Vetnuus | February 2025 15 Leading Article of propofol for induction and inhalation anesthetics for anesthesia maintenance may cause vasodilation and could lead to depressed cardiac contractility (Berry, 2015). Together, the reduced oxygen delivery to muscles could promote rhabdomyolysis, or the breakdown of skeletal muscle fibers, and lead to kidney injury from the breakdown products (e.g., myoglobin) (Bailey et al., 2012). Thus, there is a need to understand the physiologic impactsof anesthesia in dolphins, as well as develop strategies to reduce anesthesia-associated morbidities. As noted by Ridgway and colleagues, the out-of- water induction of anesthesia abates all spontaneousventilation in dolphins (Bailey et al., 2022; McCormick & Ridgway, 2018). Mechanical ventilation is, therefore, required to prevent the pathophysiologic consequences of hypoventilation. The most employed mechanical ventilation approach in veterinary species, controlled or conventional mechanical ventilation, mirrors the normal respiratory pattern of terrestrial mammals. Dolphins, however, have an inspiratory breath-hold respiratory phenotype with significant heart rate variation during each inspiratory-to- expiratory cycle (RSA) (Fahlman et al., 2020; Le-Bert et al., 2024; McCormick, 1969). This cardiopulmonary coupling strategy may improve gas exchange in the conscious diving dolphin; however, it is completely abolished with mechanical ventilation (Fahlman et al., 2020). The uncoupling effect on efficient respiratory gas exchange under anesthesia is unknown and may be of consequence. While mechanical ventilation is a critical feature of dolphin anesthesia, it can also promote alveolar collapse (atelectasis), leading to ventilation-perfusion mismatching. While Nagel, Ridgway, and colleagues were able to mechanically mimic dolphin breathing using apneustic plateau ventilation (APV) through the modification of existing large animal ventilators, the availability of this mechanical ventilation strategy to dolphin veterinarians, as well as an understanding of its effect on respiratory gas exchange under anesthesia, are lacking. A ventilation strategy that maintains airway pressure above functional residual capacity (e.g., the point in the breathing cycle where alveoli are more prone to collapse) by decreasing lung volume and pressure from an elevated plateau pressure to an airway pressure at or slightly above functional residual capacity was recently described and tested on pigs, horses, and dolphins (Bratzke et al., 1998; Bukoski et al., 2022, 2024; Le-Bert et al., 2024). In these studies, the authors compared the cardiopulmonary effects of apneustic anesthesia ventilation (AAV) and conventional mechanical ventilation (CMV) in 12 adult pigs, 10 healthy adult horses, and 10 healthy adult bottlenose dolphins. In the horse and pig studies, the authors found that AAV resulted in significantly higher respiratory system dynamic compliance (change in lung volume over the change in pleural pressure) and lower venous admixture, or physiologic shunt (Bukoski et al., 2022, 2024). In dolphins, AAV resulted in higher arterial oxygen tension and reduced alveolar dead space ventilation (Le-Bert et al., 2024). Thus, this ventilation strategy demonstrated some physiologic advantages for cardiopulmonary function while mechanically ventilating anesthetized dolphins and warrants further investigation. Another significant challenge to the physiology of anesthesia in dolphins is the impact of gravity on a species that evolved in a buoyant ocean environment (Le-Bert et al., 2024). When dolphins are removed from the neutrally buoyant environment, as is often necessary for medical and surgical procedures, the influence of gravity on hemodynamic variables may become an important factor (Figure 1). Resulting pressure gradients across dolphin tissues could contribute to whole body fluid shifts and blood flow redistribution when out of water for anesthetic procedures. Gravityinduced hemodynamic shifts will be discussed in the next section and should be considered and mitigated in anesthetized dolphins when possible. 4.CARDIOPULMONARY ADAPTATIONS RELEVANT TO ANESTHESIA IN BOTTLENOSE DOLPHINS While cetaceans evolved for life in diverse aquatic habitats, all cetacean species rely on intermittent surfacing to breathe air. Consequently, prolonged intervals of breath-holding required for locomotion and foraging impact respiratory gas exchange and metabolism (Noren et al., 2012). As such, cetaceans developed specialized anatomic characteristics and physiologic adaptations which must be considered during anesthesia. Here, we expand upon select cardiovascular and pulmonary adaptations to diving and breath-holding activities and how these adaptations may influence dolphin responses to anesthetic agents. FIGURE 1 A significant challenge to general anesthesia in dolphins is the impact of gravity on a species that evolved in a buoyant ocean environment. For this reason, Ridgway would perform surgical approaches to the dolphin ear in a partially suspended state—a surgical table-tank. The water in the surgical table-tank was also heated to assist with thermoregulation of core body temperature (Image courtesy of the U.S. Navy's Marine Mammal Program). >>>16
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