FRANK J. MILNE STATE-OF-THE-ART LECTURE


the effects became apparent after a shorter time interval and persisted longer.” Detomidine and romifidine followed in the 1980’s and 1990’s, respec- tively.17,18 Equipotent IV doses for the drugs are reported to be xylazine 1 mg/kg; detomidine 10 mi- crog/kg; and romifidine 80 microg/kg.18 Romifidine produces less lowering of the head and less ataxia than xylazine or detomidine at clinically recom- mended doses, which may make the degree of seda- tion produced more difficult to judge.18 Butorphanol was approved for use in the horse in


the 1980s and remains the only opioid agonist with such a designation.19 Approved as an analgesic drug for the treatment of abdominal pain, butorpha- nol’s primary current use is in combination with


alpha2 agonists as an adjunct for standing chemical restraint. Robertson documented the cardiopulmo- nary, sedative, and analgesic effects of butorphanol given 5 minutes after xylazine by making a skin incision 5 minutes later.19 The combination pro- duced 15 to 30 minutes of analgesia with minimal and transient hemodynamic effects and insignifi- cant respiratory depression. The authors stated, “The analgesic effects of each drug are additive and provided good chemical restraint.” Butorphanol was not a controlled substance when first mar- keted for use in the horse. That fact, and papers like this documenting its efficacy led to its wide- spread adoption. The analgesic and other effects of butorphanol, flunixin, levorphanol, morphine, and xylazine were examined in ponies by Kalpravidh in 1984.20 This paper used the pony’s response to repeatable stimuli to examine and quantify the analgesic effects of drugs. A heat lamp model was used to produce acute superficial pain and a cecal balloon model was used to assess acute visceral pain. Analgesia was assessed by measuring how long it took for the pony to react to the stimulus. Xylazine produced the highest pain threshold for 60 minutes with signifi- cant effects lasting 180 minutes and 240 minutes for superficial pain and visceral pain, respectively. Morphine was the next best drug for superficial pain but produced only slight effects for visceral pain. Butorphanol was the best drug after xylazine for visceral pain. Interestingly, flunixin was not shown to be effective for acute pain in this model, but it is widely used as an initial treatment in many in- stances. Modifications of this methodology con- tinue to be used widely to assess analgesic effects of drugs. A number of drugs and drug combinations have been reported including alpha2 adrenergic ago- nists combined with acepromazine,21 morphine,22,23 ketamine,24 buprenorphine,25 lidocaine,26 meperi- dine,27 and methadone.27


5. Intravenous Anesthetics and Adjuncts


The skeletal muscle relaxant guaifenesin (then known as glyceryl guaiacolate ether) was introduced into equine anesthesia in 1968 by Gertsen and Tillot- son.28 Guaifenesin was administered rapidly in 5%


solutions alone or in combination with thiamylal or pentobarbital after sedation with acepromazine and meperidine. Apnea did not occur at induction and at least three times the calculated dose was required to produce death. The authors reported that guaifenesin produced “excellent results when used as the sole anesthetic agent in short procedures” and the technique had a “wide margin of safety and consistently excellent results.” Although guaifene- sin remains a component of many intravenous an- esthetic combinations, its use as a sole agent is discouraged because of its minimal analgesic effects. Likely among the most significant reports in the


history of equine anesthesia was published in 1977 by Muir describing the use of xylazine and ketamine for short-term anesthesia.29 This study investi- gated the anesthetic and cardiopulmonary effects of xylazine and ketamine with xylazine given intrave- nously 5 minutes prior to intravenous ketamine. Recumbency occurred within 30 to 60 seconds of ketamine administration and anesthesia lasted ap- proximately 16 minutes with a mean time to stand- ing of 26 minutes. The quality of recovery was excellent with most horses standing on their first attempt. Hemodynamic variables including car- diac output and ventilation were well maintained while arterial oxygenation was decreased. Analge- sia and muscle relaxation were described as good. The authors stated, “Xylazine and ketamine appears to be a safe and practicable anesthetic regimen in the horse.” This paper provided a safe, humane alterna- tive to just physical restraint or the administration of succinylcholine for short-term “recumbent restraint” for procedures like castration. There have been a number of modifications of this technique since its


introduction but xylazine or another alpha2 agonist and ketamine still form the basis of the majority of equine anesthetic protocols whether the procedures are conducted in the hospital setting or in the field. Substitution of detomidine for xylazine produces similar quality induction of anesthesia but is asso- ciated with poorer recoveries with ataxia.30,31 Re- placing xylazine with romifidine is not recommend- ed unless muscle relaxants, such as guaifenesin or midazolam, are co-administered.32 The addition of butorphanol to xylazine does not increase the dura- tion of anesthesia but the quality of anesthesia is improved.31 Equine veterinarians needed a method to extend xylazine and ketamine anesthesia when the surgical task could not be completed in a 15- to 20-minute period. The question of how to safely extend the anesthetic period was answered with the publication of a paper by Greene in 1986 describing the use of guaifenesin in combination with ketamine and xyla- zine (Triple Drip or G-K-X) for total intravenous anesthesia (TIVA) of 2 hours’ duration.33 Anes- thetic conditions were good and the ponies stood without help in 15 to 30 minutes after the infusion was stopped. This paper provided important infor- mation about an anesthetic technique that contin-


AAEP PROCEEDINGS  Vol. 66  2020 155


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