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.