Comparative assessment of physiological homeostasis in zoo mammals under general anaesthesia : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Science in Wildlife Health at Massey University, Palmerston North, Manawatū, New Zealand
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2020
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Massey University
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During the anaesthesia of dangerous zoo animals, additional anaesthetic risks to patients arise from precautions required to assure human safety. Evaluating the safety of anaesthetic approaches for these animals is not straightforward, with challenges associated with identifying disturbances to physiological homeostasis, attributing these disturbances to particular interventions and relating them to morbidity endpoints. In zoo animal anaesthesia, more reliance is necessarily placed on observational studies to quantify risks related to immobilisation regimens and to refine and improve the ways anaesthesia is subsequently delivered. This study applied an opportunistic approach, using readily accessible monitoring methods to investigate general changes to physiological homeostasis occurring under zoo anaesthesia in twenty-six individuals. This included comparative analysis of nine large mammal species, of diverse phylogeny and dietary ecology. In eleven large felids representing three different Felidae species included in the study, assessment was then undertaken of changes in metabolic acid-base status, the contribution of anions that are not normally measured to these changes and their possible association with mean arterial blood pressure preceding blood sampling episodes. Strong evidence was found for time effects during anaesthesia on a variety of the measures of homeostasis that were monitored. Within the broad species ranges that were apparent in the homeostatic parameters measured, many unexpected findings were manifest in the way that different species and/or different conspecific individuals responded to anaesthesia. Within a species, differences were sometimes apparent in spite of relatively minor variations in the immobilisation protocols used, as was evident within the Panthera genus. Clinically significant alterations noted included blood pressure changes, changes to ventilation, possible ventilation-perfusion mismatches, alterations in acid-base status and occasional but concerning instances of acidaemia, hypoglycaemia, hypoxaemia and hyperkalaemia. All animals survived anaesthesia and did not show any apparent morbidity, limiting our ability to determine the pathological effects, if any, of the changes seen. Based on extrapolations from other mammals, some disturbances may have caused pathology or mortality if they had become more sustained or progressive. Extracellular (blood) base deficits exceeding -7mmol/L were common in the anaesthetised large felids, suggesting that these animals commonly displayed a state of metabolic acidosis under anaesthesia based on proposed definitions from domestic felids. However, there were marked species differences in acid-base status under anaesthesia, and further analysis showed that the drivers of the changes also varied between species. Although strong cautionary caveats are associated with the low power of the analysis, the findings suggest that inter-species extrapolations of acid-base physiology will be flawed, indicating a strong need for readily accessible species-specific reference ranges. There was no evidence for an association between strong ion gap and mean arterial pressure, but blood pressure was actively managed in most of the anaesthetics which may have confounded these results. Further study of tissue perfusion states than those provided by mean arterial blood pressure will be required to evaluate if a relationship exists between strong ion gap levels and corresponding states of blood flow in zoo animal anaesthesia. Nevertheless, this study has demonstrated that strategic point of care clinical pathology tests and blood gas analysis provide practical opportunities to minimise and even prevent many physiological changes, with the potential to diminish risk to patients without placing human safety at additional risk.