Vetnews | Februarie 2025 20 « BACK TO CONTENTS These chemoreceptors normally operate in the manner of a negative feedback loop to preserve oxygen and carbon dioxide homeostasis. In 1969, McCormick evaluated the effects of mixed inspired gases on peripheral and central chemoreceptor responses of conscious dolphins (McCormick, 1969). In this study, increasing the inspired carbon dioxide to 5% (with normal oxygen levels) induced a ventilatory response characterized by an increase in respiratory rate. This ventilatory response persisted, even when inspired oxygen concentrations were artificially increased to 40%. Conversely, when inspired oxygen was reduced to 10%, respiratory rate again increased, consistent with a hypoxic chemoreflex responses in dolphins are consistent with normal mammalian peripheral and central chemoreflexes in response to increased inspired carbon dioxide or decreased inspiredoxygen concentrations at sea level. During diving behavior, these responses appear to be reversibly blunted. Pelagic marine mammals appear to tolerate arterial oxygen tensions (paO2) as low as 12 mmHg and venous oxygen tensions (pvO2) as low as 3 mmHg during free diving conditions (Meir et al., 2009; Ponganis et al., 2011). Dolphin venous oxygen tensions (pvO2) during voluntary breath holds at the surface were measured as low as 18–20 mmHg (Williams et al., 1999). It was proposed by Stephenson that dis-facilitation, rather than overt inhibition of ventilation, during diving occurs due to profound hypocapnia, decreasing the drive to breathe below the CO2 apneic threshold during long dives (Stephenson, 2005). In Stephenson’s model, the most important factor leading to dis-facilitation of respiratory drive occurs during the surface interval between dives. Stephenson speculated that surface breathing drastically decreases arterial carbon dioxide (<30 mmHg) and increases arterial oxygen (~120 mmHg), minimizing all chemoreflex drive to breathing (i.e., dis-facilitation). Similar to terrestrial mammals, the drive to breathe in pelagic marine mammals is likely influenced more by central chemoreceptor responses to carbon dioxide versus peripheral chemoreceptor response to decreased oxygen levels (Stephenson, 2005). This may be an important consideration when allowing for permissive hypercapnia during emergence from general anesthesia. A summary of the relevant, published cardiopulmonary measurements, obtained in both unanesthetized and anesthetized bottlenose dolphins is provided in Table 3 for reference. 4. CONCLUDING REMARKS The ability to promote an anesthetic state in dolphins with minimal cardiopulmonary derangements is critical to reduce anesthesiaassociated morbidities and improve clinical outcomes. Current practices in dolphin anesthesia involve use of agents that cause respiratory depression and undermine cardiac function. While many of these effects can be mitigated with controlled mechanical ventilation and chronotropic, inotropic and vasopressive agents, these interventions are often complex, requiring specialized equipment and drug delivery methods. As such, limited peerreviewed publications exist on the pharmacokinetics of anesthetic drugs in bottlenose dolphins. Therefore, extrapolation of drug dosages from terrestrial species and human medicine is often performed. As evidenced from the distinct features of dolphin cardiopulmonary physiology, as well as genomic and receptor differences that could impact binding and metabolism of anesthetic agents as outlined above, the mechanisms of anesthesia may not translate from comparative species. Consideration of the specialized structure and function of the cardiopulmonary system of dolphins should, therefore, guide anesthetic practices to minimize its effects on cardiopulmonary depression and promote hemodynamic stability in the anesthetized dolphin. For example, investigations into partial and total intravenous anesthetic techniques that would limit reliance on inhalation anesthetics may provide cardio-pulmonary sparing effects and improve perfusion states. Additionally, evaluation of mixed gas ventilation approaches, such as lower inspired oxygen concentrations, may reduce morbidities associated with absorption atelectasis. And finally, limiting the gravitational effects on dolphin circulation by investigating buoyant materials may improve hemodynamics and thus, perfusion, under general anesthesia. Though we recognize the need to improve anesthesia protocols in dolphins, a significant hurdle in the advancement of dolphin anesthesiology is the relative lack of dolphins under human care around the world. The low availability of study subjects and resource expertise makes controlled basic physiologic and pharmacologic studies difficult to complete. Bottlenose dolphins considered ‘charismatic megafauna,’ are not routinely subjected to risky and complex biomedical studies. A greater understanding of dolphin physiology will continue to contribute to improvements in anesthesia protocols and medical management and may even inspire advances in human biomedical research and health care. However, in the West, dolphins are not managed as laboratory animals and thus, opportunities for controlled biomedical studies will remain limited. Often these opportunities present by observations incidental to emergent or urgent clinical interventions and dependent on gradually evolving approaches to medical management of healthy dolphins among larger holders of this species. For these reasons, it is of utmost importance these and smaller holding institutions maintain a network of scientists and collaborators with expertise in cetacean medicine, physiology, and anesthesiology. Only with these collaborations can the practice and discipline of dolphin anesthesiology eliminate the hurdle of inexperience and advance anesthetic practices for the improvement of cetacean care around the world. v Leading Article How to cite this article: Le-Bert, C. R., Mitchell, G. S., & Reznikov, L. R. (2024). Cardiopulmonary Adaptations of a diving marine mammal, the bottlenose dolphin: Physiology during anesthesia. Physiological Reports, 12, e16183. https://doi.org/10.14814/phy2.16183 Cardiopulmonary adaptations of a diving marine mammal, the bottlenose dolphin.... <<<19
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