Breath-by-breath alveolar oxygen transfer at the onset of step exercise in humans: methodological implications

Abstract.

The effects of using different algorithms to estimate the time constant of changes in oxygen uptake at the onset of square-wave 120 W cycloergometric exercise were evaluated in seven subjects. The volume of oxygen taken up at the alveoli (VO_2Ai) was determined breath-by-breath (BB) from the volume of O₂ transferred at the mouth (VO_2mi) minus the corresponding volume changes in O₂ stores in the alveoli: VO_2Ai=VO_2mi–[V _Ai–1(FO_2Ai–FO_2Ai–1)+FO_2Ai·ΔV _Ai], where V _Ai–1 is the alveolar volume at the end of the previous breath, FO_2Ai and FO_2Ai–1 are estimated from the fractions of end-tidal O₂ in the current and previous breaths, respectively, and ΔV _Ai is the change in volume during breath i. These quantities can be measured BB, with the exception of V _Ai–1 which must be assumed. The respiratory cycle has been defined as the time elapsing between identical fractions of expiratory gas in two successive breaths. Using this approach, since FO_2Ai=FO_2Ai–1, any assumption regarding V _Ai–1 becomes unnecessary. In the present study, VO_2Ai was calculated firstly, by using this approach, and secondly by setting different V _Ai–1 values (from 0 to FRC+0.5 l, where FRC is the functional residual capacity). Values for alveolar O₂ flow (V˙O_2Ai), as calculated from the quotient of VO_2Ai divided by breath duration, were then fitted bi-exponentially. The time constant of the phase II kinetics of V˙O_2Ai (τ₂) was linearly related to V _Ai–1, increasing from 36.6 s (V _Ai–1=0) to 46.8 s (V _Ai–1=FRC+0.5 l) while τ₂ estimated using the first approach amounted to 34.3 s. We concluded that, firstly, the first approach allowed us to calculate V˙O_2A during transitions in step exercise; and secondly, when using methods wherein V _Ai–1 must be assumed, τ₂ depended on V _Ai–1.