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


ically healthy horses remains the most challenging of the common domestic species, with reported mor- tality rates of approximately 0.1 to 1%.7–9 Horses are physiologically stressed by anesthesia


hend all that the manner conveys.”14 Veterinary care improved as colleges of veterinary medicine were established and their influence spread.


to a greater degree than other species.10 Typically, a horse is anesthetized in an unaccustomed place where it is administered drugs that cause it to lose consciousness and fall to the ground or floor. Once recumbent, its mouth may be pried open and a large bore tube is inserted into the airway for a period of minutes to hours. In hospital situations, the horse is often lifted with hobbles by a hoist and positioned on a table in an unnatural position. Then, the horse is connected to a machine that delivers anes- thetic and controls its breathing. Once an intended procedure is completed, the horse is again lifted by its limbs and placed in another room (recovery stall) or space where it is expected to attain a standing position in a relatively short period of time (less than 90 minutes) while recovering from the effects of the drugs that caused it to fall down in the first place!


2. The Developing Science of Equine Anesthesia


The current practices of equine anesthesia have evolved by necessity and because of the work of a large number of dedicated individuals and groups who have provided veterinary colleagues with infor- mation. The author has chosen to highlight publi- cations that have had significant impact on the development of the practice of equine anesthesia to its current state. The publications are grouped in six subject areas and within groups are presented chronologically in an attempt to provide context. The publications were selected based on the author’s experience and his perusal of the veterinary litera- ture including works on the history of equine anes- thesia.2,11–13 Scientific articles published after the highlighted publications are discussed as a method to transition to today. Veterinary medicine gained initial prominence in


society in large part because of the importance of the horse in transportation and for work in agriculture. Prior to the development of the profession, much of medical care for horses was left to untrained indi- viduals. Books such as The Illustrated Horse Doc- tor,14 published in 1880 by Mayhew were written to “instruct the novice in such a manner as would afford a reasonable prospect of success.” Such texts covered all of the known maladies of the day includ- ing simple ophthalmia (iridocyclitis?), staggers (mycotoxin-induced incoordination?), gutta serena (blindness of unknown origin?), nasal gleet (thick nasal discharge?), and scald mouth (slobbering or frothing?). Surgeries were performed with physi- cal restraint of the horse usually in the absence of adjuvant drugs. Directions for casting the horse included: “Let it be hobbled and never, during the operation, hear any sound but soothing accents. Animals do not understand words, creatures may not be able to literally interpret; but they compre-


154 2020  Vol. 66  AAEP PROCEEDINGS 3. Early Anesthesia Texts


Sir Frederick Hobday (of the Hobday procedure on the equine throat), published an important text, An- aesthesia and Narcosis of Animals and Birds in 1915.15 Hobday was a voice for humane care of animals and promoted augmenting physical re- straint with chemical restraint, believing that sur- gical procedures should be performed as painlessly as possible. Available “narcotics” included chloral hydrate given orally or rectally. Intravenous ad- ministration was discouraged because of “violent ir- ritation if the fluid finds its way under the skin” and the potential for air embolus. The use of morphine was discouraged because it produced excitement. Practical inhalant anesthesia was limited to chloro- form by inhaler with surgical durations limited to 15 minutes. Unmedicated horses were hobbled and cast to the ground prior to application of the inhaler with the proviso that the hobbles should not be removed until the animal could rise properly. Professor John G. Wright of the University of Liv-


erpool published an early book, Veterinary Anaesthe- sia, in 1941, a text that has been revised and expanded through 11 editions, the last of which was presented in 2014.16 The first edition of this book facilitated the transition from physical restraint to chemical restraint. The authors noted that ade- quate restraint was necessary even for quite simple procedures and stated that casting prior to the ap- plication of anesthetics may be necessary to ensure safety. The use of succinylcholine for painful sur- gery was discouraged but the authors noted that its use in casting may be no more distressing than hobbles and might be less risky. Chlorpromazine, promazine, meperidine, and chloral hydrate were available for use as premedicants and basal narcot- ics. Early editions of the book addressed endo- tracheal intubation and inhalant anesthesia with chloroform, cyclopropane, and halothane using both open and closed (circle) systems. Intravenous an- esthetic techniques included the use of chloral hy- drate, chloral hydrate with magnesium sulfate, chloral hydrate with magnesium sulfate and pento- barbital (Equithesin or “Army Anesthesia”), and thiopental. Recoveries from anesthesia were ex- pected to be from 90 minutes to 2 hours in duration.


4. Sedatives and Analgesics


Xylazine was introduced as a new sedative for horses and cattle by Clarke and Hall in 1969.4 Heart rate and cardiac output decreased and arte- rial blood pressure increased transiently after drug administration. The authors noted that xylazine was a “safe, reliable, short acting sedative” that was “better than any other compound in current use.” Interestingly it was stated that “heavier doses seemed to produce no more profound sedation, but


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