FIGURE 6.

A: an example of the changes in nitrogen concentration measured at the mouth during a single breath nitrogen washout (SBNW). The expiration that follows the inspiration of a single deep breath of 100% oxygen produces a flat phase 1 as the dead space empties. This is followed by an upward sloping phase 2 where the nitrogen concentration rises swiftly followed by a much more slowly rising alveolar plateau (phase 3). That is followed by phase 4 produced by closure of the airways supplying the better ventilated regions of the lung and a greater contribution from the regions that are less well ventilated which have a higher concentration of nitrogen. B: a multiple breath nitrogen washout curve (MBNW). Tidal volume (bottom panel) and expired nitrogen (top panel) are plotted against breath number. Each expiration has its own alveolar plateau. The alveolar plateaus for the 1st and 20th breath are shown in the inset normalized for the mean expired nitrogen concentration. The initial phase 3 slope is influenced by inhomogeneity in the acinar compartment, and the progressive increase in slope as a function of breath number is influenced by convective inhomogeneity. Calculating Sacin and Scond can indicate whether the origin of the inhomogeneity is in the peripheral or more central airways. [B from Verbanck et al. (127).]