Inspired volume dependence of the slope of the alveolar plateau.

Two series of experiments were performed in nine healthy subjects. In series I, 96 single breath tests were performed in two subjects. 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. The following experimental conditions were considered variable: inspired volume with preinspiratory lung volume equal to residual volume or functional residual capacity (FRC), both with and without 10 sec breath-holding. In series II, each of the other 7 subjects performed 12 single breath N2 tests in which the subject inspired both 1 L and an inspiratory capacity (IC) of O2 from FRC with without 10 sec breath-holding. The main experimental observation is that without breath-holding the absolute value of the slope of the alveolar plateau is larger for 1 L inspiration than for an IC inspiration, whereas this volume dependence was smaller after 10 sec of breath-holding. Based on Milic-Emili model of the lung and on simulations of a multibranch point model of the acinus we concluded that, in so far as these models adequately describe gas distribution in the lung, intraregional parallel units presenting a sequential convective flow play an important role in the genesis of the alveolar plateau.