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

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 (VO2Ai) was determined breath-by-breath (BB) from the volume Of 02 transferred at the mouth (VO2mi)) minus the corresponding volume changes in O-2 stores in the alveoli: VO2Ai = VO2mi-[VAi-1(FO2Ai-FO2Ai-1) + FO(2Ai)DeltaV(Ai)], where VAi-1 is the alveolar volume at the end of the previous breath, FO2Ai and FO2Ai-1i are estimated from the fractions of end-tidal O-2 in the current and previous breaths, respectively, and DeltaV(Ai) is the change in volume during breath i. These quantities can be measured BB, with the exception of VAi-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 FO2Ai= FO2Ai-1, any assumption regarding VAi-1 becomes unnecessary. In the present study, VO2Ai was calculated firstly, by using this approach, and secondly by setting different VAi-1 values (from 0 to FRC+0.51, where FRC is the functional residual capacity). Values for alveolar O-2 flow (VO2Ai), as calculated from the quotient Of VO2Ai divided by breath duration, were then fitted biexponentially. The time constant of the phase 11 kinetics Of (V)over dotO(2Ai) (tau(2)) was linearly related to VAi-1, increasing from 36.6 s (VAi-1 = 0) to 46.8 s (VAi-1 = FRC + 0.51) while tau(2) estimated using the first approach amounted to 34.3 s. We concluded that, firstly, the first approach allowed us to calculate O-2A during transitions in step exercise; and secondly, when using methods wherein VAi-1 must be assumed, tau(2) depended on VAi-1.