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


ues to be widely used, particularly in out-of-hospital settings. The addition of a muscle relaxant like guaifenesin to xylazine-ketamine anesthesia ex- tended the anesthetic period and provided improved anesthetic conditions. While guaifenesin is useful in this setting, its use is cumbersome because dilute solutions (5% to 10%) are required to avoid hemoly- sis and vasculitis after IV administration, resulting in the need to administer a volume of 0.5 to1Lof solution to have a significant effect. An alternative method of providing muscle relaxation was ad- dressed in a 1990 report from Brock that compared diazepam to guaifenesin as the anesthetic adjunct.34 This paper described the use of diazepam as a re- placement for guaifenesin as part of an induction technique prior to maintenance with halothane. It followed an earlier paper suggesting that the ad- dition of diazepam improved xylazine/ketamine an- esthesia in the field.35 The authors found that a 0.1 mg/kg dose of diazepam was useful and “produces a quality of anesthetic induction, transition, and re- covery comparable to guaifenesin.”34 The required volume of diazepam was smaller (approximately 10 mL or less) but was a controlled substance, requiring strict documentation of use. Subsequent papers have investigated the use of midazolam, another benzodiaz- epine, and have found it to be similarly convenient and useful.36,37 The proprietary combination of tiletamine and


zolazepam administered after alpha2 adrenergic agonists produces qualitatively similar anesthesia to xylazine-diazepam-ketamine and is of longer du- ration but the quality of recovery is reduced.38 Propofol, a widely used drug in other species, is not widely used in horses. Initial reports suggested that propofol, after xylazine, was a satisfactory tech- nique but subsequent investigations raised concerns with excitement on induction, significant respira- tory depression, and hypoxemia.39–41 More re- cently, the combination of propofol and ketamine after xylazine sedation has been investigated and may have some applicability, particularly in- hospital settings where ventilation can be easily assisted.42


6. Inhalant Anesthetics and the Pathophysiology of Anesthesia and Recumbency


Important information concerning the cardiac and respiratory effects of recumbency in anesthetized horses were addressed by Gillespie and co-workers in 1969.43 This paper was accompanied by a paper by the same authors in the British literature and documented that anesthetized horses have large dif- ferences between their alveolar and arterial oxygen tensions.44 Cardiac output was consistently re- duced and calculated pulmonary shunt averaged 14% of pulmonary blood flow. Potential causes of the dysfunction were postulated to be maldistribu- tion of perfusion due to gravity; hypoventilation of the down lung; development of atelectasis; and de- creases in cardiac output. The dysfunction oc-


156 2020  Vol. 66  AAEP PROCEEDINGS


curred relatively rapidly with the assumption of recumbency and was not corrected by the delivery of large tidal volumes. These papers revealed that recumbency associated with anesthesia produced significant depression of both cardiovascular and respiratory function that needed to be counteracted. These papers introduced a subject and line of inves- tigation that have dominated the equine anesthesia literature since their publication. The recognition that anesthesia, particularly in-


halant anesthesia, causes ventilatory compromise led to the development of equipment to provide as- sisted or controlled ventilation. No commercial company produced equipment capable of delivering an adequate tidal volume in an appropriate time frame so investigators developed equipment from available parts used in humanmedicine. An early paper (1975) by Thurmon utilized a Bird Mark IX respirator powered by an air compressor to compress a multivolume bellows in order to ventilate anesthe- tized horses.45 A rebreathing bag was hermetically sealed in a 12-gallon steel drum (bag in a barrel). The opening of the rebreathing bag was connected to a large animal circle anesthesia machine. This pa- per provided a blueprint for veterinarians to con- struct ventilators of sufficient capacity to ventilate an adult horse. The advent of mechanized assisted ventilation allowed for increased complexity and du- ration of surgery because manual compression of a large animal rebreathing bag to assist ventilation for more than a few minutes is exhausting and thus impractical. The equipment required to anesthetize large ani-


mals had to be developed de novo because the tidal volumes and respiratory velocities inherent in the respiration of the horse could not be accommodated by the available equipment. The dynamics of establishing sufficient percent-


ages of the anesthetic gases to provide anesthesia were considerably different than those seen in hu- man patients or small animals. Steffey addressed these differences with his paper of 1977 titled, “Rate of Change of Halothane Concentration in a Large Animal Circle Anesthetic System.”46 This paper demonstrated how to rapidly develop sufficient con- centrations of inhalant anesthetics to maintain anesthesia. Specifically, large-animal anesthetic sys- tems at maintenance flow rates (3 L/min) require 30 minutes to reach concentrations sufficient to main- tain anesthesia. Most intravenous induction tech- niques provide only 20 minutes of anesthesia thus strategies to more rapidly increase the concentra- tion of inhalant are required. A suggested strategy was “to initiate anesthesia with high delivered fresh gas flow rates (i.e., 8 to 12 l/min) then after 10 to 15 minutes reduce them to low maintenance flows (about 3 to 6 l/min).” This paper gave veterinarians a way to safely and rapidly develop concentrations of inhalant anesthetics that produce anesthetic states. Largely because of this paper, flowmeters on large-


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