A Review of the American College of Veterinary Anesthesiologists Guidelines for Anesthesia of Horses

LAMENESS—SURGERY A Review of the American College of Veterinary Anesthesiologists Guidelines for Anesthesia of Horses John A. E. Hubbell, DVM, MS, Diplomate ACVA The risk of death associated with anesthesia in horses is ϳ1%. The American College of Veterinary Anesthesiologists has developed guidelines for the anesthesia of horses that provide a pathway for the safe anesthesia of horses, and if followed, they provide a measure of support for veterinarians should their methods be questioned when anesthetic complications occur. Author’s address: Department of Veterinary Clinical Sciences, The Ohio State University, 601 Vernon L. Tharp Street, Columbus, OH 43210; e-mail: [email protected]. © 2008 AAEP. 1. Introduction anesthetic regimen, monitoring and supportive care, use of injectable adjuncts, local and regional anes- Estimates of the risk of death associated with elec- 4 tive general anesthesia in the horse range from 0.1% thesia, and recovery. Attorneys representing (1 in 1000) to 1% (1 in 100).1,2 The risk of death plaintiffs in malpractice litigation may refer to the increases for anesthesia Ͼ1 hr in duration and var- guidelines and question omissions from the sug- gested procedures when determining if standards of ies with operation type.3 Anesthesia is safest in practice have been maintained. The equine guide- horses 2–7 yr of age, and mortality rate is decreased lines supplement another ACVA document covering when horses receive pre-anesthetic sedatives and suggestions for monitoring veterinary patients; this tranquilizers, particularly acepromazine. IV anes- document includes the establishment of a medical thesia seems to be safer than inhalant anesthetics; record and regular recordings of intraoperative however, this is difficult to interpret, because proce- values.5 dures performed during IV anesthesia tend to be This paper will address the contents of the guide- shorter than those performed under inhalant lines and how equine veterinarians might use them 1 anesthesia. to improve their patient care and provide justifica- The American College of Veterinary Anesthesiol- tion for their methodologies when necessary. The ogists (ACVA) recognized that horses provide chal- performance of appropriate evaluations and treat- lenges beyond those of other domestic animals, and ments for the betterment of our equine patients is in response, they established an Equine Standards the primary goal, but it must be recognized that Committee to formulate guidelines for equine anes- maintaining good medical records and obtaining in- thesia. The guidelines concisely describe recom- formed consent are integral to the sustainability of mendations for pre-operative evaluation, selection of any medical practice.6 Anesthetic complications NOTES 48 2008 ր Vol. 54 ր AAEP PROCEEDINGS LAMENESS—SURGERY and death are one of the top eight categories of The physical examination should emphasize the car- claims submitted to the Professional Liability Insur- diovascular and respiratory systems. Palpating ance Trust of the American Veterinary Medical As- the rate, strength, and rhythmnicity of the periph- sociation (AVMA), which comprises 6% of a recently eral pulse and checking the color of the mucous reported series of cases.7 Good medical records and membranes gives evidence of the integrity of the appropriate client communication are important fac- cardiovascular system. Auscultation provides ad- tors in reducing claims. A phrase repeated often in ditional information; however, it can be somewhat the legal world is, “if it wasn’t written down, it didn’t confusing, because up to 80% of horses have cardiac 9 happen.” murmurs. Fever is frequently a sign that a horse is incubating respiratory disease or another infec- 2. Guidelines and Interpretation tion that might be exacerbated by the stress of anesthesia. The amount of laboratory work I. Pre-operative Evaluation recommended depends on the results of the physical A. History examination. A packed cell volume and plasma to- 1. Response to prior sedation tal protein provide information on hydration, oxy- 2. History of any significant illness or injury gen-carrying capacity, and overall health. Total 3. Current problem white counts (high or low) and a fibrinogen level are valuable indices of present or impending disease. The medical history frequently provides significant Physical findings such as marked bradycardia, information regarding the risk of anesthesia. The weakness, tenting of the skin, weakness, or ataxia attending veterinarian may have a continuing rela- should prompt the veterinarian to include further tionship with the client and the horse, but some diagnostic tests. long-standing conditions (minor ataxia, respiratory An assessment of the anesthetic risk should be noise, or hyperkalemic periodic paralysis) that are established and communicated to the owner before 10 being managed chronically become more important the pre-operative evaluation. This communica- when facing anesthesia; therefore, a discussion of tion is separate from the usual communications their implications for anesthesia would be part of about prognosis (such as 60% of horses return to informed consent. The drug, dose, and route of ad- soundness after this operation), because the impact ministration selected for pre-anesthetic medication of an anesthetic death is more acute. Signed or may be dependent on previous responses and the witnessed informed consent acknowledging the risks of anesthesia is an important step in risk man- temperament of the horse. Some horses cannot be 11 approached with a syringe and needle. In those agement. instances, oral detomidine (0.06 mg/kg) can be squirted into the mouth like a paste.8 A history of II. Selection of Anesthetic Regimen exercise intolerance indicates further evaluation of A. Appropriate regimen the cardiovascular and respiratory systems. A his- 1. Physical status of the patient 2. Duration of anesthesia required tory of recent respiratory disease is of particular 3. Number and skill of personnel available concern. Usually, anesthesia makes respiratory 4. Safety of facility/location where anesthe- disease worse. Anesthesia-induced reductions in sia (including induction and recovery) mucociliary clearance, cough reflex, drying of the will be performed airway, and positioning may contribute to exacerba- 5. Anesthetic equipment available tions of clinical and subclinical respiratory disease. 6. Monitoring equipment available An assessment of the anticipated duration of anesthesia gives further information with regard to the The safety of any anesthetic procedure is primarily degree of risk. determined by the skill and experience of the veter- B. Physical examination inarians in charge and the quality and quantity of 1. Temperature, pulse rate, and respiratory people assisting them. Fortunately, most equine rate patients undergoing anesthesia are healthy and tol- 2. Evaluation of all organ systems with a erate short anesthetic episodes well. The antici- focus on the presence or absence of car- pated duration of anesthesia determines both the diovascular and respiratory abnormali- risk and the type of anesthetics best used. It is ties important to cover the “what ifs” before anesthesia 3. Capillary refill time and mucous mem- is induced including the question of what will hap- brane color pen if the procedure takes longer than I think it will. C. Laboratory blood work Equine veterinarians are best protected when 1. Order and/or perform any necessary they use drugs that are approved for use in the horse blood work with doses and routes indicated on the labels. Un- 2. Recommended tests, if any, will depend fortunately, only a limited number of anesthetic on the physical status of the patient and drugs are approved for equine use in the United the procedure to be performed States, and fewer are currently marketed. Ap- AAEP PROCEEDINGS ր Vol. 54 ր 2008 49 LAMENESS—SURGERY proved drugs for standing chemical restraint include experience when anesthetized in lateral or dorsal acepromazine (IV, IM, or SQ), promazine (PO, IV, or recumbency. When oxygen is supplied and ventila- IM), xylazine (IV or IM), triflupromazine (IV or IM), tion is assisted, TIVA can safely be used for longer detomidine (IV or IM), romifidine (IV), pentazocine periods. However, the quality of recovery is re- (IV), and butorphanol (IV). IV agents approved for duced with extended anesthetic periods; this is use in the horse include chloropent (chloral hydrate, caused by accumulation of the drug before it is magnesium sulfate, and pentobarbital), guaifenesin, cleared through hepatic metabolism. thiamylal, thialbarbitone, and pentobarbital. In- C. Inhalant anesthesia halant anesthetics approved for use in the horse 1. Preferred for lengthy procedures (Ͼ1hof include methoxyflurane, isoflurane, and halothane. anesthesia time) Additionally, the number of non-marketed but ap- 2. Requires additional equipment compared proved drugs actually equals the number of those with TIVA marketed. Diazepam, ketamine, thiopental, tilet- 3. Commonly used inhalants include halo- amine-zolazepam, and sevoflurane are not labeled thane, isoflurane, and sevoflurane for use in the horse, but a basis for their use can be found in the literature.12–15 They are used widely Inhalant anesthesia and the associated supplemen- and thus, would seem to fall under the reasonable tal oxygen tensions provide greater control of anes- extra-label use that is encompassed in normal stan- thetic depth. The use of inhalants requires a dard of care. greater level of

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