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LAMENESS—

A Review of the American College of Veterinary Anesthesiologists Guidelines for 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 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, 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 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 ; 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 an- esthesia. 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 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, 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 anes- thesia 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), (PO, IV, or recumbency. When is supplied and ventila- IM), (IV or IM), triflupromazine (IV or IM), tion is assisted, TIVA can safely be used for longer detomidine (IV or IM), romifidine (IV), periods. However, the quality of recovery is re- (IV), and (IV). IV agents approved for duced with extended anesthetic periods; this is use in the horse include chloropent ( hydrate, caused by accumulation of the drug before it is sulfate, and ), guaifenesin, cleared through hepatic metabolism. , 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 . 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. , , thiopental, tilet- 3. Commonly used inhalants include halo- amine-, 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 monitoring, because anesthetics are One area of potential concern is the tendency for continually administered. The ACVA recommends equine veterinarians to combine sedatives (xylazine monitoring of arterial blood pressure when inhal- and detomidine), tranquilizers (acepromazine), and ants are used (see below). (butorphanol) into regimens to produce stand- ing chemical restraint. The potential number of com- III. Monitoring and Supportive Care binations would seem endless if you were to consider A. IV catheterization is recommended for the variations in the doses. Label recommendations cau- administration of anesthetic drugs, fluids, tion against some combinations (the package insert for and/or supportive medications xylazine says it should not be used in combination with tranquilizers), but in some instances, those same The use of IV catheters helps to insure that anes- combinations are promoted in the literature.16–18 thetic medications are given appropriately and It is the author’s impression that the veterinarian is makes it more convenient when additional drugs are safest when using approved drugs only. The recom- required intraoperatively. The reduction in the use mendation of a drug combination, supported in the of and thiopental has reduced the literature, provides some justification for its use. likelihood of severe sequelae to perivascular admin- B. Total IV anesthesia (TIVA) istration of anesthetic drugs; however, guaifenesin 1. Recommended for procedures expected to given outside of the vein can cause swelling.19 be Յ1 h in duration Additionally, the use of a properly functioning cath- 2. Muscle relaxation may not be as profound eter eliminates one potential reason (extravascular compared with inhalant anesthesia administration) for lack of a drug effect. 3. Anesthetic agents may be administered B. Proper position and padding is vital to aid in as intermittent boluses or as an IV infu- the prevention of muscle or nerve injury sion C. TIVA 1. Oxygen source with flowmeter for nasal The majority of equine anesthetic episodes are of insufflation, if indicated short duration and occur outside of a hospital set- 2. Endotracheal tubes and demand valve ting. Injectable anesthetics are most suitable for readily available to ventilate, if neces- use in this arena. Most current anesthetic tech- sary niques use ketamine as the primary anesthetic 3. Pad/cloth for face and eye agent. Ketamine is relatively supportive of cardio- vascular and respiratory function and recoveries Apnea during field anesthesia is relatively rare, be- from a single administration are usually strong and cause the use of succinylcholine chloride and barbi- crisp.10 The primary complications associated with turate solutions has largely been replaced with ketamine anesthesia are insufficient muscle relax- ketamine combinations. Horses can be ventilated ation and inadequate duration of action. These in an emergency situation by readapting a stomach complications can be offset by insuring that horses tube onto an oxygen source, placing the stomach are fully sedate (usually using an alpha-2 tube into the nasal cavity, and occluding the nos- such as xylazine or detomidine) before administer- trils.20 When the chest rises to a normal level, the ing the ketamine and by incorporating a muscle nostrils are released, and the animal exhales. This relaxant such as diazepam or guaifenesin.11 The pattern can be repeated until the oxygen source is recommendation for limiting TIVA to Յ1 h stems exhausted. Alternatively, an endotracheal tube from the difficulties with oxygenation that horses can be placed, and the lungs can be inflated using a

50 2008 ր Vol. 54 ր AAEP PROCEEDINGS LAMENESS—SURGERY demand valve connected to an oxygen source. It is F. Monitoring of the respiratory system the author’s impression that few equine veterinari- 1. Observation of and ans in ambulatory practice routinely carry a com- rhythm pressed oxygen source. 2. Pulse oximetry, if indicated D. Inhalant anesthesia 3. Capnometry, if indicated (please note 1. Appropriately sized cuffed endotracheal that end tidal carbon-dioxide tensions tube frequently underestimate arterial car- 2. Oxygen source and anesthesia machine bon-dioxide tensions in anesthetized 3. Means to scavenge anesthetic waste horses) 4. Arterial blood analysis, if indicated 4. Means to manually or mechanically ven- 5. is treated with either as- tilate, if necessary sisted or controlled ventilation

Endotracheal tubes are necessary for the safe ad- Observation and recording of respiratory rate and ministration of inhalant agents as a component of rhythm are sufficient for most short-term anesthe- delivery of the gases. These tubes also minimize sia. Apnea is rare in the absence of exposure of personnel to trace anesthetic gas levels. use, and respiratory rates in the range of 4–12 Waste anesthetic gases should be removed, prefera- breaths/min are common. Pulse oximeters are eas- bly by an active system (one that uses suction). ily applied and provide information on The Occupational Safety and Health Administration and hemoglobin saturation; unfortunately, they re- has set guidelines for the maximum exposure levels quire frequent repositioning. For longer proce- of personnel to the inhalants at 2 dures, capnometry estimates the arterial partial ppm.21 Veterinarians routinely anesthetizing pressure of , which provides informa- horses for Ͼ1 h should consider the purchase of an tion about the adequacy of ventilation, but it does anesthetic machine with a ventilator. not replace arterial blood gas analysis.25 Horses E. Monitoring of the cardiovascular system anesthetized for Ͼ30 min will hypoventilate. Hy- 1. Digital pulse palpation poventilation is relatively well tolerated for periods 2. Capillary refill time and mucous mem- Յ1 h. Horses anesthetized for Ͼ45 min benefit brane color from controlled ventilation as long as arterial blood 3. Electrocardiogram, if indicated pressures are maintained. Early ventilation is as- 4. Arterial blood pressure, if indicated sociated with the highest levels of oxygenation; thus, (strongly recommended when inhalation if long procedures are anticipated, controlled venti- anesthesia is used) lation should be instituted as soon as arterial blood 5. should be treated with ap- pressures are stabilized.26 In an emergency situa- propriate medication (fluids, inotropes, tion, spontaneous ventilation might be stimulated etc.) by the administration of doxapram (0.2–1.0 mg/kg, IV).27 Irreversible changes in cerebral function occur within 5 min of the cessation of blood flow. There- IV. Injectable Adjuncts During Anesthesia fore, cardiovascular function should be assessed at A. May be useful to provide additional anesthe- least that frequently and recorded at 10-min inter- sia, analgesia, or muscle relaxation during vals. Palpation of pulses allows determination of anesthesia heart rate as well as some subjective information B. May be administered as a bolus or with cer- about the strength of cardiac contractions and arte- tain medications, and may be given as a rial blood pressure. Determining and recording constant-rate infusion heart rate is sufficient basal cardiovascular monitor- C. Common adjuncts ing for short anesthetic procedures. Determination 1. Opioids (e.g., butorphanol) of arterial blood pressure becomes more critical as 2. Ketamine the anesthetic duration increases. Monitoring ar- 3. Local anesthetics (IV or as a local/re- terial blood pressure and maintaining mean arterial gional technique) blood pressure in excess of 60 mmHg is critical for 4. Muscle relaxants success during inhalant anesthesia and TIVA of a. Diazepam or longer duration. Mean arterial blood pressures b. Guaifenesin Ͻ60 mmHg are associated with an increased inci- c. Neuromuscular blocking agents (con- dence of rhabdomyolysis.22 Arterial blood pres- trolled ventilation and monitoring of sures are best measured by arterial cannulation. neuromuscular function is required Indirect methods can be used, but the values pro- during paralysis) duced are best used for trending blood pressure rather than for absolute values.23 Hypotension Adjuncts are primarily used as a method to reduce should be treated with IV fluids and inotropes, as the requirement for inhalant anesthetic during sur- necessary.24 gery by providing analgesia. is thought

AAEP PROCEEDINGS ր Vol. 54 ր 2008 51 LAMENESS—SURGERY to increase motility in colic patients and has been 3. Sedatives and/or may be ad- shown to reduce the requirement for inhalant anes- ministered during the recovery period to thetics.28 The use of neuromuscular blocking aid in a smooth transition to standing drugs is usually limited to ophthalmologic and some complex orthopedic procedures. All recoveries from anesthesia should be attended. Approximately 30% of anesthesia-related deaths in 1 V. Local and Regional Analgesia/Anesthesia horses occur during the recovery phase. In most instances, horses recover within1hofthediscon- A. May be chosen as the sole technique for cer- 29 tain procedures tinuation of the anesthetic. Recoveries from B. Depending on the temperament of the pa- short anesthetics are usually uncomplicated. Re- coveries from inhalant anesthesia with isoflurane or tient and type of procedure, chemical re- sevoflurane are shorter than those from halothane straint may also be used in combination anesthesia. Many horses benefit from the admin- with a local or regional technique istration of a small dose of (0.2 mg/kg xy- C. May also be used as an adjunct to general lazine, IV, or 0.04 mg/kg detomidine, IV) to slow anesthesia recovery and allow for additional exhalation of the D. Choice of local anesthetics includes lido- gases.30 The horse should be attended until it can caine, mepivacaine, and bupivacaine; the stand unassisted. addition of epinephrine (5 ␮g/ml) may help to improve the quality and duration of an- 3. Conclusion esthesia E. Local and regional techniques In summary, the ACVA guidelines for equine anes- 1. Local infiltration (e.g., line block or ring thesia provide a pathway to increase the safety of block) anesthetized horses. Also, the documentation of 2. Peripheral what was done and when it was done provides pro- tection for the veterinarian should a crisis ensue. 3. Intra-articular block 4. Paravertebral block References 5. Epidural analgesia/anesthesia 1. Johnston GM, Eastment JK, Wood JLN, et al. The confiden- a. Local anesthetics tial enquiry into Perioperative Equine Fatalities (CEPEF); b. Alpha-2 mortality results of Phases 1 and 2. Vet Anaesth Analg 2002;29:159–170. 2. Bidwell LA, Bramlage LR, Rood WA. Fatality rates associ- Local and regional techniques are low-risk anes- ated with equine general anesthesia, in Proceedings. 50th thetic adjuncts for either standing or general anes- Annual American Association of Equine Practitioners Con- thetic procedures; however, excessive doses or vention 2004;492–493. 3. Johnston GM, Taylor PM, Holmes MA, et al. Confidential volumes (epidural) can cause ataxia and recum- enquiry of perioperative equine fatalities (CEPEF-1): pre- bency. Epinephrine should not be added if the area liminary results. Equine Vet J 1995;27:193–200. to be desensitized is peripheral (such as an ear), 4. Martinez EA, Wagner AE, Driessen B, et al. American College because the vasoconstriction could cause necrosis. of Veterinary Anesthesiologists guidelines for anesthesia in horses. Available online at http://www.acva.org/diponly/ac- tion/Guidelines Anesthesia Horses 041227.htm. Accessed on VI. Recovery January 17, 2008. 5. Suggestions for monitoring anesthetized veterinary patients. A. TIVA J Am Vet Med Assoc 1995;206:956–937. 1. If in a padded, confined area (recovery 6. Flemming DD, Scott JF. The informed consent doctrine: stall), no assistance may be needed what veterinarians should tell their clients. J Am Vet Med 2. If in an open (outside) area, relatively Assoc 2004;224:1436–1439. soft (grass) areas should be available that 7. Meagher DM. A review of equine malpractice claims, in Proceedings. 51st Annual American Association of Equine are free of obstacles (trees, fences); assis- Practitioners Convention 2005;508–514. tance should be provided to prevent too 8. Ramsay EC, Geiser D, Carter W, et al. Serum concentra- much momentum tions and effects of detomidine delivered orally to horses in a. Control head and protect eyes three different mediums. Vet Anaesth Analg 2002;29:219– 222. b. Assist on tail (if possible) 9. Kriz N, Hodgson DR, Rose RJ. Prevalence and clinical im- B. Inhalant anesthesia portance of heart murmurs in racehorses. J Am Vet Med 1. Depending on temperament, physical Assoc 2000;216:1441–1445. status, inhalant used, surgical procedure 10. Wilson JF. Medical records: content, requirements, and legal implications. In: Wilson JF, Rollin BE, Garbe JAL, performed, and design of recovery stall, eds. Law and ethics of the veterinary profession. Yardley, the horse may recover either unassisted PA: Priority Press Limited, 1993;316–367. or with assistance on the head and/or tail 11. Bonvicini KA. Tools for enhancing communication: an 2. If recovery is unassisted, the patient overview of risk management in equine practice, in Proceed- ings. 52nd Annual American Association of Equine Practi- should be observed as often as needed to tioners Convention 2006;181–187. be able to identify if the horse unexpect- 12. Hubbell JAE, Muir WW. Xylazine and -zolaz- edly requires assistance epam anesthesia in horses. Am J Vet Res 1989;50:737–742.

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13. Muir WW, Skarda RT, Milne DW. Evaluation of xylazine halothane-anesthetized horses. Am J Vet Res 1987;48:192– and ketamine hydrochloride for anesthesia in horses. Am J 197. Vet Res 1977;38:195–201. 23. Riebold TW, Evans AT. Blood pressure measurement in the 14. Brock N, Hildebrand SV. A comparison of xylazine-diaze- anesthetized horse, comparison of four methods. Vet Surg pam-ketamine and xylazine-guaifenesin-ketamine in equine 1985;14:332–337. anesthesia. Vet Surg 1990;19:468–474. 24. Swanson CR, Muir WW, Bednarski RM, et al. Hemody- 15. Grosenbaugh DA, Muir WW. Cardiorespiratory effects of namic responses in halothane-anesthetized horses given in- sevoflurane, isoflurane, and halothane anesthesia in horses. fusions of or dobutamine. Am J Vet Res 1985;46: Am J Vet Res 1998;59:101–106. 365–370. 16. Nolan AM, Hall LW. Combined use of sedatives and 25. Cribb PH. Capnographic monitoring during anesthesia in horses. Vet Rec 1984;114:63–67. with controlled ventilation in the horse. Vet Surg 1988;17: 17. Muir WW, Skarda RT, Sheehan W. Hemodynamic and re- 48–52. spiratory effects of a xylazine-acetylpromazine drug combi- 26. Day TK, Gaynor JS, Muir WW, et al. Blood gas values nation in horses. Am J Vet Res 1979;40:1518–1522. during intermittent positive pressure ventilation and spon- 18. Robertson JT, Muir WW. A new drug combination taneous ventilation in 160 anesthetized horses positioned in in the horse. Am J Vet Res 1983;44:1667–1669. lateral or dorsal recumbency. Vet Surg 1995;24:266–276. 19. Funk KA. Glyceryl guaicolate: some effects and indica- 27. Wernette KM, Hubbell JAE, Muir WW, et al. Doxapram: tions in horses. Equine Vet J 1973;5:15–19. cardiopulmonary effects in the horse. Am J Vet Res 1986; 20. Riebold TW, Evans AT, Robinson NE. Emergency ventila- 47:1360–1361. tion in the horse, in Proceedings. 25th Annual Convention 28. Doherty TJ, Frazier DL. Effect of intravenous lidocaine on of the American Association of Equine Practitioners 1980;25: halothane minimum alveolar concentration in ponies. Equine 113–122. Vet J 1998;30:300–303. 21. Anesthetic gases: guidelines for workplace exposures. 29. Whitehair KJ, Steffey EP, Willits NH, et al. Recovery of Available online at http://www.osha.gov/dts/osta/anesthetic- horses from inhalation anesthesia. Am J Vet Res 1993;54: gases/index.html. Accessed on January 28, 2008. 1693–1702. 22. Grandy JL, Steffey EP, Hodgson DS, et al. Arterial hypo- 30. Hubbell JAE. Recovery from anaesthesia in horses. Equine tention and the development of postanesthetic myopathy in Vet Edu 1999;11:160–167.

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