Respiratory sinus arrhythmia and submersion bradycardia in bottlenose dolphins (<i>Tursiops truncatus</i>)

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 (<i>f</i>_H) in six adult male bottlenose dolphins (<i>Tursiops truncatus</i>) spontaneously breathing at the surface to quantify the relationship between respiration and <i>f</i>_H, and compared this with <i>f</i>_H during submerged breath-holds. We found that dolphins exhibit a pronounced respiratory sinus arrhythmia (RSA) during surface breathing, resulting in a rapid increase in <i>f</i>_H after a breath followed by a gradual decrease over the following 15-20 s to a steady <i>f</i>_H that is maintained until the following breath. RSA resulted in a maximum instantaneous <i>f</i>_H (i<i>f</i>_H) of 87.4±13.6 beats min^-1 and a minimum i<i>f</i>_H of 56.8±14.8 beats min^-1, and the degree of RSA was positively correlated with the inter-breath interval (IBI). The minimum i<i>f</i>_H during 2 min submerged breath-holds where dolphins exhibited submersion bradycardia (36.4±9.0 beats min^-1) was lower than the minimum i<i>f</i>_H observed during an average IBI; however, during IBIs longer than 30 s, the minimum i<i>f</i>_H (38.7±10.6 beats min^-1) was not significantly different from that during 2 min breath-holds. These results demonstrate that the <i>f</i>_H 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 <i>f</i>_H variability and emphasize the need to understand its relationship to submersion bradycardia.