VENTILATION HETEROGENEITY IN THE ACINAR AND CONDUCTIVE ZONES OF THE NORMAL AGEING LUNG Online Supplement

1. Principle and computation of Sacin and Scond from phaseIII slope analysis : The MBW normalized slope analysis technique identifies the various mechanisms of ventilation heterogeneity generating a phaseIII slope, attributing these to either the conductive airways (where convection predominates) or the acinar air spaces (where diffusion and convection interact). In the conductive lung zone, a non zero phaseIII slope is due to flow asynchrony between lung units with a different specific ventilation (by differential convection into each unit). Around the entrance of the acinus, a structural asymmetry in terms of heterogeneity in the cross section of any two daughter branches, or in the volume of the two subtended units, results in a non zero phaseIII slope, by the interplay of convection and diffusion. The solid lines in the right panel of Figure OS1 illustrate that the former mechanism generates a steady increase in Sn, while the latter quickly obtains a horizontal Sn asymptote. Considering these two Sn constituents for each breath to be approximately additive, it is possible to define two characteristic indices Sacin and Scond representing ventilation heterogeneity generated at branch points in the acinar and conductive lung zones, respectively (left panel of Figure OS1). Scond can then defined as the slope of the regression line of Sn versus