Ameliorating the adverse cardiorespiratory effects of chemical immobilization by inducing general anaesthesia in sheep and goats: implications for physiological studies of large wild mammals
Chemical immobilization is necessary for the physiological study of large wild animals. However, the immobilizing drugs can adversely affect the cardiovascular and respiratory systems, yielding data that do not accurately represent the normal, resting state. We hypothesize that these adverse effects can be ameliorated by reversing the immobilizing agent while holding the animal under general anaesthesia. We used habituated sheep Ovis aries (N = 5, 46.9 ± 5.3 kg body mass, mean ± SEM) and goats Capra hircus (N = 4, 27.7 ± 2.8 kg) as ungulate models for large wild animals, and measured their cardiorespiratory function under three conditions: (1) mild sedation (midazolam), as a proxy for the normal resting state, (2) immobilization (etorphine and azaperone), and (3) general anaesthesia (propofol) followed by etorphine antagonism (naltrexone). Cardiac output for both sheep and goats remained unchanged across the three conditions (overall means of 6.2 ± 0.9 and 3.3 ± 0.3 L min−1, respectively). For both sheep and goats, systemic and pulmonary mean arterial pressures were significantly altered from initial midazolam levels when administered etorphine + azaperone, but those arterial pressures were restored upon transition to propofol anaesthesia and antagonism of the etorphine. Under etorphine + azaperone, minute ventilation decreased in the sheep, though this decrease was corrected under propofol, while the minute ventilation in the goats remained unchanged throughout. Under etorphine + azaperone, both sheep and goats displayed arterial blood hypoxia and hypercapnia (relative to midazolam levels), which failed to completely recover under propofol, indicating that more time might be needed for the blood gases to be adequately restored. Nonetheless, many of the confounding cardiorespiratory effects of etorphine were ameliorated when it was antagonized with naltrexone while the animal was held under propofol, indicating that this procedure can largely restore the cardiovascular and respiratory systems closer to a normal, resting state.
KeywordsCardiac output Etorphine Mammal Opioid Propofol Ventilation
The authors acknowledge the expertise and contribution made by the academics, technicians, and volunteers at the School of Physiology, and the Central Animal Service, at the University of the Witwatersrand. We thank especially David Gray, Zipho Zwane, Robyn Hetem, Benjamin Rey, Nico Douths, Peter Kamerman, Richard McFarland, Hilary Lease, Peter Buss, Michelle Miller, Tapiwa Chinaka, and W. Maartin Strauss. We also thank Tobias Wang of Aarhus University, and one anonymous reviewer, for providing valuable feedback on the manuscript. This research was supported by an Australian Research Council Discovery Project Award to R. S. Seymour, S. K. Maloney, and A. P. Farrell (DP-120102081). E. P. Snelling holds a South African Claude Leon Foundation Postdoctoral Fellowship. A. P. Farrell holds a Canada Research Chair and is supported by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada.
EPS, RSS, LCRM, AF, AH, DM, APF, M-AC, and SKM: conception and design of research; EPS, RSS, LCRM, AF, AH, DM, APF, M-AC, and SKM: performed experiments; AI: analysed data; AI, EPS, RSS, LCRM, and SKM: interpreted results of experiments; AI: prepared figures; AI, EPS, RSS, and SKM: drafted the manuscript; AI, EPS, RSS, AF, LCRM, APF, and SKM: edited and revised the manuscript; AI, EPS, RSS, LCRM, AF, AH, DM, APF, M-AC, and SKM: approved the final version of the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. All procedures performed in studies involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted.
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