VN March 2023

Vetnuus | Maart 2023 10 TABLE 4. Mean (SD) values from12 immobilized black rhinoceroses (Diceros bicornis) in Namibia during investigations comparing the physiology of lateral position with aerial suspension by the feet. The animals were in lateral recumbency or suspended by their feet before data were collected. Each rhinoceros received both treatments sequentially, in random order. These data are subdivided by the order (first or second) in which each treatment (lateral recumbency or suspension by the feet) was applied. P a CO 2 (arterial carbon dioxide pressure) and P A O 2 (alveolar oxygen pressure) are the only parameters where the order of treatment had a significant effect. The alpha value for comparisons was 0.05. a Time to collect data = time in posture before data were collected. * = significant difference depending onwhether the animals were in lateral recumbency for the first treatment or for the second treatment. Mean calculated allometric estimates of carbon dioxide production were 1.7 mL/kg/ min (0.05) for Vallo0.8CO 2 and 2.2 mL/kg/min (0.07) for Vallo1.0CO 2 . Mean VCO 2 measured in lateral recumbency was 2.3 mL/kg/min (0.44), which was 0.5 mL/kg/min greater (P=0.0001) than Vallo0.8CO 2 , but mean VCO 2 was not significantly different from Vallo1.0CO 2 (P=0.238). The allometrically predicted pulmonary variables, ValloT, ValloE, and falloR, were 10 mL/kg, 94 mL/kg/min, and 8.7 breaths per min, respectively (Schmidt-Nielsen 1984). By inspection, measured VT was similar to ValloT, but measured VE and fR were much less than the corresponding allometric normal values (Table 3). DISCUSSION Our findings allowed us to reject our first hypothesis and showed that suspending immobilized black rhinoceros by their feet for 10 min did not impair pulmonary function more than did lateral recumbency. All immobilized black rhinoceroses in our study were severely hypoxemic and hypercapnic regardless of whether they were suspended by their feet or lying in lateral recumbency. The severity of the hypoxemia and hypercapnia concurs with observations that others have made in freeranging black rhinoceroses captured using potent opioids (Kock et al. 1990; Fahlman et al. 2016). In this study, suspension by the feet was actually associated with very slightly better respiratory gas exchange, as suggested by numerically small, but statistically significant, greater mean P a O 2 and lower mean P a CO 2 in this position. Although improvements inarterial blood gases are numerically slight, the increment in mean P a O 2 and the decrementin mean P a CO 2 associated with suspension by the feet may have biological andclinical significance in animals such as these that are already severely hypoxemic and hypercapnic. These improvements with suspension may be especially critical because translocation of rhinoceroses exacerbates water loss, mobilizes energy reserves, damages muscles, and leads to oxidative stress (Pohlin et al. 2020). Because VT, VE, and fR values were similar in the two test postures, the slightly smaller mean P a CO 2 in suspended rhinoceroses may be attributable to greater alveolar ventilation and correspondingly smaller dead space ventilation in this posture compared with lateral recumbency (Radcliffe et al. 2014). The black rhinoceroses had slightly greater pHa when they were suspended compared with lateral recumbency. Because base excess and lactate were not significantly different between the two postures, the greater pHa was most likely due to respiratory compensation via improved alveolar ventilation and lesser P a CO 2 when they were suspended. Suspension resulted in a slightly greater mean P a O 2 compared with lateral recumbency. However, because P( A-a )O 2 was not significantly different between the postures, the greater mean P a O 2 did not appear to be caused by less venous admixture in suspended rhinoceroses. Therefore the greater P a O 2 with suspension might be due to the slightly greater P A O 2 in suspended animals, which is consistent with the improved alveolar ventilation postulated above for animals in suspension. The S a O 2 was 8% greater when the rhinoceroses were suspended. Although our data do not allow us to confirm it, this small increment in S a O 2 might make a biologically important contribution to oxygen availability in these severely hypoxemic animals. Because measured VCO 2 was 35% greater than Vallo0.8CO 2 but was not significantly different from Vallo1.0CO 2 , our data must be considered equivocal regarding the postulated hypermetabolic effect of immobilization in black rhinoceros (our second hypothesis). In white rhinoceros, VCO 2 was 4.2 (1.1), 3.9 (0.7), and 4.6 mL/kg/min (0.5) at 30, 40, and 50 min, respectively, after being given etorphine alone; these values are consistent with a markedly hypermetabolic state in white rhinoceroses (Boesch 2020). Our measured values for VCO 2 in immobilized black rhinoceros are considerably less than those reported in white rhinoceros after etorphine. This difference in VCO 2 Order of treatment Lateral first (n = 6) Lateral second (n = 6) P Suspended first (n = 6) Suspended second (n = 6) P P a CO 2 (mm Hg) 58 (4.2) 52 (4.5) 0.025* 53 (2.6) 50 (5.1) 0.167 P A O 2 (mm Hg) 49 (5.2) 57 (5.5) 0.025* 55 (3.2) 59 (6.3) 0.177 Time to collect data (min) a 26 (8.9) 12 (1.2) 0.004* 14 (2.6) 12 (4.0) 0.414 Leading Article