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 subjects 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 (FA) change at constant volume plus the volume change (deltaVAi) at constant fraction i.e. VAi-1(FAi-FAi-1)+FAi x deltaVAi, where VAi-1 is the end-expiratory volume at the beginning of the breath. These quantities, except for VAi-1, can be measured on a single-breath (breath-by-breath) basis and VAi-1 set equal to the subject's functional residual capacity (FRC, Auchincloss model). Alternatively, the respiratory cycle can be defined as the interval elapsing between two equal expiratory gas fractions in two successive breaths (Grønlund model G). In this case, Ft1 = Ft2 and thus the term VAi-1 (FAi-FAi-1) vanishes. In the present study, average alveolar O2 uptake (VO2,A) and CO2 output (VCO2,A) were equal in both approaches whereby the mean signal-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 the questionable assumption that the inter-breath variability of alveolar gas transfer is minimal. It is concluded that the only promising approach for assessing "true" single-breath alveolar gas transfer is that originally proposed by Grønlund.