Aerosol Derived Airway Morphometry at Different Levels of Lung Inflation

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Abstract
Monodisperse aerosol particles can be used to estimate intrapulmonary air-space dimensions. However, effective air-space diameters (EAD) measured by this technique as a function of volumetric lung depth are dependent on lung inflation, which may be different between subjects with different lung volumes. In an attempt to achieve better comparability of EAD among subjects, the influence of the level of lung inflation on air-space dimensions was investigated in a group of 9 healthy subjects. The results showed that EAD increased with lung volume nearly linearly in peripheral lung regions whereas the increase in proximal regions was steeper and more heterogeneous within the group. This result can be understood when two mechanisms for a change in air-space diameter in a fixed volumetric lung depth are assumed: (1) a real increase in air-space caliber, (2) an apparent increase induced by a change of the anatomical site associated with a fixed volumetric lung depth (proximal shift). When the volumetric lung depth is normalized to the actual lung volume a visible reduction of the influence of lung volume on the measurement of EAD is observed. Differences in EAD among individuals are now better detectable, even when lung volumes are not comparable. Although this normalization considerably reduces the inter- and intraindividual scattering of EAD, the observation that the magnitude of the change in EAD with lung volume is different for different volumetric compartments of the lungs indicates that the assumptions underlying the normalization approach are not valid in a strict sense. Therefore, the normalization procedure presented in this paper cannot be used for physiological studies of airway distension.