Coupling of ventilation and gas exchange during transitions in work rate by humans

The dynamic responses of expired ventilation (VE) following transitions in work rate were studied during step increases and decreases, and during pseudorandom binary sequence (PRBS) changes in work rate. Six healthy men completed multiple repetitions of each test type under conditions of hypoxia (fractional inspired O2, FIO2 = 0.14), room air and hyperoxia (FIO2 = 0.70). Neither potentiation nor attenuation of the carotid chemoreceptors, with hypoxia or hyperoxia respectively, influenced the magnitude or duration of the phase 1 response at the onset of exercise. However, the off-transient phase 1 was smaller with hypoxia than normoxia or hyperoxia. The time course of phase 2 was faster in hypoxia and slower in hyperoxia, but there was symmetry between on- and off-responses. This was counter to predictions of asymmetry based on the short-term potentiation hypothesis. Tight coupling existed for breath-by-breath VE relative to CO2 output during PRBS exercise, especially in hypoxia (r2 = 0.40 +/- 0.08; 0.64 +/- 0.03; and 0.71 +/- 0.05 for hyperoxia; normoxia; and, hypoxia), but poorer coupling existed to O2 uptake, especially in hyperoxia (r2 = 0.09 +/- 0.04; 0.37 +/- 0.05; and 0.46 +/- 0.07). These data are consistent with neural factors in phase 1, and humoral mediation of phase 2 responses during dynamic exercise in healthy subjects.