Article ID: RP010807
Although closing volume is regarded as a clinical test for early detection of peripheral airway closure, its ground is not clear. There have been no simulation studies for phase IV in the single-breath nitrogen washout (SBNW) curve in spite that several mathematical models for phase III have been proposed. We modeled the lung tissue deformation during slow expiration in which the lung tissue was regarded as a porous elastic body like a sponge. We assigned the maximum tissue density of lung parenchyma over which the lung tissue could not be contracted, according to several experimental reports in literatures. SBNW curves were then simulated by computing expired air volumes and its nitrogen concentrations for respective acini in the lung model. The simulated SBNW curves well reproduced phase IV, cardiac oscillation, and its postural changes. We found that the higher lung compliance increased closing volume but decreased residual volume. The smaller maximum tissue density generated larger closing volume and larger residual volume. It suggested that phase IV reflected the alveolar contractility and the increase of closing volume in emphysema could be explained by insufficient contraction of alveoli. We also found that the distribution of maximum tissue density affected the onset of Phase IV. A constant value of maximum tissue density generated a clear onset of phase IV, but a wide distribution of maximum tissue density corresponding to peripheral airway closure obscured its onset. It suggested that the airway closure was not necessary for phase IV appearance both in normal and emphysematous lung.
