Procedural Sedation in the Horse

Introduction

Procedural sedation is performed in standing horses undergoing various diagnostic or surgical procedures as an alternative to general anesthesia. It has been recently reported that the mortality rate of general anesthesia in horses is higher than standing sedation. Procedural sedation is a technique of administering sedatives, with or without systemic analgesic drugs or locoregional techniques, to facilitate standing procedures while maintaining cardiovascular compensatory mechanisms, airway control and oxygenation.

Procedural sedation in the standing horse is not without it challenges. The goal is to have the horse calm and comfortable and not reacting to manipulation or noxious stimuli. However, if the level of sedation and muscle relaxation is excessive, severe ataxia or the horse falling can result in patient or staff injury. By combining two or more drugs with different pharmacologic effect, a lower dose requirement for each individual drug can decrease their side effects and mitigate complication risk.

Procedures

Diagnostic procedures that can be performed in the sedated standing horse include radiography, ultrasound, magnetic resonance imaging, and endoscopy. Surgical procedures, when appropriate analgesia is provided along with sedation, include dental and sinus procedures, cutaneous mass removal, tracheostomy, laparoscopic procedures, and perineal surgery.

Patient Selection

When selecting which patients to perform procedural sedation while standing, their behavior and temperament must be considered. Horses that are fractious, highly stressed, or not used to handling are considered poor candidates for procedural sedation. Not only will these patients likely resist handling and restraint, but their high sympathetic tone will also reduce the effects of administered sedatives. Increasing the dose administered may result in oversedation, increasing the risk of severe ataxia or falling.

A thorough physical examination is recommended prior to sedation to identify patients with cardiovascular disease or dehydration that may be at greater risk of adverse complications.

Location

Procedural sedation should be performed in a quiet area without bright lights, sudden or loud noises, and staff or patient traffic. Ideally, a dedicated room for standing procedures is used that has stocks with metal bars so that there is patient access from all sides. While sedated, the use of blinders and ear plugs can help to reduce environmental stimulation.

Preparation and Planning

Preparation of the patient for procedural sedation should include the following:

• Placement of intravenous catheter for drug and fluid administration
• Lavage of the oral cavity to remove food and debris
• Placement of a urinary catheter for procedures anticipated to be longer than 60 minutes

When planning for a standing procedure, there needs to be a protocol if the horse should fall to the ground. If this occurs, induction of general anesthesia should be rapidly performed. Calculate and have readily available the appropriate doses of injectable induction drugs (i.e.: ketamine and diazepam). Several sizes of endotracheal tubes, oxygen source, and the means to support ventilation must also be available.

Drug Selection

When selecting a drug protocol for procedure sedation in a standing horse, the goal is to provide appropriate and reliable sedation, little to no ataxia, and adequate analgesia. Utilizing a multimodal strategy instead of administration of a single drug will provide appropriate sedation and analgesia with fewer side effects. Below is a brief description of the clinical effects of commonly used drugs for procedural sedation:

Acepromazine

Acepromazine is a phenothiazine tranquilizer that is administered to provide a calming and anxiolytic effect, decreased dose requirements of other sedatives used in combination, but no analgesia. Phenothiazines act by blocking the action of neurotransmitters causing alpha-adrenergic blockade and vasodilation. Hypotension can occur and is of most concern when there is significant hypovolemia (hemorrhage or dehydration). Due to the splenic dilating effect, acepromazine should be avoided in nephrosplenic entrapment procedures. Priapism and paraphimosis have been reported. Therefore, the administration of acepromazine in breeding stallions is not recommended.

Alpha-2 agonist drugs

The alpha-2 agonist drugs, xylazine, detomidine, and romifidine, are approved for use in horses in the USA and are the most common components of a procedural sedation protocol. Alpha-2 agonist drugs produce sedation and analgesia, as well as muscle relaxation. A ceiling sedative effect is seen, whereby increasing the dose extends the duration of action but not the level of sedation.

Xylazine is useful for procedures involving the head, neck and forelimbs but should be avoided as the sole agent for procedures involving the rear limbs because rapid arousal and kicking can occur. Xylazine can increase intrauterine pressure and myometrial contractility and should not be administered during the last trimester of pregnancy. Detomidine is a more specific alpha-2 receptor with a duration of action approximately twice as long as xylazine. Romifidine has a similar duration of action as detomidine with less severe ataxia than other alpha-2 agonist drugs.

Following administration of an initial intravenous bolus, often in combination with an opioid, additional doses (one-quarter to one-half the initial dose) can be administered as needed to extend the sedative effects or administered as a continuous infusion, to avoid the peaks and troughs of the clinical effect from repeat boluses.Common side effects following alpha-2 agonist drug administration include arterial hypertension, bradyarrhythmias, decreased cardiac output, reduced GI motility, sweating, hyperglycemia, and polyuria.

Opioids

Opioids provide analgesia and potentiate the effects of sedatives and tranquilizers. When combined with an alpha-2 agonist drug and administered as a bolus or continuous infusion, opioids significantly reduce the effective dose requirement of either drug. The potential side effects of opioids include excitement, head shaking, pacing, and decreased GI motility. Because of the risk of histamine release, intravenous opioid administration should be by slow injection. Opioids administered most commonly in horses as a part of a procedural sedation protocol include butorphanol, morphine, and methadone.

Ketamine

Ketamine, at subanesthetic doses, provides adjunctive analgesia in standing horses. Following a subanesthetic bolus, the duration of action is 30 minutes. Therefore, a continuous infusion can be administered for a prolonged procedure when the analgesia provide by the alpha-2 agonist and opioid is insufficient. A ‘ketamine stun’ (0.1 mg/kg IV) can be an effective technique when the horse becomes suddenly aroused during a standing procedure. With a rapid onset and profound sedation, the horse may remain still for about 15 minutes.

Lidocaine

Lidocaine can provide analgesia and sedation when administered as a component of a multimodal procedural sedation protocol. A bolus loading dose is usually followed by a continuous infusion. However, an important consideration is lidocaine’s low therapeutic index. Toxic plasma levels can develop during a prolonged infusion. Clinical signs of local anesthetic toxicity include ataxia and muscle fasciculations, which may progress to seizures, respiratory arrest, and cardiovascular collapse in severe overdoses. If minor symptoms are observed, the lidocaine infusion rate should be reduced or discontinued.

Locoregional techniques

Depending on the planned procedure, a locoregional anesthesia/analgesia technique can be used to reduce the required level of sedation and improve patient comfort during standing procedures. Techniques include caudal or intercoccygeal epidural administration, specific nerve blocks for dental, sinus, and ophthalmic procedures, and intravenous regional anesthesia for distal limb procedures. Numerous combinations including local anesthetic drugs, combined with an alpha-2 agonist or an opioid are commonly used.

Monitoring and Support

Patient monitoring during procedural sedation should include a dedicated anesthesia team member observing pulse and respiratory rate, electrocardiography, and arterial blood pressure. This individual should continually assess the patient’s level of sedation and analgesia and adjust sedative and analgesic drugs as needed. Intravenous administration of a balanced electrolyte solution at a rate of 5 mL/kg/hr is recommended.

Conclusion

For procedural sedation in the standing horse, there is no single ideal drug. A multimodal protocol that includes sedative and analgesic drugs and, when possible, an appropriate locoregional technique, will allow for the ability to perform many diagnostic and surgical procedures. For specific drug combination and dosing recommendations, please see the additional resources listed below.

Additional Resources

Campoy L, Sedgwick SR. Standing Sedation and Iocoregional Analgesia in Equine Dental Surgery. Vet Clin North Am Equine Pract. 2020 Dec;36(3):477-499.

Creighton CM. Standing Sedation. In: Doherty T, Valverde A, Reed RA, editors. Manual of Equine Anesthesia and Analgesia. 2nd Edition. Hoboken (NJ): Wiley-Blackwell 2022 p. 333-338.

Gozalo-Marcilla M, Bettschart-Wolfensberger R, Johnston M, Taylor PM, Redondo JI. Data Collection for the Fourth Multicentre Confidential Enquiry into Perioperative Equine Fatalities (CEPEF4) Study: New Technology and Preliminary Results. Animals (Basel). 2021 Aug 30;11(9):2549.

Guedes A. How to maximize standing chemical restraint. AAEP proceedings. Lexington, Kentucky: American Association of Equine Practitioners. 2013;59:461-3.

Hubbell JAE. Practical Standing Chemical Restraint of the Horse. AAEP proceedings. Las Vegas, Nevada: American Association of Equine Practitioners. 2009;55:2-6.

Labelle AL, Clark-Price SC. Anesthesia for ophthalmic procedures in the standing horse. Vet Clin North Am Equine Pract. 2013 Apr;29(1):179-91.