Differential kinetics of the cardiac, ventilatory, and gas exchange variables during walking under moderate hypoxia

Abstract

We investigated the effects of moderate hypoxia (FiO2 = 15%) on different kinetics between pulmonary ventilation (VE) and heart rate (HR) during treadmill walking. Breath-by-breath VE, oxygen uptake (VO2), carbon dioxide output (VCO 2), and HR were measured in 13 healthy young adults. The treadmill speed was sinusoidally changed from 3 to 6 kmh-1 with four oscillation periods of 1, 2, 5, and 10 min. The amplitude (Amp), phase shift (PS) and mean values of these kinetics were obtained by harmonic analysis. The mean values of all of these responses during walking at a sinusoidally changing speed became greater under hypoxia compared to normoxia (FiO2 = 21%), indicating that moderate hypoxia could achieve an increased energy expenditure (increased VO2 and VCO2) and hyperventilation. The Amp values of the VE, VO2, and VCO2 kinetics were not significantly different between normoxia and hypoxia at most periods, although a significantly smaller Amp of the HR was observed at faster oscillation periods (1 or 2 min).The PS of the HR was significantly greater under hypoxia than normoxia at the 2, 5, and 10 min periods, whereas the PS of the VE, VO2, and VCO 2 responses was not significantly different between normoxia and hypoxia at any period. These findings suggest that the lesser changes in Amp and PS in ventilatory and gas exchange kinetics during walking at a sinusoidally changing speed were remarkably different from a deceleration in HR kinetics under moderate hypoxia.