IN-DEPTH: ANESTHESIA AND PAIN MANAGEMENT

Balancing Total Intravenous Anesthesia and Inhalant Anesthesia in Horses

Ann E. Wagner, DVM, MS, Diplomate ACVA, ACVP

Author’s address: Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, Colorado 80523; e-mail: [email protected]. © 2009 AAEP.

1. Introduction anesthesia are often smoother and more controlled 5 Intravenous drugs such as chloral hydrate, pento- than recoveries from inhalant anesthesia. Other barbital, and thiamylal were once commonly used advantages of TIVA include the minimal equipment required compared with inhalant anesthesia and for general anesthesia of horses but were often as- 6 sociated with prolonged or violent recover- potentially reduced stress. The development of re- ␣ ies. When inhalant anesthetics such as halothane versible or shorter-acting drugs such as 2 agonists, were introduced in the 1960s, the use of IV drugs for ketamine, and propofol has facilitated the use of 1 TIVA for longer procedures than previously consid- maintenance of anesthesia became less common. 7 In recent years, interest in IV anesthetic techniques ered safe. has been renewed, both by recognition of the rela- This article will review the current state of TIVA in horses, as well as how certain IV sedatives, anes- tively high risk of morbidity or mortality with inhal- thetics, and analgesics are being used to supplement ant techniques and by the development of injectable and improve inhalant anesthesia. drugs with fewer undesirable side effects and poten- tially improved recovery characteristics.2 2. Short-Term IV Anesthesia (≤20 min) In 2002, a large, multi-center prospective study In the United States, the most common technique reported peri-anesthetic mortality for horses to be used for short-term IV anesthesia is xylazine seda- 0.9% (1 death per 111 anesthetized horses).3 tion followed by ketamine induction, with or without Subsequently, one individual equine hospital re- diazepam for improved muscle relaxation. Xyla- ported a much lower mortality rate of 0.12–0.24% (1 zine-ketamine or xylazine-diazepam-ketamine pro- death per 417–833 cases).4 In the multi-center vides an average of 16 min of surgical anesthesia, study, anesthesia induced and maintained using with recovery to standing in ϳ25–32 min.8–10 It only IV drugs (total intravenous anesthesia [TIVA]) has been suggested that duration of surgical anes- was reported to be associated with a lower risk of thesia is longest in grade horses and shorter in Thor- death (0.3%, or 1 death per 321 cases) than anesthe- oughbreds.9 Most equine practitioners find both sia induced with IV drugs and maintained with in- induction and recovery with this drug regimen to be halant anesthesia.3 Recoveries from short-term IV satisfactory and reasonably safe. For good quality

NOTES

AAEP PROCEEDINGS ր Vol. 55 ր 2009 7 IN-DEPTH: ANESTHESIA AND PAIN MANAGEMENT

Table 1. Adjuncts for Improved Quality of Anesthesia

Listed below are suggested adjuncts for improving quality of anesthesia in horses that are too light or too tense after an ␣-2 agonist ϩ ketamine Ϯ diazepam induction and do not respond sufficiently to additional increments of ␣-2 agonist or ketamine Systemically administered drugs Thiopental ϳ1–2 mg/kg ͓5–10 ml of 10% thiopental͔ IV (augments CNS depression and muscle relaxation) Guaifenesin ϳ25–50 mg/kg ͓250–500 ml of 5% guaifenesin͔ IV (augments muscle relaxation only) Butorphanol ϳ0.01–0.02 mg/kg ͓0.5–1 ml͔ augments analgesia (more noticeable during IV anesthesia than during gas anesthesia) Locally or regionally administered drugs Castration Lidocaine infiltration of testicle Ϯ spermatic cord augments analgesia, improves cremaster relaxation, and prolongs surgical anesthesia time 15–20 ml 2% lidocaine injected directly into each testicle; some surgeons inject additional 5 ml into spermatic cord also Wounds Specific nerve blocks if applicable (digital, etc.) Perilesional infiltration with lidocaine Eye Specific nerve blocks if applicable Topical anesthetic on globe

of surgical anesthesia, it is important to ensure that Thiopental, once the “gold standard” of anesthetic sedation and muscle relaxation are adequate; there- induction agents, is no longer commonly used for fore, in general, a higher dose of xylazine (1.1 mg/kg short-term anesthesia in horses. Although with or ϳ500 mg per 450-kg horse) than would typically appropriate premedication such as xylazine, thio- be used for standing sedation is recommended. Ad- pental inductions are quite predictable and smooth, ditional methods for improving muscle relaxation recoveries from short-term thiopental anesthesia and quality of IV anesthesia are listed in Table 1. tend to be somewhat uncoordinated.10 In addition, Although xylazine is the most common pre-medi- thiopental can be very irritating if accidentally in- cation in the United States, other ␣2 agonists may jected perivascularly, and the volume typically re- be used and may afford a slightly longer anesthesia quired to anesthetize a 450-kg horse (ϳ27 ml of 10% time. For example, detomidine (0.02 mg/kg, IV) fol- solution) makes perivascular injection a consider- lowed by ketamine (2.2 mg/kg, IV) produced recum- able risk, unless an IV catheter is in place. bency of slightly longer duration (average, 27 min) Interestingly, results of a controlled study sug- than xylazine (1.1 mg/kg, IV) and ketamine (2.2 gested that thiopental produced better-quality in- mg/kg, IV) (average, 23 min). However, muscle re- ductions than did ketamine and that combining laxation during anesthesia and recovery quality either ketamine or thiopental in various proportions were both slightly less satisfactory with detomidine- with propofol improved both induction and recovery ketamine than with xylazine-ketamine.11 quality.10 Propofol is an induction agent that has Romifidine is a newer ␣2 agonist associated with a become extremely popular both in humans and in longer duration and purportedly less ataxia than small animals, with generally smooth inductions xylazine. Both of these characteristics could be and particularly rapid and smooth recoveries, with beneficial for short-term IV anesthesia. A compar- none of the residual “hangover” often associated ison of romifidine (0.1 mg/kg, IV) to xylazine (1.1 with other anesthetics. In the equine study, com- mg/kg, IV), in combination with diazepam and ket- binations of 1⁄4 induction dose of propofol with 3⁄4 amine, showed that both regimens produced excel- induction dose of ketamine or thiopental produced lent quality anesthesia, with good induction and the smoothest inductions. However, the best-qual- recovery quality, and good muscle relaxation. The ity recoveries were achieved by combining 3⁄4 dose of romifidine combination produced about 5 additional propofol with 1⁄4 dose of ketamine or thiopental. min of surgical anesthesia (21 min) compared with One might then think that combining 1⁄2 dose of the xylazine combination (16 min). Recovery re- propofol with 1⁄2 dose of the others would result in quired about 44 min for the romifidine group com- both excellent inductions and excellent recoveries, 8 pared with 32 min for the xylazine group. and indeed, the “1⁄2-1⁄2” combinations did result in Butorphanol has been used as an adjunct for IV both induction and recovery quality that appeared to anesthesia. A comparison of xylazine-ketamine or be slightly better than either ketamine or thiopental detomidine-ketamine, with and without the addition alone.10 of butorphanol (0.04 mg/kg, IV, or 18 mg per 450-kg Unfortunately, there are issues related to propofol horse), suggested that butorphanol may improve administration in horses that detract from its use- muscle relaxation and surgical conditions and pro- fulness as a sole induction agent. Inductions by long recumbency time slightly.11 propofol alone may be quite violent, with paddling of

8 2009 ր Vol. 55 ր AAEP PROCEEDINGS IN-DEPTH: ANESTHESIA AND PAIN MANAGEMENT the limbs and uncoordinated muscle activity.12 pared by adding 4 ml of ketamine (100 mg/ml) and 1 Premedication with either xylazine or detomidine ml of detomidine (10 mg/nl) to reconstitute a 5-ml does not prevent excitatory behavior and muscle bottle of tiletamine-zolazepam. Horses were first activity.13 With the currently available propofol sedated with xylazine 0.44 mg/kg, IV (ϳ200 mg for a concentration of 10 mg/ml, the required induction dose 450-kg horse), and the combination TZKD was ad- (2 mg/kg) for a typical horse is nearly 100 ml, making ministered at 3 ml/450 kg, IV. This volume re- it inconvenient and time consuming to administer. sulted in individual drug doses of tiletamine- Respiratory depression may be profound and may ne- zolazepam at 0.67 mg/kg, ketamine at 0.53 mg/kg, cessitate assisted ventilation.12,13 Despite its reputa- and detomidine 0.013 mg/kg. The authors reported tion for rapid recoveries in most species, a single that advantages of this combination included the induction dose of propofol in horses (after xylazine small volume of drug to administer, predictable and premedication) is associated with slightly longer times rapid induction, good muscle relaxation and analge- to standing (ϳ33 min) than a single dose of ketamine sia that lasted longer than that associated with xy- (ϳ25 min).10,13 lazine-ketamine, and acceptable cardiorespiratory The use of tiletamine-zolazepam, another option values. All horses stood satisfactorily in two or for induction of anesthesia of horses, was first de- fewer attempts within 40 min after induction.15 scribed 20 yr ago.14 Tiletamine is a dissociative anesthetic similar to ketamine, and zolazepam is a 3. TIVA (>20 min) benzodiazepine similar to diazepam. Tiletamine When a procedure requires Ͼ15–20 min, surgical and zolazepam are marketed in the United States as anesthesia can be prolonged by administering addi- the 50:50 combination Telazol,a formulated as a tional IV anesthetics, either as intermittent boluses powder that must be reconstituted before use. A or constant rate infusions (CRIs) (Table 2). To gain 500-mg vial contains 250 mg of tiletamine and 250 an extra 3–5 min of surgical anesthesia, additional mg of zolazepam, and the recommended doses are ␣2 agonist and ketamine can be administered at given as milligram per kilogram of both drugs com- approximately one third the original induction bined. After sedation with xylazine, anesthesia in- doses. These boluses can be repeated several times duced with tiletamine-zolazepam, at doses of 1.1, as needed. 1.65, or 2.2 mg/kg, IV, resulted in surgical anesthe- When a procedure is anticipated to take Ͼ30 min, sia of ϳ20 min, lateral recumbency of 30- to 39-min it is advisable to use a CRI to maintain anesthesia to duration, and recovery to standing in 32–46 min; provide a more stable plane of anesthesia. Since thus, tiletamine-zolazepam produces longer dura- the early 1980s, the most common technique used tion of anesthesia than does ketamine. Quality of for this purpose has been “triple drip” or “GKX,” a anesthesia was judged to be best with tiletamine- combination of guaifenesin (50 mg/ml), ketamine zolazepam doses of 1.65 or 2.2 mg/kg, IV. However, (1–2 mg/ml), and xylazine (0.5 mg/ml).16 For more PaO2 decreased more than with xylazine-ketamine painful or noxious procedures, the higher concentra- anesthesia.14 In addition, recoveries from short- tion of ketamine (2 mg/ml) is recommended, whereas term xylazine-tiletamine-zolazepam anesthesia for restraint involving minimal stimulus, the lower tend to be less satisfactory than recoveries from ketamine concentration (1 mg/ml) may suffice. xylazine-ketamine anesthesia.11 In the United Although GKX can be used for induction of anesthe- States, tiletamine-zolazepam is usually more expen- sia, this induction involves a relatively prolonged sive than ketamine-diazepam. Potential advan- period of ataxia, and most practitioners prefer to tages of tiletamine-zolazepam include the slightly induce with xylazine-ketamine or xylazine-diaze- longer duration of anesthesia and the very small pam-ketamine and then use GKX for maintenance. volume required for injection, which may be advan- Typical infusion rates vary from 1.5 to 2.2 ml/kg/h, tageous for certain situations, such as darting feral depending on the procedure and the individual’s horses. response, as well as the concentration of ketamine in A combination of tiletamine-zolaepam-ketamine- the GKX. detomidine (TZKD) has been used for induction of Guaifenesin-ketamine-detomidine (GKD) is an- anesthesia in horses.15 The combination was pre- other combination used for TIVA. After sedation

Table 2. Simplified Continuous Rate Infusions

A simple way to deliver CRIs without a fluid pump is to add 1 h worth of the appropriate drug(s), based on the individual patient’s weight, to a 250-ml bag of IV fluid. At 1 drop/s (assuming an IV set that delivers 15 drops/ml), the contents of the bag will be delivered in ϳ1h. For example, to achieve a xylazine-ketamine (XK) infusion in a 450-kg horse: X: 450 kg ϫ 2.1 mg/kg/h ϭ 945 mg/h ϭ 9.5 ml/h K: 450 kg ϫ 7.2 mg/kg/h ϭ 3240 mg/h ϭ 32.4 ml/h Therefore, remove ϳ40–45 ml from a 250-ml bag of IV fluid; replace with 950 mg (9.5 ml) xylazine and 3240 mg (32.4 ml) ketamine; and deliver at 1 drop/s (assuming 15 drops/ml).

AAEP PROCEEDINGS ր Vol. 55 ր 2009 9 IN-DEPTH: ANESTHESIA AND PAIN MANAGEMENT with detomidine (0.02 mg/kg, IV) and induction with ␮g/kg, IV) 10 min after terminating the CRIs and ketamine (2 mg/kg, IV), anesthesia was maintained stood in an average of 42 min.20 In another study, in ponies for 90 min with GKD (guaifenesin 100 horses were sedated with medetomidine (5 ␮g/kg, mg/ml, ketamine 4 mg/ml, and detomidine 0.04 mg/ IV), induced with midazolam (0.04 mg/kg, IV) and ml; note these concentrations of guaifenesin and ketamine (2.5 mg/kg, IV), given a loading dose of ketamine are about twice the usual concentrations propofol (0.5 mg/kg, IV), and maintained on either used in GKX), at an infusion rate of 0.8 ml/kg/h for propofol (0.22 mg/kg/min) or a combination of ket- the first 60 min and 0.6 ml/kg/h for the final 30 amine (1 mg/kg/h), medetomidine (1.25 ␮g/kg/h), min.17 The ponies took longer to stand (average, 46 and propofol (0.14 mg/kg/min). Most horses be- min) compared with ponies anesthetized with halo- came apneic and hypoxemic and were therefore me- thane (33 min), but recovery quality was generally chanically ventilated. After just under2hof good to excellent. Oxygen was supplemented dur- anesthesia, recoveries were actually better quality ing anesthesia.17 and faster (average, 62 min) in the horses receiving An alternative to guaifenesin-based combinations the combination compared with propofol only (aver- is a CRI of xylazine and ketamine. After a stan- age, 87 min), leading the authors to suggest that dard induction, an infusion of xylazine at 35 ␮g/kg/ propofol was the drug most responsible for prolong- min (2.1 mg/kg/h) and ketamine at 120 ␮g/kg/min ing recoveries.21 (7.2 mg/kg/h) can be administered to maintain an- The time limit for safely maintaining anesthesia esthesia for up to 70 min.18 using TIVA is uncertain. Ponies maintained with Xylazine-ketamine CRIs are associated with very GKX for 120 min were able to stand without help smooth recoveries, but after 70 min of anesthesia, within 30 min after terminating the infusion, “and the time required for the horse to recover to stand- postanesthetic problems were not encountered.” ing is generally longer (50–55 min) compared with However, these ponies weighed only 150–275 kg, recoveries from inhalation anesthesia (15–40 min, they were endotracheally intubated, and they re- depending on the specific inhalant used).18 ceived supplemental oxygen.16 Some of the other Other alternatives for TIVA include various com- CRI techniques described here have been used to binations of ␣2 agonists with propofol. Detomidine maintain anesthesia for Ͼ2 h in full-size horses, but (0.015 mg/kg, IV) and propofol (2 mg/kg, IV) were again, the horses were intubated, breathing high used to induce anesthesia in horses for experimental inspired oxygen, often mechanically ventilated, and abdominal surgery, with anesthesia maintained by positioned on padded surgery tables. Without sup- propofol infusion at an average of 0.18 mg/kg/min. plemental oxygen, an anesthetized, recumbent However, the combination was not recommended for horse is likely to be hypoxemic (arterial PO2 in the clinical use, because inductions were associated range of 40–60 mm Hg),8,19 which increases the risk with some excitement, and respiratory depression of damage to muscles and other vital organs from and hypoxemia occurred.19 inadequate oxygen delivery (Table 3). Because Medetomidine, an ␣2 agonist developed for use in TIVA is often performed under field conditions, small animals, has also been combined with propofol without supplemental oxygen or a well-padded sur- for TIVA in horses. In a clinical study of horses, face, a common recommendation is to limit anesthe- medetomidine (7 ␮g/kg, IV) was given for sedation, sia time to ϳ60 min.22 and because of poor inductions with propofol, anes- thesia was induced using ketamine (2 mg/kg, IV) 4. Intravenous Adjuncts During Inhalant Anesthesia and diazepam (0.02 mg/kg, IV); anesthesia was Partly because inhaled anesthetics are associated maintained by CRIs of medetomidine (3.5 ␮g/kg/h, with several disadvantages—including cardiovascu- IV) and propofol (0.1 mg/kg/min, IV). Duration of lar depression, an increased stress response, and anesthesia averaged just under 2 h, but the longest variable recovery quality —and partly because of was nearly 4 h. About one half of the horses were increasing interest in improving pain management, apneic and needed mechanical ventilation. Horses a number of anesthetic or analgesic adjuncts are were given an additional dose of medetomidine (2 being used to augment inhalation anesthesia in

Table 3. When to Supplement Oxygen During Anesthesia

During general anesthesia, supplementation of oxygen is recommended, although rarely used in field situations. Anesthesia and

recumbency cause ventilation-perfusion mismatch in the lungs that lowers PaO2 considerably. Even if you do not normally supplement oxygen, consider doing it when: Anesthesia and recumbency will be prolonged Ͼ 30 min The horse has respiratory abnormalities such as COPD The horse has cardiac abnormalities such as a murmur The horse is very young (neonate) or very old

Although typically oxygen may be delivered by nasal insufflation (10–15 l/min), intubation and manual or mechanical ventilation may be necessary for some TIVA procedures, especially those involving propofol.

10 2009 ր Vol. 55 ր AAEP PROCEEDINGS IN-DEPTH: ANESTHESIA AND PAIN MANAGEMENT horses. The use of multiple agents in this manner Some investigators have used multiple IV drugs is termed “balanced anesthesia” or “partial intrave- combined with inhalant anesthesia in a true “PIVA” nous anesthesia” (PIVA). The usual goals of add- technique. One protocol supplemented halothane ing these adjuncts are to decrease the requirement anesthesia with a CRI of ketamine and guaifenesin for the inhalant (therefore minimizing cardiovascu- that was gradually decreased over time (mean infu- lar depressant effects), provide analgesia, and im- sion rates varied from 39 ␮g/kg/min and 1 mg/kg/ prove recoveries. min at 20 min, respectively, to 13 ␮g/kg/min and 0.3 Intravenous lidocaine has been shown to decrease mg/kg/min at 120 min). This protocol was associ- the maximum alveolar concentration (MAC) for in- ated with a 50% reduction in halothane require- halant anesthetics in a dose-dependent fashion. ment, fewer “top-up” doses of ketamine, and less After a loading dose of 2.5 or 5 mg/kg, IV, lidocaine need for dobutamine to support blood pressure. infusions of either 50 or 100 ␮g/kg/min decreased Recovery times and quality were “acceptable” and halothane MAC by 30–70%.23 Because of the pur- similar to horses recovering from halothane only.33 ported benefit of lidocaine infusions on gastrointes- Another example of PIVA is infusion of guaifen- tinal motility, lidocaine CRIs are frequently esin, ketamine, and medetomidine in combination administered to horses anesthetized for colic sur- with sevoflurane. A solution of guaifenesin (ϳ50 gery but are also used during other surgeries. For g/l), ketamine (ϳ2 g/l), and medetomidine (ϳ2.4 routine clinical use as an anesthetic adjunct, a load- mg/l) was administered at 0.5 ml/kg/h. Horses re- ing dose is probably not necessary, because a lido- ceiving this combination during sevoflurane anes- caine CRI reaches steady state in ϳ20 min.24 thesia maintained excellent quality of anesthesia, Because of concerns about potential residual ataxia did not require dobutamine for blood pressure sup- in recovery after IV lidocaine,25 it is recommended port, and required fewer attempts to stand in recov- that lidocaine CRI be terminated ϳ15 min before the ery compared with horses maintained with end of anesthesia. sevoflurane only.34 Ketamine CRIs are also used to supplement inhal- ant anesthesia. A plasma ketamine concentration of 5. Intravenous Anesthetics at the End of Inhalant ϳ11 ␮g/ml was associated with halothane MAC reduc- Anesthesia tion of ϳ37% and corresponding improvement in car- Because inhalant anesthesia is often associated with diac output.26 Approximately 30% MAC reduction less-than-ideal recovery quality, a variety of tech- might be achieved by administration of 50 ␮g/kg/min niques have been used in an attempt to improve (3 mg/kg/h).27 Because ketamine has antagonist recoveries. The most common involve administra- properties at spinal cord n-methyl-d-aspartate tion of an ␣2 agonist at the end of anesthesia or on (NMDA) receptors associated with the pain “wind-up” the horse’s arrival at the recovery stall. The use of phenomenon, even infusion rates Ͻ50 ␮g/kg/min may xylazine (0.1 mg/kg, IV), detomidine (2 ␮g/kg, IV), or be associated with improved pain management, al- romifidine (8 ␮g/kg, IV) after isoflurane anesthesia though evidence of this benefit specific to horses is not improved recovery and reduced ataxia compared available. Ketamine infusion rates of 10–50 ␮g/kg/ with isoflurane alone, with each of the drugs being min (0.6–3 mg/kg/h) are typically used clinically. similarly effective.35 Clinically, a slightly higher Medetomidine CRIs have gained some popularity dose of xylazine (0.22 mg/kg, IV or ϳ100 mg per as an adjunct to inhalant anesthesia in horses. 450-kg horse) is often used to improve recovery. Infusion rates of 3.5–5 ␮g/kg/h (ϳ1.6–2.3 mg/h for a Acepromazine is also used to modify recovery, al- 450-kg horse) reduce MAC by ϳ25%.27 An added though there is no documented proof of its benefit. benefit of medetomidine CRI as a supplement to Clinical experience suggests that a very small dose isoflurane anesthesia is reported to be improved re- of acepromazine (0.007 mg/kg, IV, or ϳ3mgper covery quality.28 450-kg horse), given about 20 min before the end of In dogs, opioids are a mainstay of anesthesia and anesthesia (to allow enough time for onset of effect), perioperative pain management, but use of opioids in may result in calmer, more controlled recoveries. horses is much more controversial, not only because of However, even after administration of aceproma- undesirable side effects such as central nervous sys- zine, many horses still benefit from additional seda- tem excitation and impaired gastrointestinal function, tion with xylazine in recovery. but also because of inconsistent results from studies of Part of the recent interest in TIVA is attributable to various opioids’ effect on MAC for inhalants. Butor- the good-to-excellent recovery quality typically associ- phanol (0.05 mg/kg or ϳ22.5 mg per 450-kg horse) had ated with TIVA; however, as previously mentioned, no reliable effect: it increased halothane MAC in there is still concern about maintaining anesthesia three of six ponies tested, decreased it in one pony, and Ͼ1–1.25 h using TIVA. A compromise might be to did not change it in two others.29 Alfentanil and mor- use inhalant anesthesia for maintenance during the phine likewise had no predictable beneficial effect on majority of a long procedure and then terminate deliv- the MAC for isoflurane in horses.30,31 A more recent ery of the inhalant and continue anesthesia using IV study reported that a high dose of fentanyl reduced drugs for the last 30 min or so, the goal being to allow isoflurane MAC but that the effect was less than that the horse to eliminate the inhalant completely before of fentanyl in humans or dogs.32 awakening from the IV anesthetics. A recent exper-

AAEP PROCEEDINGS ր Vol. 55 ր 2009 11 IN-DEPTH: ANESTHESIA AND PAIN MANAGEMENT imental study of seven horses used CRIs of xylazine 15. Muir WW, Gadawski JE, Grosenbaugh DA. Cardiorespira- (20 ␮g/kg/min) and ketamine (60 ␮g/kg/min) to con- tory effects of a tiletamine/zolazepam-ketamine-detomidine combination in horses. Am J Vet Res 1999;60:770–774. tinue anesthesia for 30 min after 90 min of isoflurane 16. Greene SA, Thurmon JC, Tranquilli WJ, Benson GJ. Car- anesthesia. Although no statistically significant im- diopulmonary effects of continuous intravenous infusion of provement in recovery quality was shown, the average guaifenesin, ketamine, and xylazine in ponies. Am J Vet Res scores for quality of standing and overall quality of 1986;47:2364–2367. recovery suggested the technique might have bene- 17. Taylor PM, Kirby JJ, Shrimpton DJ, Johnson CB. Cardiovas- 36 cular effects of surgical castration during anaesthesia main- fit. The authors have continued to use this tech- tained with halothane or infusion of detomidine, ketamine and nique for specific clinical cases, such as horses that guaifenesin in ponies. Equine Vet J 1998;30:304–309. have undergone orthopedic procedures in which a 18. Mama KR, Wagner AE, Steffey EP, et al. Evaluation of smooth recovery is especially important. xylazine and ketamine for total intravenous anesthesia in horses. Am J Vet Res 2005;66:1002–1007. 19. Matthews NS, Hartsfield SM, Hague B, et al. Detomidine- 6. Summary propofol anesthesia for abdominal surgery in horses. Vet Surg 1999;28:196–201. To date, there are numerous drug combinations that 20. Bettschart-Wolfensberger R, Kalchofner K, Neges K, et al. can be used to produce general anesthesia in horses. Total intravenous anaesthesia in horses using medetomidine Currently, there is no single perfect method that is and propofol. Vet Anaesth Analg 2005;32:348–354. uniformly reliable and safe, particularly for long- 21. Umar MA, Yamashita K, Kushiro T, et al. Evaluation of total intravenous anesthesia with propofol or ketamine-me- term anesthesia. However, the development of detomidine-propofol combination in horses. J Am Vet Med newer injectable drugs and continuing studies into Assoc 2006;228:1221–1227. their use for TIVA or PIVA promise further improve- 22. Equine Monitoring Guidelines: Anesthesia in Horses. ments in the near future. Available at http://www.acva.org/diponly/action/Guidelines_ Anesthesia_Horses_041227.htm. 23. Doherty TJ, Frazier DL. Effect of intravenous lidocaine on References and Footnote halothane miimum alveolar concentration in ponies. Equine Vet J 1998;30:300–303. Accessed on June 25, 2009. 1. Hubbell JAE. A short history of equine anesthesia in Pro- 24. Feary DJ, Mama KR, Wagner AE, et al. 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