VN March 2023

Vetnews | March 2023 11 between species might be due to black rhinoceros being inherently less susceptible than white rhinoceros to the hypermetabolic effects of etorphine. Although it is possible that the azaperone tranquilizer coadministered with etorphine in the black rhinoceroses muted an increase in metabolic rate from the etorphine, others have reported no relief from hypoxemia when azaperone is given with etorphine to white rhinoceroses (de Lange et al. 2017). The measured VCO 2 reported here in black rhinoceroses reinforces clinical experience suggesting that there are differences between the two species of African rhinoceros in the way they respond to immobilization with drug combinations that include l-opioid agonists. Hypercapnia (mean P a CO 2 .50 mm Hg) observed in rhinoceroses in both positions is compatible with impaired alveolar ventilation. Measured VE in these rhinoceroses was 63% of the allometrically predicted normal value and consistent with hypoventilation. Measured VT was similar to that predicted allometrically; however, measured fR was 63% of that predicted for normal awake rhinoceroses (Citino and Bush 2007), suggesting that the hypoventilation is due principally to inhibition of breathing pattern generation. In other species, l-opioids primarily inhibit respiratory pattern generation rather than tidal volume, thus it is likely that the hypoventilation in our rhinoceroses was attributable to the etorphine component of the immobilizing drug combination resetting the rhinoceros’ carbon dioxide-sensing mechanism and slowing their breathing rate (Lalley 2003). The randomly assigned treatments in a paired cross-over design aided detection of small significant differences. However, this model produced a limitation to the study, particularly related to the six animals that were in lateral recumbency first. Because all 12 animals went into lateral recumbency after darting, the six animals assigned to be in lateral recumbency first were not suspended at all before the first data were collected, whereas the other six animals were suspended before any data were collected. Also the six animals assigned to be in lateral recumbency first were in lateral recumbency for longer than the corresponding cohort that was in lateral recumbency as their second treatment. Only P a CO 2 and P A O 2 were affected by order of treatment, such that those animals that were in lateral recumbency first had higher P a CO 2 and lower P A O 2 . This observation might be due to prolonged time in lateral recumbency without autonomic perturbation associated with posture change before the observations were made or to some other factor. Dead space fraction in laterally recumbent rhinoceroses in our study was similar to that reported previously with the same equipment in black rhinoceroses (Radcliffe et al. 2014). Such a large dead space fraction is unusual. Because the Enghoff method used here substitutes P a CO 2 for alveolar PCO 2 in the Bohr equation, it is likely to overestimate dead space when intrapulmonary shunt is large or there are large areas of lung with low V/Q ratio (Wagner et al. 2008). Although shunt fraction was not measured here, the low P a O 2 and large P( A-a )O 2 suggest that shunt fraction was considerable in these rhinoceroses, making such overestimation of total respiratory dead space likely. A more accurate measure of dead space uses volumetric capnography, which requires simultaneous measurement of expired volume and expired carbon dioxide; unfortunately, the equipment for this was not available in the field. Enghoff dead space, as measured here, should be viewed as a composite of dead space ventilation plus intrapulmonary venous admixture (Suarez- Sipmann et al. 2013). That being the case, values in excess of 50% for Enghoff dead space, as we report, might be evidence of severe, but nonspecific, impairment of pulmonary gas exchange. These experiments suggest that the pulmonary system of immobilized black rhinoceros is no more compromised by suspension by the feet for 10 min than it is by lying in lateral recumbency. However, because helicopter translocation of black rhinoceroses usually takes longer than 10 min, future experiments should be directed to studying how longer periods of suspension by the feet influence thesevere hypoxia and hypercapnia observed here. Such studies should also determine whether the small improvements in P a O 2 and P a CO 2 we observed with suspension are consistent. v Leading Article