New perspectives in breath-by-breath determination of alveolar gas exchange in humans

Alveolar gas transfer over a given breath (i) was determined in ten subject s at rest and during steady-state cycling at 60, 90 or 120 W as the sum of volume of gas transferred at the mouth plus the changes of the alveolar gas stores. This is given by the gas fraction (F-A) change at constant volume plus the volume change (DeltaV(Ai)) at constant fraction i.e. VAl-1(F-Ai-FA i-1)+F-Ai.(Delta VAi), where VAi-1 is the end-expiratory volume at the begi nning of the breath. These quantities, except for VAi-1 can be measured on a single-breath (breath-by-breath) basis and VAi-1 equal to the subject's f unctional residual capacity (FRC, Auchincloss model). Alternatively, the re spiratory cycle can be defined as the interval elapsing between two equal e xpiratory gas fractions in two successive breaths (Gronlund model G). In th is case, F-t1=F-t2 and thus the term VAi-1 (F-Ai-FAi-1) vanishes. In the pr esent study, average alveolar O-2 uptake ((V) over dot O-2,O-A) and CO2 out put ((V) over dot CO2,A) were equal in both approaches whereby the mean sig nal-to-noise ratio (S/N) was 40% larger in G. Other approaches yield steady state S/N values equal to that obtained in G, although they are based on t he questionable assumption that the inter-breath variability of alveolar ga s transfer is minimal. It is concluded that the only promising approach for assessing "true" single breath alveolar gas transfer is that originally pr oposed by Gronlund.