Purpose: The majority of highly trained endurance athletes with a maximal oxygen uptake greater than 60 mL·min-1·kg-1 develop exercise-induced hypoxemia (EIH). Yet some of them apparently do not. The pathophysiology of EIH seems to be multifactorial, and one explanatory hypothesis is a relative hypoventilation. Nevertheless, conflicting results have been reported concerning its contribution to EIH. The aim of this study was to compare the cardiorespiratory responses to maximal exercise of highly trained endurance athletes demonstrating the same aerobic capacity without EIH (N athletes) and with EIH (H athletes). Methods: Ten N athletes and twelve H athletes performed an incremental exercise test. Measurements of arterial blood gases and cardiorespiratory parameters were performed at rest and during exercise. Results: All athletes presented a significant decrease in PaO2 (P < 0.05) from rest up to 80% V(dot)O2max associated with an increase in PaCO2, both findings consistent with a relative hypoventilation. Then the H athletes, who had a greater training volume per week and a higher second ventilatory threshold than the N athletes (respectively, 17 ± 1.1 vs 13.1 ± 0.7 h·wk-1; 91.8 ± 1.7 vs 86.1 ± 1.8% V(dot)O2max), presented a continuous PaO2 decrease up to V(dot)O2max. This was associated with a widening (Ai-a)DO2. Conclusion: This study showed that a relative hypoventilation, probably induced by a high level of endurance training, induced hypoxemia in all athletes. However, a nonventilatory mechanism, perhaps related to the volume of training, seemed to affect gas exchanges beyond the second ventilatory threshold in the H athletes, thereby enhancing EIH.
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