Progressive changes of oxygenation, diving response, and involuntary breathing movements during repeated apneas

This study investigated whether trained freedivers can sustain a 1:1 apnea-to-recovery ratio without progressive arterial or cerebral oxygen desaturation.

21 trained freedivers (6 females) performed 7 static apneas of fixed 2-min duration, each followed by 2-min of rest, in a supine laboratory setting. Arterial oxygen saturation (SpO₂) and heart rate (HR) were measured continuously. Near-infrared spectroscopy (NIRS) assessed cerebral and peripheral muscle oxygenation. A chest force sensor recorded involuntary breathing movements (IBM). End-tidal CO₂ (EtCO₂) was measured pre- and post apnea.

SpO₂ declined most during the first apnea (94 ± 3 %) but stabilized thereafter (p < 0.005). Lowest HR increased from 61 ± 15 to 65 ±13 bpm across the series (p = 0.02), and the intial apnea tachycardia declined by 10 bpm (p = 0.012). Cerebral oxygenation increased above baseline only during the first apnea (1.0 ± 2.3 %); in subsequent apneas it remained stable, although slightly below baseline. Muscle oxygenation declined during all apneas but was more pronounced in the first (-6.7 ± 3.1 %). IBM onset was progressively delayed; 63 % of participants showed no IBM during the final apnea. EtCO₂ increased after each apnea by ≈ 1.0kPa (p < 0.001) but did not change progressively across the series.

A 1:1 apnea-to-recovery ratio was physiologically sustainable in trained freedivers at rest, without inducing progressive oxygen desaturation. The initial apnea elicited the strongest oxygen-conserving responses, which progressively attenuated across the series, suggesting that physiological regulation during repeated submaximal apneas is adaptable to meet situation-specific demands. The progressive IBM delay despite stable CO₂ levels suggests additional mechanisms beyond chemoreflex-driven stimulation of breathing may contribute to ventilatory drive.