Early ventilation-heart rate breakpoint during incremental cycling exercise.

Previous observations having reported a transient hypoxia at the onset of incremental exercise, we investigated the existence of concomitant ventilatory and heart rate (HR) breakpoints. 33 subjects executed a maximal cycling exercise with averaging for successive 5-s periods of HR, ventilation, tidal volume (V T ), mean inspiratory flow rate (V T /Ti), and end-tidal partial pressures of O 2 (P ET O 2 ) and CO 2 . In 10 subjects, the transcutaneous partial pressure of O 2 (PtcO 2 ) was recorded and the venous blood lactic acid (LA) concentration measured. At the beginning of exercise, P ET O 2 decreased, reaching a nadir, then progressively increased until the exercise ended. PtcO 2 varied in parallel. Whether or not a 0-W cycling period preceded the incremental exercise, the rate of changes in V E , V T , V T /Ti and HR significantly increased when the nadir PO 2 was reached. The ventilatory/ HR breakpoint was measured at 33±4% of VO 2 max, whereas the ventilatory threshold (V Th ) was detected at 67±4% of VO 2 max and LA began to increase at 45 to 50% of VO 2 max. During incremental cycling exercise, we identified the existence of HR and ventilatory breakpoints in advance of both lactate and ventilatory thresholds which coincided with modest hypoxia and hypercapnia.