGH THIS BLOG POST.