and increased locomotor activity may be seen when opioids are used in the absence of adequate sedation (e.g. without an alpha‐2 agonist or acepromazine), particularly at high doses and in the absence of pain [4]. The degree to which behavioral opioid side effects manifest is individually variable and is possibly related to genetic mu opioid receptor polymorphism [5].
A phenomenon termed “epinephrine reversal” may occur with administration of acepromazine alone to an already excited horse. Drug‐induced alpha‐1 adrenergic blockade unmasks beta‐2 adrenergic activation produced by circulating catecholamines, causing significant decreases in vascular resistance that result in severe hypotension and collapse [6].
Intra‐arterial injection of sedative drugs typically causes near‐immediate agitation, collapse, and convulsions that may resolve over the course of several minutes to an hour. Death can occur acutely, or long‐term neurological side effects may dictate euthanasia [7].
Recumbency can be seen following weakness or paralysis of the hind limbs induced by migration of drugs injected into the inter‐coccygeal (caudal) epidural space to the lumbar region, where the motor nerves to the pelvic limbs originate. This is most likely to occur with high volumes of local anesthetics, whose mechanism of action includes both sensory and motor blockade, but has been reported with alpha‐2 agonists [8].
While there is little work documenting the incidence of serious drug‐related complications during standing procedures, in the authors’ experience “breaks” in horse behavior happen with some regularity. These can occur at any time during a procedure and be very sudden in nature, and can include aberrations such as horses dramatically “exiting” the stocks. Though plenty has been written about the commonly used sedatives, there is no data to support the superiority of one drug or combination of drugs over another for all possible procedures. Much success relates to the experience of the person administering the sedation and the individual characteristics of the horse.
Prevention
Patients for standing procedures should be carefully selected, as some horses are not amenable to standing surgery even when sedated with standard drug dosages. The surrounding environment should be closely monitored and efforts made to reduce ambient noise, provide footing with traction, and limit distractions.
Thoughtful selection of sedative and analgesic drugs should be based on patient and procedure, including the use of local anesthetic techniques when possible. Delivery of sedative drugs via an intravenous catheter is preferred rather than “off the needle” to reduce risk of inadvertent intra‐arterial injection, particularly when multiple doses of sedatives must be given.
Using opioids such as butorphanol or morphine as a part of the sedation protocol in combination with an alpha‐2 agonist does not guarantee prevention but is thought to reduce the likelihood of sudden movements/phantom kick behaviors. To avoid hind limb paralysis, the volume of local anesthetics injected into the caudal epidural space should be limited to less than 10 ml in an adult normal size horse [9].
Treatment
If a horse becomes recumbent, the area around the horse should be cleared of equipment and non‐essential personnel. Depending on the cause of collapse, some horses may stand immediately. Some may be extremely agitated while others calmer. Supportive care may be required (e.g. large volume intravenous fluids, padding, extraction from a particular position, or physical assistance to stand) while drugs are metabolized or in some cases reversed. If the horse has sustained serious injury, general anesthesia may be required to assess and/or treat the horse or complete the procedure.
Expected outcome
If potential behavioral problems are recognized early and the procedure can be halted or different drugs or techniques selected, there may be no long‐term consequences to the horse. However, catastrophic injury can occur, in some cases necessitating extensive further treatments (e.g. long bone fracture repair) or euthanasia.
Complications During General Anesthesia
Unanticipated Movement
Definition
Unexpected movement (e.g. of the limbs or head) during anesthesia may be considered a complication, as it may result in injury to the horse or personnel and damage to surgical and anesthesia equipment.
Risk factors
Inadequate dosing of anesthetic drugs
Lack of experience monitoring or attention to anesthetic depth
Ophthalmic procedures
Pathogenesis
Movement occurs primarily as a result of inadequate depth of anesthesia. However, it is the authors’ experience that some horses will move without warning, even when maintained at what appears to be an appropriate plane of anesthesia based on objective and subjective monitoring parameters.
Prevention
While it seems obvious that anesthesia depth influences movement on the surgical table and the answer is to keep the horse at a deeper plane of anesthesia, this is confounded by the negative effects of the inhalation agents on cardiorespiratory function and the potential for other complications (even death) with an anesthetic overdose.
The use of adjunctive medications with analgesic or inhaled anesthetic sparing properties may be beneficial [10–13]. Infusions of ketamine, lidocaine, alpha‐2 adrenergic agonists (e.g. xylazine, detomidine, romifidine, medetomidine, dexmedetomidine), guaifenesin, benzodiazepines, and propofol have been used for this purpose. When considering the use of any adjunctive drug, the cardiorespiratory effects must be weighed against the overall health status of the horse. For example, while alpha‐2 agonists have been shown to provide analgesia, reduce the minimum alveolar concentration (MAC) of inhalant anesthetics, and improve recovery quality in horses, they also cause significant decreases in cardiac output. Other drugs may have better cardiovascular effects but negatively influence recovery quality. Benefits and risks of individual drugs should be evaluated in context of the individual case.
The literature is inconclusive with regard to the benefit of systemically administered opioids as an anesthetic adjunct [14]. Mu opioids largely tend to increase MAC for inhaled anesthetics or do not enhance inhalant anesthetic sparing properties of other infusions [15–17]. Provision of regional opioids (e.g. intra‐articular or epidural administration) has been definitively shown to be beneficial for analgesia [18–20].
The use of regional anesthesia techniques (e.g. intra‐testicular block for castration [21], distal limb blocks) can minimize the potential for horse movement while also providing analgesia and a reduced need for systemically administered anesthetics.
For procedures in patients where involuntary movement is difficult to manage, neuromuscular blocking drugs could be administered to prevent further movement. The anesthetist is cautioned to ensure that horses are concurrently administered appropriate doses of anesthetic and analgesic drugs, as neuromuscular blocking drugs do not have either of these properties. The use of neuromuscular blockade alone to immobilize a horse should be considered inhumane. Additionally, the ability to provide positive pressure ventilation is critical.
Monitoring
