New acquisitions in the assessement 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 O-2 transfer (VO2A) in humans. VO2A is the difference of the O-2 volume transferred at the mouth minus the alveolar O-2 stores changes. These are given by the alveolar volume change at constant O-2 fraction (FAiO2 DeltaV(Ai)) plus the O-2 alveolar fraction change at constant volume [VAi-1(F-Ai-FAi-1)O-2], where VAi-1 is the alveolar volume at the beginning of the breath i. All these quantities can be measured BB, with the exception of VAi-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 O-2 fractions in two subsequent breaths (Gronlund algorithm, GR). In this case, FAiO2=FAi-1O2 and the term VAi-1(F-Ai-FAi-1)O-2 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 VAi-1 values in estimating the time constant of alveolar O-2 uptake ((V)over dotO(2A)) kinetics at the onset of 120 W step exercise were evaluated. (V)over dotO(2A) was calculated by using GR and by using (in AU) VAi-1 values ranging from 0 to FRC +0.5 l. The time constant of the phase II kinetics (tau(2)) of (V)over dotO(2A) increased linearly, with VAi-1 ranging from 36.6 s for VAi-1=0 to 46.8 s for VAi-1=FRC+0.5 l, whereas tau(2) amounted to 34.3 s with GR. We concluded that, when using AU in estimating (V)over dotO(2A) during step exercise transitions, the tau(2) value obtained depends on the assumed value of VAi-1.