FRANK J. MILNE STATE-OF-THE-ART LECTURE 8. The Recovery Period


A recent publication suggests that the portion of morbidity and mortality once associated with the maintenance of anesthesia has been shifted to the recovery phase, presumably due to better anesthetic agents and improved monitoring.9 Recovery re- mains the most problematic part of the anesthetic period, primarily driven by the horse’s apparent de- sire to stand soon after regaining consciousness. A number of strategies have been devised to facili- tate the safe return to a standing posture, but none is without complication. The most comprehensive study of recovery was published by Whitehair et al in 1993.52 The paper reported an extensive labora- tory study of horses recovering from inhalant anesthesia without other drugs. Horses were anes- thetized on 3 occasions: twice with halothane (1- and 3-hour duration) and once with isoflurane (3- hour duration). Horses were placed in a recovery stall and allowed to recover undisturbed other than for sample collection. Rate of elimination of the inhalants was measured and the time to recovery events (eyelid, ear, head, and limb movement, and others) were recorded. The importance of this pa- per is that it provided a context for evaluating the events that occur during the transitions between the anesthetized and awake states. Useful signs for gauging the progress of the recovery included in- creasing eyelid and eyeball movement, ear move- ment, swallowing, lifting of the head, and limb movement. This information allows personnel as- sisting recovery from anesthesia to better determine when a horse is ready to stand. Further, the study provided basic information for the development of subsequent research on methods to modify the re- covery period.53


9. Morbidity and Mortality


Equine anesthesia is more perilous than anesthesia of the other domestic species as previously noted. A landmark series of papers on morbidity and mor- tality was published by Johnston, beginning in 1995 with a preliminary report.54 This paper estab- lished mortality rates for anesthesia in the horse. The authors collected and analyzed data on 41,824 anesthetic episodes from 129 surgical centers. The overall death rate within 7 days of anesthesia was 1.9%. Death rate of non-colic patients was 0.9%. Cardiovascular arrest or collapse during anesthesia and fractures or myopathies in recovery accounted for 33% and 32% of deaths, respectively, with the remainder due to various causes. Horses anesthe- tized for fracture repair, horses anesthetized outside of normal working hours, and foals less than 1 month of age were at increased risk. Risk in- creased with age with horses older than 14 years. Lack of any preanesthetic sedation increased risk and acepromazine administered as a single drug (not in combination) reduced risk. Inhalant anes- thesia without premedication was at higher risk and TIVA of shorter duration was at less risk. By com-


158 2020  Vol. 66  AAEP PROCEEDINGS


parison, the mortality rate in humans was cited as 1 in 10,000. Others have examined the issue and reported rates between 0.08 and 1.8% depending on the design of the study.7,9 As previously noted, a recent paper from 2015, reported on mortality rates, suggesting that the incidence has not changed in 20 years.7,9


10. The Stress of Anesthesia


Stress is a significant component of equine anesthe- sia. The most comprehensive examination of this phenomena was conducted by Taylor10 and pub- lished in a series of articles highlighted by the 1989 publication, “The Equine Stress Response to Anaes- thesia.” In a series of accompanying papers, 6 po- nies were anesthetized at least 12 times using a variety of anesthetic drugs, techniques, and inter- ventions (ventilation, cardiovascular support). In- dices of stress (serum cortisol, catecholamines, insulin, glucose) were measured prior to and for 24 hours after each anesthetic episode. Ponies anes- thetized with sodium thiopental alone and with TIVA showed little to no evidence of stress. All other anesthetic techniques were stressful. Nor- malization of ventilation and arterial blood pres- sures did not reduce stress. This series of papers established that anesthesia (without surgery) is a stress for equines. Subsequent papers have largely confirmed these findings with the suggestion that TIVA is less stressful than inhalant anesthesia or suppresses the stress response.55 The validity of this suggestion is unproven, largely because most procedures completed using TIVA are of shorter du- ration, making parallel comparisons difficult.


11. The Things We Know We Know: The Current Practice of Equine Anesthesia


The advances that have been made in the period of “modern” anesthesia have dramatically improved the ability of veterinarians to provide care to equine patients. Many of these advances are the result of the development of molecular biology and molec- ular pharmacology which fostered the discovery of the mechanism of action of many anesthetic drugs and paved the way for the formulation of new drugs. The use of drugs in the horse has generally followed their use in humans and other animals but occasionally developed simultaneously (ketamine)


or predated use in humans (alpha2 agonists).11 For example, when xylazine was first marketed for sedation of the horse, the fact that the majority of the drug’s effects were produced via stimulation of


alpha2 receptors was not known because the exis- tence of alpha2 receptors was unknown until Langer discovered them in 1974. This knowledge facili-


tated the development of detomidine, romifidine, and dexmedetomidine, and the antagonist, atipam- ezole. In addition, the development and commer- cialization of anesthetic equipment specifically designed for large animals, cost-effective catheters for IV and intra-arterial use, and clinically useful


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