Re-evaluating the significance of the dive response during voluntary surface apneas in the bottlenose dolphin, Tursiops truncatus

The dive response is well documented for marine mammals, and includes a significant reduction in heart rate (f(H)) during submersion as compared while breathing at the surface. In the current study we assessed the influence of the Respiratory Sinus Arrhythmia (RSA) while estimating the resting f(H) while breathing. Using transthoracic echocardiography we measured f(H), and stroke volume (SV) during voluntary surface apneas at rest up to 255 s, and during recovery from apnea in 11 adult bottlenose dolphins (Tursiops truncatus, 9 males and 2 females, body mass range: 140-235 kg). The dolphins exhibited a significant post-respiratory tachycardia and increased SV. Therefore, only data after this RSA had stabilized were used for analysis and comparison. The average (+/- s.d.)f(H), SV, and cardiac output (CO) after spontaneous breaths while resting at the surface were 44 +/- 6 beats min(-1), 179 +/- 31 ml, and 7909 +/- 18141 min(-1), respectively. During the apnea the f(H),SV, and CO decreased proportionally with the breath-hold duration, and after 255 s they, respectively, had decreased by an average of 18%, 1-21%, and 12-37%. During recovery, the f(H), SV, and CO rapidly increased by as much as 117%, 34%, and 190%, respectively. Next, f(H),SV and CO rapidly decreased to resting values between 90-110 s following the surface apnea. These data highlight the necessity to define how the resting f(H) is estimated at the surface, and separating it from the RSA associated with each breath to evaluate the significance of cardiorespiratory matching during diving.