Modeling the diving bradycardia: Toward an "oxygen-conserving breaking point"?

Area of Science:

• Physiology
• Human Diving Science
• Cardiovascular Regulation

Background:

• Exponential decay models inadequately describe heart rate (HR) kinetics during prolonged static breath holding (BH).
• Accurate HR quantification during extended BH is crucial for understanding physiological responses.

Purpose of the Study:

• To develop a novel, meaningful model for quantifying HR kinetics during prolonged static BH.
• To investigate HR dynamics in both air and immersed conditions.

Main Methods:

• Nonlinear regression analysis was employed to model beat-to-beat HR reduction in 11 trained divers during prolonged static BH.
• Heart rate variability indices (RMSSD, DFAα1) and peripheral oxygen saturation (SpO2) were analyzed to correlate with HR changes.

Main Results:

• A tri-phasic HR model was established, featuring an initial exponential drop, a linear rise, and a subsequent linear drop.
• A significant "oxygen-conserving breaking point" was identified at similar SpO2 levels in air and immersion.
• The HR drop post-breaking point correlated with significant shifts in cardiac autonomic regulation.

Conclusions:

• The tri-phasic model provides accurate HR kinetics during prolonged BH.
• The identified "oxygen-conserving breaking point" suggests a unique adaptive mechanism against hypoxic damage in human diving bradycardia.