Postinduction butorphanol administration alters oxygen consumption to improve blood gases in etorphine-immobilized white rhinoceros

Abstract

ObjectiveTo investigate the effects of postinduction butorphanol administration in etorphine-immobilized white rhinoceros on respiration and blood gases. Study designRandomized crossover study. AnimalsA group of six sub-adult male white rhinoceros. MethodsEtorphine, or etorphine followed by butorphanol 12 minutes after recumbency, was administered intramuscularly [2.5 mg etorphine, 25 mg butorphanol (1000–1250 kg), or 3.0 mg etorphine, 30 mg butorphanol (1250–1500 kg)]. Sampling started at 10 minutes after initial recumbency, and was repeated at 5 minute intervals for 25 minutes. Arterial blood gases, limb muscle tremors, expired minute ventilation and respiratory frequency were measured at each sampling point. Calculated values included alveolar–arterial oxygen gradient [P(A-a)O2], expected respiratory minute volume (V˙e), tidal volume (Vt), oxygen consumption (V˙O2) and carbon dioxide production (V˙CO2). ResultsEtorphine administration resulted in an initial median (range) hypoxaemia [arterial partial pressure of oxygen 25.0 (23.0–28.0) mmHg], hypercapnia [arterial partial pressure of carbon dioxide 76.2 (67.2–81.2) mmHg], increased P(A-a)O2 [41.7 (36.6–45.1) mmHg, V˙O2 [11.1 (10.0–12.0) L minute−1] and muscle tremors. Butorphanol administration was followed by rapid, although moderate, improvements in arterial partial pressure of oxygen [48.5 (42.0–51.0) mmHg] and arterial partial pressure of carbon dioxide [62.8 (57.9–75.2) mmHg]. In rhinoceros administered butorphanol, V˙O2 [4.4 (3.6–5.1) L minute−1] and V˙CO2 [4.2 (3.8–4.4) L minute−1] were lower than in those not administered butorphanol. Increased arterial oxygen tension was associated with lower oxygen consumption (p=0.002) which was positively associated with lower muscle tremor scores (p<0.0001). Conclusions and clinical relevanceHypoxaemia and hypercapnia in etorphine-immobilized rhinoceros resulted from an increased [P(A-a)O2] and increased V˙O2 and V˙CO2 associated with muscle tremors. Rather than being associated with changes in V˙e, it appears that improved blood gases following butorphanol administration were a consequence of decreased V˙O2 associated with reduced muscle tremoring.