Asymmetric and Axisymmetric Constant Curvature Liquid-Gas Interfaces in Pulmonary Airways

Estimating the diameter of airways susceptible for collapse using crackle sound

Airways that collapse during deflation generate a crackle sound when they reopen during subsequent reinflation. Since each crackle is associated with the reopening of a collapsed airway, the likelihood of an airway to be a crackle source is identical to its vulnerability to collapse. To investigate this vulnerability of airways to collapse, crackles were recorded during the first inflation of six excised rabbit lungs from the collapsed state, and subsequent reinflations from 5, 2, 1, and 0 cmH(2)O end-expiratory pressure levels. We derived a relationship between the amplitude of a crackle sound at the trachea and the generation number (n) of the source airway where the crackle was generated. Using an asymmetrical tree model of the rabbit airways with elastic walls, airway vulnerability to collapse was also determined in terms of airway diameter D. During the reinflation from end-expiratory pressure = 0 cmH(2)O, the most vulnerable airways were estimated to be centered at n = 12 with a peak. Vulnerability in terms of D ranged between 0.1 and 1.3 mm, with a peak at 0.3 mm. During the inflation from the collapsed state, however, vulnerability was much less localized to a particular n or D, with maximum values of n = 8 and D = 0.75 mm. Numerical simulations using a tree model that incorporates airway opening and closing support these conclusions. Thus our results indicate that there are airways of a given range of diameters that can become unstable during deflation and vulnerable to collapse and subsequent injury.

The estimation of lung mechanics parameters in the presence of pathology: a theoretical analysis

Mechanical lung function is frequently assessed in terms of lung resistance (R (L)), lung elastance (E (L)), and airway resistance (R (aw)). These quantities are determined by measuring input impedance at various oscillation frequencies, and allow lung tissue resistance (R (t)) to be estimated as the difference between R (L) and R (aw). These various parameters change in characteristic ways in the presence of lung pathology. In particular, the ratio R (t)/E (L) (known as hysteresivity, (eta) has been shown both experimentally and in numerical simulations to increase when regional heterogeneities in mechanical function develop throughout the lung. In this study, we performed an analytical investigation of a two-compartment lung model and showed that while heterogeneity always leads to an increase in E (L), eta will increase only initially. When heterogeneity becomes extreme, eta stops increasing and starts to decrease. However, there are no experimental reports of eta decreasing under conditions in which heterogeneity would be expected to exist. We speculate that this is because liquid bridges invariably form across airway lumen that narrow to a certain point, thereby preventing them from achieving arbitrarily small non-zero radii. We also show that recruitment of closed lung units during lung inflation may lead to variables responses in both eta and E (L).