Discussion

Diving heart rate profile

The instantaneous heart rate generally followed an abrupt exponential pattern previously reported for bottlenose dolphins and other species, including the California sea lion and the Weddell seal. Despite individual variations in instantaneous heart rate, the overall pattern of the diving bradycardia was consistent across all dolphins.

Influence of depth on heart rate and HRV

Other studies have also found that heart rate decreased with depth in various marine species, including bottlenose dolphins, Weddell seals, harbor seals, California sea lions, narwhals, and loggerhead turtles. Nevertheless, comparing the strength of these relationships across studies is challenging due to differences in analytical methods. Additionally, these studies did not account for the confounding effect of dive duration or pre-dive breathing frequency when evaluating heart rate at different depths. The findings of the present study indicate that depth has a direct influence on heart rate independently of the dive duration and activity, supporting the hypothesis that hydrostatic pressure influences cardiovascular regulation via lung volume changes. 

In this study, CVRR and RMSSD decreased from the pre-dive to the diving phase, indicating that the heart beats with fewer fluctuations in the inter-beat interval, consistent with a reduction (or even suppression) of sympathetic activity during diving. This suppression could be partly attributed to the fact that the animals were static while performing the apneas at various depths. CVRR showed a significant decrease from 0.5 m to both 5 m and 10 m, but no significant difference between 5 m and 10 m, suggesting that the regulatory mechanism reaches a steady state by 5 m and maintains similar autonomic control at greater depths. In contrast, RMSSD did not differ significantly across depths, indicating that vagal (parasympathetic) modulation increases upon apnea onset and is consistently maintained regardless of the dive depth.

Such mechanisms would ensure efficient cardiovascular control throughout the entire period the animal cannot access oxygen, independently of the depth reached during the dive. 

Conditioned variation in heart rate

The existence of somatic motor control over heart rate responses during diving has been suggested in breath-hold diving vertebrates. No significant differences were found in pre-dive heart rate and breathing frequency across depths for all tested dolphins, suggesting that dolphins did not anticipate the dive depth and duration in this study.

Influence of pre-diving breathing frequency on diving heart rate and HRV

As expected, pre-dive breathing frequency affected heart rate at the surface, but also during diving. The results of this study support the hypothesis that surface-induced cardiovascular patterns driven by respiratory tachycardia – or respiratory sinus arrhythmia in other mammals – affect diving physiology. Therefore, studies evaluating the degree of diving bradycardia should account for breathing frequency.

Influence of apnea duration on diving heart rate and HRV

Heart rate and CVRR showed a progressive decrease during the apnea, whereas RMSSD was not influenced by dive duration. This suggests that while the overall cardiovascular response becomes less variable over time, the short-term regulation of heart rate remains consistent, regardless of the duration of the apnea. Additionally, it may also indicate that the finer-scale autonomic adjustments governing RMSSD may be influenced by factors beyond dive duration alone, such as individual physiological variability or pre-dive state.

Several studies have demonstrated that heart rate decreases with increasing dive duration in marine mammals, including bottlenose dolphins, elephant seals, grey seals, harbor seals, Weddell seals, California sea lions, and harbor porpoises. This decline in heart rate with increasing dive duration is primarily attributed to a greater vagal response, which helps to conserve and optimize the use of available oxygen stores, maximizing time underwater and forage efficiency.