Chemoreflex Drive and the Dynamics of Ventilation and Gas Exchange during Exercise at Hypoxia

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Abstract
We tested the hypothesis that the promotion of hypoxic ventilatory responsiveness (HVR) and/or hypercapnic ventilatory responsiveness (HCVR) mostly acting on the carotid body with a changing work rate can be attributed to faster hypoxic ventilatory dynamics at the onset of exercise. Eleven subjects performed a cycling exercise with two repetitions of 6 minutes while breathing at FIO2 = 12%. The tests began with unloaded pedaling, followed by three constant work rates of 40%, 60%, and 80% of the subject's ventilatory threshold at hypoxia. Reference data were obtained at the 80% ventilatory threshold work rate during normoxia. Using three inhaled 100% O2 breath tests, a comparison of hypoxia and normoxia revealed an augmentation of HVR in hypoxia, which then significantly increased proportionally with the increase in work rate. In contrast, HCVR using three inhaled 10% CO2 breath tests was unaffected by the difference in work rate at hypoxia but did exceed its level at normoxia. The decrease in the half-time of hypoxic ventilation became significant with an increase in work rates and was significantly lower than at normoxia. Using a multiregression equation, HVR was found to account for 63% of the variance of hypoxic ventilatory dynamics at the onset of exercise and HCVR for 9%. O2 uptake on-kinetics and off-kinetics under hypoxic conditions were significantly slower than under normoxic conditions, whereas they were not altered by the changing work rates at hypoxia. These results suggest that the faster hypoxic ventilatory dynamics at the onset of exercise can be mostly attributed to the augmentation of HVR with an increase in work rates rather than to HCVR. Otherwise, O2 uptake dynamics are affected by the lower O2, not by the changing work rates under hypoxic conditions.

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