Bronchial length and diameter behavior during bronchial collapse in excised dog lungs

We compared changes in bronchial diameters (Dbr) and lengths (Lbr) in six excised dog lobes at 4 volume levels during deflation from 30 cm H2O under two conditions. First, intrabronchial and alveolar pressures were kept equal to zero during lung deflation; then, bronchial collapse was induced by negative intrabronchial pressure at several constant lung volumes. During the two procedures the major intrapulmonary bronchi were isolated from the rest of the lung by 10-15 beads blocking all tributary bronchi as previously described. Lbr was estimated with a linear displacement transducer connected by a thin rod to the most distal bead. Mean Dbr was calculated from Lbr and bronchial volume at each lung recoil pressure (PL) and at each intrabronchial pressure (Pbr). When PL decreased from full inflation in Condition 1, Dbr and Lbr decreased almost proportionately; however, in Condition 2 when PL (and thus lung volume) was kept constant and Pbr was reduced from 0 to -80 cm H2O, Lbr was reduced by only 5% (mean) whereas Dbr was reduced by 70% (mean) from initial near-homogeneous values. This suggested that, due to the interdependence in intact lungs, Lbr in intact lungs is almost constrained by changes in lung volume. By contrast, Dbr can undergo large reductions and the stiffness of intact bronchi is related mainly to PL. Thus, the intact bronchus has some of the characteristics of a thick-walled cylinder (where the wall includes surrounding parenchyma) in which longitudinal strain is small by comparison with circumferential strain.