Respiratory sinus arrhythmia and submersion bradycardia in bottlenose dolphins (Tursiops truncatus)

Abstract

Among the many factors that influence the cardiovascular adjustments of marine mammals is the act of respiration at the surface, which facilitates rapid gas exchange and tissue re-perfusion between dives. We measured heart rate (fH) in six, adult male bottlenose dolphins (Tursiops truncatus) spontaneously breathing at the surface to quantify the relationship between respiration and fH, and compared this to fH during submerged breath-holds. We found that dolphins exhibit a pronounced respiratory sinus arrhythmia (RSA) during surface breathing resulting in a rapid increase in fH after a breath followed by a gradual decrease over the following 15-20 seconds to a steady fH that is maintained until the following breath. RSA resulted in a maximum instantaneous fH (ifH) of 87.4±13.6 beats min−1, a minimum ifH of 56.8±14.8 beats min−1, and the degree of RSA was positively correlated with the inter-breath interval (IBI). The minimum ifH during 2-minute, submerged breath-holds where dolphins exhibited submersion bradycardia (36.4±9.0 beats min−1) was lower than the minimum ifH observed during an average IBI, however during IBIs longer than 30 seconds, the minimum ifH (38.7±10.6 beats min−1) was not significantly different from that during 2-minute breath-holds. These results demonstrate that the fH patterns observed during submerged breath-holds are similar to those resulting from RSA during an extended IBI. Here we highlight the importance of RSA in influencing fH variability and emphasize the need to understand its relationship to submersion bradycardia.