Vetnews | Februarie 2025 18 « BACK TO CONTENTS Leading Article during general anesthesia. In a limited number of reported outof- water, non-diving, conscious dolphins, the normal mean arterial blood pressure ranged from 120 to 140 mmHg (Ridgway & McCormick, 1971). While no studies exist specifically measuring vascular resistance in resting dolphins, genomic studies of diving marine mammals point to evolutionary pressure on endothelin pathway genes (EDN1, EDN2, EDN3, EDNRA, EDNRB). Further, there is a genetic loss of a renal amino acid transporter of arginine reabsorption (SLC6A18), which would reduce production of the vasodilatory signaling molecule nitric oxide and, thus, be a potential mechanism for promoting vasoconstriction (Hindle, 2020; Huelsmann et al., 2019; Tian et al., 2016). Additional genetic markers contributing to efficient peripheral vasoconstriction in diving dolphins are those of their intrinsic coagulation pathway. Genes encoding coagulation factor XII are absent in dolphins. Specifically, the loss of the kallikrein B1 gene protects against thrombus formation, while key coagulation factors of the extrinsic pathway required for hemostasis of damaged tissue are not lost (Huelsmann et al., 2019; Kokoye et al., 2016; Semba et al., 1998, 2000). Since vasoconstriction-induced reduction in blood vessel diameter during diving typically increases risk for thrombus formation in other mammals, loss of these genes likely provide an evolutionary advantage to cetaceans (Haulena & Schmitt, 2018; Kokoye et al., 2016). 4.1.2 Respiratory sinus arrhythmia and cardiopulmonary coupling in dolphins Respiratory sinus arrhythmia (RSA) is the variation of heart rate with the inter-breath interval and is seen in many species, including humans. RSA is frequently used as an index of cardiac vagal tone and overall fitness and health. Many marine mammal species, including bottlenose dolphins, demonstrate pronounced RSA during surface breathing, resulting in dramatic changes in instantaneous heart rate throughout the inter-breath interval (Blawas, Nowacek, et al., 2021; Cauture et al., 2019; Fahlman, Miedler, et al., 2019). However, heart rate patterns during prolonged breath-holds are similar to those resulting from RSA during extended inter-breath intervals (Blawas, Nowacek, et al., 2021). Cardiorespiratory, coupling such as RSA has been proposed in dolphins as a physiologic strategy to optimize gas exchange during the surface interval between prolonged breath-hold dives (Fahlman et al., 2020; Fahlman, Miedler, et al., 2019; Giardino et al., 2003; Hayano et al., 1996). 4.2 Respiratory system adaptations Beyond the cardiovascular system, the respiratory system in cetaceans also displays specialized physiologic and anatomic adaptations suitable for a completely aquatic life. For example, the cetacean blowhole is a result of migration of nasal passages to the top of the forehead (Berta et al., 2014). The trachea of cetaceans tends to be short and wide and holds up to 4% of total lung volume (Davenport et al., 2013; Piscitelli et al., 2013). The compliant spiraling rings of the trachea and bronchi are also unique; they retain enough rigidity to remain patent, but enough flexibility to withstand the extreme compression experienced during a dive (Denk et al., 2020; Moore et al., 2013). Cetacean lungs are unilobular and lie dorsal to the heart while enclosed within a complete mediastinum. The lungs exhibit morphological advantages essential for the explosive, intermittent ventilation observed during surfacing intervals. An abundance of elastic fibers, high pulmonary compliance, collateral ventilation adaptations, and cartilaginous reinforcement of bronchi and bronchioles, allow cetacean alveoli to undergo compression and collapse at extreme hydrostatic pressures (Piscitelli et al., 2013). In larger airways and the extra-pulmonary bronchi, smooth muscle replaces elastic layers. The presence of structurally reinforced airways allows for the accumulation of air at high pressures within the dead space when non-reinforced alveoli would collapse. Alveolar collapse could protect against nitrogen gas absorption or gas emboli when ascending from a dive. Some investigators hypothesize that these attributes, along with evidence of hypoxic pulmonary vasodilation rather than vasoconstriction, and intrapulmonary arteriovenous shunts, enable functional pulmonary shunting at any depth in cetaceans; this ability for pulmonary shunts may not be fully dependent on hydrostatic compression (Garcia Parraga et al., 2018). Thus, even with alveolar collapse, extensive collateral ventilation plus hypoxia-induced vasodilation may enable continued gas exchange in upper parts of the lung (Gompelmann et al., 2013). 4.2.1 Mechanics of breathing in the dolphin The inspiratory, breath-holding breathing pattern of cetaceans is an essential feature required for underwater feeding activities. The defining characteristic of volitional breathing in bottlenose dolphins is a rapid exhalation of large air volumes (~90%–95% of total lung capacity) through the blowhole within approximately 0.26–0.5 s, followed by a slower inspiratory phase ending with blowhole closure. Thus, the respiratory system maintains in an inflated state with the inspired air until the next respiratory cycle (Cotten et al., 2008; Fahlman et al., 2015, 2017; Fahlman, Brodsky, et al., 2019; Piscitelli et al., 2013; Ridgway, 1972). Existing physiologic measures of lung function and mechanics should be considered when applying mechanical ventilation during general anesthesia of dolphin patients (Table 3). Dolphin airway (alveoli) opening pressures, for example, are 21–25 cm H2O, with maximum lung volume achieved at pressures around 30 cm H2O (Piscitelli et al., 2010). Tidal volumes reported in the literature range from 15 to 22 mL/kg for dolphins resting at the surface (terrestrial mammals are about 7.7 mL/kg), while breathing frequency can be significantly variable, averaging between 0.9 and 3.6 breaths/minute in the resting dolphin (Fahlman et al., 2015; Mortola & Seguin, 2009; Piscitelli et al., 2013; Ridgway, 1972). Total lung capacity in bottlenose dolphins is reported to be between 40 and 138 mL/kg (lung mass ~2.7% of total body mass); however, some of these measurements were acquired from excised lungs, possibly contributing to the wide ranges reported (Fahlman et al., 2015, 2017; Mortola & Seguin, 2009; Piscitelli et al., 2010, 2013). Cardiopulmonary adaptations of a diving marine mammal, the bottlenose dolphin.... <<<17
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