New acquisitions in the assessment of breath-by-breath alveolar gas transfer in humans

We summarise recent results obtained in testing some of the algorithms utilised for estimating breath-by-breath (BB) alveolar O₂ transfer (VO_2A) in humans. VO_2A is the difference of the O₂ volume transferred at the mouth minus the alveolar O₂ stores changes. These are given by the alveolar volume change at constant O₂ fraction (F _AiO₂ ΔV _Ai) plus the O₂ alveolar fraction change at constant volume [V _Ai−1(F _Ai−F _Ai−1)O₂], where V _Ai−1 is the alveolar volume at the beginning of the breath i. All these quantities can be measured BB, with the exception of V _Ai−1, which is usually set equal to the subject’s functional residual capacity (FRC) (Auchincloss algorithm, AU). Alternatively, the respiratory cycle can be defined as the time elapsing between two equal O₂ fractions in two subsequent breaths (Grønlund algorithm, GR). In this case, F _AiO₂=F _Ai−1O₂ and the term V _Ai−1(F _Ai−F _Ai−1)O₂ disappears. BB alveolar gas transfer was first determined at rest and during exercise at steady-state. AU and GR showed the same accuracy in estimating alveolar gas transfer; however GR turned out to be significantly more precise than AU. Secondly, the effects of using different V _Ai−1 values in estimating the time constant of alveolar O₂ uptake (V̇O_2A) kinetics at the onset of 120 W step exercise were evaluated. V̇O_2A was calculated by using GR and by using (in AU) V _Ai−1 values ranging from 0 to FRC +0.5 l. The time constant of the phase II kinetics (τ₂) of V̇O_2A increased linearly, with V _Ai−1 ranging from 36.6 s for V _Ai−1=0 to 46.8 s for V _Ai−1=FRC+0.5 l, whereas τ₂ amounted to 34.3 s with GR. We concluded that, when using AU in estimating V̇O_2A during step exercise transitions, the τ₂ value obtained depends on the assumed value of V _Ai−1.