Diffusion-dependent contribution to the slope of the alveolar plateau

Two healthy subjects performed single breath tests, the inspired gas containing 90% O2, 5% He and 5% SF6. The slope of the alveolar plateau was computed for N2, HE and SF6. The following experimental conditions were considered variable: (1) preinspiratory lung volume (PILV) with inspired volume (VT) equal to 1 L, (2) VT with PILV equal to functional residual capacity (FRC) with and without 10 sec of breath-hold (BH) time, (3) VT with PILV = FRC + 1 L, (4) flow with VT = 1 L and PILV = FRC with and without 10 sec end inspiration BH and (5) BH time with VT = 1 L and PILV = FRC. The slope dependence on the different variables was compared with simulations performed with two multibranch point models of the human acinus with different asymmetries. The model simulations give a coherent picture of all the experimental observations. The present work gives support to previous analyses which suggest that inter-dependence of transport by convection and diffusion in the periphery of the lung is a major component of the slope of the alveolar plateau.