Preinspiratory lung volume dependence of the slope of the alveolar plateau.

Nine healthy subjects performed He-SF6 single-breath washouts. The inspired gas contained 90% O2, 5% He and 5% SF6 and the slope of the alveolar plateau for N2, He and SF6 was computed. Each subject performed three times in the seated position four different maneuvers: inspiration of both 1 L and a full inspiration from a preinspiratory lung volume (PILV) equal to residual volume (RV) + 1 L and RV + 2 L. The main experimental observation was that for an inspired volume of 1 L, the slopes decreased by approximately a factor of 2 when PILV increased from RV + 1 L to RV + 2 L, the He-SF6 slope difference decreasing significantly more. Previous results can be explained if intraregional parallel units play an important role in the genesis of the alveolar plateau. Furthermore, comparisons with simulations of a multibranch-point model of the acinus suggest that those units are, at least in part, intra-acinar and that the acini do not expand homogeneously. The present work also suggests that the more informative maneuver to obtain data reflecting acinar behavior from single-breath washouts, consists in an inspiration not larger than 1 L from a PILV situated between functional residual capacity and closing volume.