Oxygen-enriched air is commonly used in the sport of SCUBA-diving and might affect ventilation and heart rate, but little work exists for applied diving settings. We hypothesized that ventilation is decreased especially during strenuous underwater fin-swimming when using oxygen-enriched air as breathing gas. Ten physically-fit divers (age: 25±4; 5 females; 67±113 open-water dives) performed incremental underwater fin-swimming until exhaustion at 4 m water depth with either normal air or oxygen-enriched air (40% O2) in a double-blind, randomized within-subject design. Heart rate and ventilation were measured throughout the dive and maximum whole blood lactate samples were determined post-exercise. ANOVAs showed a significant effect for the factor breathing gas (F(1, 9)=7.52; P=0.023; η2 p=0.455), with a lower ventilation for oxygen-enriched air during fin-swimming velocities of 0.6 m·s−1 (P=0.032) and 0.8 m·s−1 (P=0.037). Heart rate, lactate, and time to exhaustion showed no significant differences. These findings indicate decreased ventilation by an elevated oxygen fraction in the breathing gas when fin-swimming in shallow-water submersion with high velocity (>0.5 m·s−1). Applications are within involuntary underwater exercise or rescue scenarios for all dives with limited gas supply.

The positive effects of combined hyperoxia and physical exercise on physiological parameters and cognitive functioning are established for normobaric laboratory contexts. Still, increased practicability exists in hyperbaric settings like underwater activities and SCUBA diving, where environmental and sport-specific factors might moderate effects. Improved cognition, reduced ventilation (V ̇E), and lower blood lactate concentrations [Lac -] are highly relevant, especially during high-stress and rescue scenarios. Fifteen participants performed 3 × 8 min of continuous underwater fin-swimming at 25 % (low), 45 % (moderate), and 75 % (vigorous) heart rate reserve (HRR) in each test. Three separate test days differed solely by the inspiratory oxygen partial pressure (P I O 2 : 29 kPa, 56 kPa, and 140 kPa). V ̇E was measured continuously, whereas breathing gas analysis, blood sampling, and Eriksen Flanker tasks for inhibitory control (100 stimuli) were performed post-exercise. Two-way ANOVAs with repeated measures on the factors P I O 2 and exercise intensity analyzed physiological outcome variables and reactions times (RT) and accuracy (ACC) of inhibitory control. V ̇E was significantly reduced for 140 kPa during moderate and vigorous and for 56 kPa during vigorous compared to 29 kPa. 56 kPa and 140 kPa showed no differences. [Lac -], post-exercise V ̇CO 2 , and velocity were unaffected by P I O 2 . Faster RTs but lower ACC of inhibitory control were observed following exercise at 75 % HRR compared to rest, 25 %, and 45 % HRR, while P I O 2 produced no effects. Underwater performance in hyperoxia presents reduced V ̇E, possible by dampened chemoreceptor sensitivity, and effects on cognition that differ from laboratory results and emphasise the moderating role of sport-specific factors. Highlights. Hyperoxia-induced reductions in V ̇E with 56 and 140 kPa P I O 2 during constant submaximal finswimming intensity compared to air might be prominently caused by peripheral chemoreceptor suppression. . No difference between 56 and 140 kPa was detected, indicating a P I O 2 threshold limiting further hyperoxic influence on V ̇E. O 2 supply might sufficiently cover metabolic demands of submaximal exercise with 56 kPa, while further reductions in V ̇E could be observed only by severely higher P I O 2 . . Cognitive performance by inhibitory control was unaffected by P I O 2 . Faster RTs but lower ACC were observed following vigorous exercise (75 % HRR) compared to rest, low, and moderate exercise.

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