Journal of Zoo and Wildlife Medicine 45(2): 263–271, 2014 Copyright 2014 by American Association of Zoo Veterinarians PULMONARY DEAD SPACE IN FREE-RANGING IMMOBILIZED BLACK RHINOCEROSES (DICEROS BICORNIS) IN NAMIBIA Robin W. Radcliffe, D.V.M., Dipl. A.C.Z.M., Peter Morkel, vdB. B.V.Sc., Mark Jago, B.V.Sc., Arthur A. Taft, Ph.D., R.R.T., Pierre du Preez, M.Sc., Michele A. Miller, D.V.M., Ph.D., Dedi Candra, D.V.M., Daryl V. Nydam, D.V.M., Ph.D., Jason S. Barry, M.A. and Robin D. Gleed, B.V.Sc., M.R.C.V.S., Dipl. A.C.V.A.A. Abstract: It was observed previously that end-expired carbon dioxide (PE ´ CO2) decreased when immobilized black rhinoceroses (Diceros bicornis) were moved from sternal to lateral recumbency. These experiments were designed to test whether greater alveolar ventilation or greater pulmonary dead space in lateral recumbency explains this postural difference in PE ´ CO2. Twenty-one (9 male, 12 female; 15 [3.5–26] yr old) wild black rhinoceroses were immobilized with etorphine and azaperone and positioned in either sternal or lateral recumbency. All rhinoceroses were hypoxemic and had lactic and respiratory acidemia. The animals in lateral recumbency were more acidemic, had higher lactate, and lower arterial oxygen that those in sternal recumbency; however, arterial carbon dioxide was similar between groups. Both PE ´ CO2 and mixed expired carbon dioxide pressure were lower in lateral than sternal recumbency. Although there was no difference in tidal volume or arterial carbon dioxide, both the breathing rate and minute ventilation were greater in lateral recumbency. The physiologic dead space ratio and dead space volume were approximately two times larger in lateral recumbency; hence, the decrease in PE ´ CO2 in lateral recumbency can be attributed to increased dead space ventilation not increased alveolar ventilation. Positioning immobilized rhinoceroses in lateral recumbency does not confer any advantage over sternal in terms of ventilation, and the increase in minute ventilation in lateral recumbency can be considered an energetic waste. Although arterial oxygen was superior in sternal recumbency, further studies that measure oxygen delivery (e.g., to the muscles of locomotion) are warranted before advice regarding the optimal position for immobilized rhinoceroses can be given with confidence. Key words: Anesthesia, black rhinoceros, capnography, dead space, Diceros bicornis, oxygenation, posture. INTRODUCTION The black rhinoceros (Diceros bicornis) is criti- cally endangered, with just over 5,000 animals surviving in the wild in southern Africa. Optimal management of rhinoceros populations requires capture of individuals for procedures, including translocation to minimize regional genetic homo- geneity and for repatriation. However, in such a small population, occasional morbidity and mor- tality associated with capture is unacceptable. 15 The harsh nature of black rhinoceros’ habitat usually requires that free-ranging animals be immobilized by darting from a helicopter; this procedure inevitably induces an extreme flight response. Established immobilization protocols for black rhinoceroses use potent opioids, such as etorphine, injected by dart; unfortunately, these drugs produce important side effects, including hypoventilation, hypoxemia, hypercapnea, hyper- tension, and acidemia that may contribute to mortality during capture. 11,10,3,4,6,19 These pharma- cologic perturbations exacerbate the effects of the exertion that usually precedes capture. In large quadrupeds, both the pulmonary and cardiovascular systems are affected by posture under anesthesia. 24,25 In anesthetized horses (Eq- uus caballus) that have not undergone exertion, alveolar ventilation and arterial oxygenation are greater in sternal recumbency than in lateral recumbency. 9 On the other hand, in anesthetized adult elephants, the pulmonary system is gener- ally considered to be more disadvantaged in sternal recumbency than it is in lateral recumben- cy. 12,15 The optimal posture for rhinoceroses under anesthesia has not been determined. From the Cornell University College of Veterinary Medicine, Tower Road, Ithaca, New York 14853, USA (Radcliffe, Nydam, and Gleed); Private Consultant, P.O. Box 260, Kakamas 8870, South Africa (Morkel); Namibia Ministry of Environment and Tourism, Private Bag 13306, Windhoek, Namibia (Jago and duPreez); Georgia Regents University, Department of Respiratory Therapy, EC-4318, Augusta, Georgia 30912, USA (Taft); Palm Beach Zoo, 1301 Summit Boulevard, West Palm Beach, Florida 33405, USA (Miller); Yayasan Badak Indonesia (Rhino Founda- tion of Indonesia), Gedung Badan Planologi, Kehutanan, Jalan Juanda 100, Bogor 16122, Indonesia (Candra); and the Cornell Statistical Consulting Unit, Cornell University, Savage Hall, Ithaca, New York 14853, USA (Barry). Present address (Miller): Rare Species Conservatory Foundation, 122 E Road, Loxahatchee, Florida 33470, USA. Correspondence should be directed to Dr. Radcliffe (rwr32@cornell.edu). 263