Investigating Bottlenose Dolphin (Tursiops truncatus) Cardiac Frequency and Cardiac Contractility Using a Novel Physio-logging Tag

Abstract

Vertebrate animals undergo a constellation of physiological responses when they experience submersion. These responses, collectively known as the dive response, include apnea (breath-hold), bradycardia (a reduction in heart rate), and peripheral vasoconstriction (the restriction of oxygenated blood to organs critical to life). Cetaceans, the order of mammals that includes whales, dolphins, and porpoise, are obligate air-breathing mammals and one of the few mammalian taxa to become fully aquatic. Given this evolutionary trajectory, cetaceans are an excellent model for investigating the physiological extremity of the dive response. One limiting factor in dive response research involving cetaceans is the relative lack of non-invasive physio-logging devices that can be attached in free-swimming animal contexts. To address this gap, my collaborators and I invented a new multi-sensor, suction cup-attached device called the FaunaTag. The FaunaTag was custom-built to enable non-invasive collection of cardiovascular physiological data in cetacean species. Equipped with a novel contact sensor column that interfaces with the body surface of the tagged animal, the FaunaTag's near-infrared spatially-resolved diffuse reflectance bio-optical sensor and its accelerometer and gyroscope sensors were used to investigate aspects of the dive response in bottlenose dolphins (Tursiops truncatus), the most accessible and well-studied member of the cetacean order.In the first set of experimental trials, I used the FaunaTag and a new methodological approach to investigate the extent to which dolphin cardiac heart rate changes during alternating bouts of stationary surface free-breathing and submerged apnea. In these trials, the FaunaTag and its unique contact sensor measured the vibrations associated with the cardiac cycle at the dolphin's chest wall. These vibrations were used to compute instantaneous heart rate and instantaneous kinetic energy associated with cardiac contractility. During these trials, we also tested the efficacy of the FaunaTag's near-infrared bio-optical sensor to measure dolphin heart rate before, during, and after apnea, with the FaunaTag placed at a variety of body locations, and the extent to which optically-computed heart rate estimates matched the cardiac frequency estimates calculated from cardiac vibrations. I found that instantaneous heart rate estimates measured in this study were consistent with the heart rates computed using electrocardiography in previous studies involving these same animals. I also observed expected patterns of bradycardia during extended apneas, respiratory sinus arrhythmias following respiration events, and a return to a baseline heart rate shortly after respiration. I also found that instantaneous kinetic energy of cardiac contraction varies between free-breathing and breath-holding trial phases, with a decline to a stable apneic baseline during submerged breath-holds, followed by a steep rise following cessation of apnea and an eventual return to a variable but reduced post-apnea baseline. The FaunaTag's near-infrared spectroscopy performed poorly at dorsal body locations, detected 60% of the matched heartbeats while attached to the cardiac window of the bottlenose dolphin, and achieved a match rate exceeding 90% in the best trial. Future efforts involving the FaunaTag will feature an improved bio-optical sensing module which may resolve poor optical cardiography at the dorsal surfaces of the dolphin body and other cetacean species.