Cardiorespiratory coupling in cetaceans; a physiological strategy to improve gas exchange?

In the current study we used transthoracic echocardiography to measure stroke volume (SV), heart rate (f_H) and cardiac output (CO) in adult bottlenose dolphins (Tursiops truncatus), a male beluga whale calf [Delphinapterus leucas, body mass (M_b) range: 151-175 kg] and an adult female false killer whale (Pseudorca crassidens, estimated M_b: 500-550 kg) housed in managed care. We also recorded continuous electrocardiogram (ECG) in the beluga whale, bottlenose dolphin, false killer whale, killer whale (Orcinus orca) and pilot whale (Globicephala macrorhynchus) to evaluate cardiorespiratory coupling while breathing spontaneously under voluntary control. The results show that cetaceans have a strong respiratory sinus arrythmia (RSA), during which both f_H and SV vary within the interbreath interval, making average values dependent on the breathing frequency (f_R). The RSA-corrected f_H was lower for all cetaceans compared with that of similarly sized terrestrial mammals breathing continuously. As compared with terrestrial mammals, the RSA-corrected SV and CO were either lower or the same for the dolphin and false killer whale, while both were elevated in the beluga whale. When plotting f_R against f_H for an inactive mammal, cetaceans had a greater cardiac response to changes in f_R as compared with terrestrial mammals. We propose that these data indicate an important coupling between respiration and cardiac function that enhances gas exchange, and that this RSA is important to maximize gas exchange during surface intervals, similar to that reported in the elephant seal.