Effect of edema and height on bronchial diameter and shape in excised dog lung.

Mechanical interactions among the artery, bronchus, and lung parenchyma cause the bronchus to be pulled from a circular cross-section in such a way that the largest diameter of the bronchus ( Db2 ) lies along a line joining the centers of bronchus and artery. To effect these interactions, the peribronchovascular interstitial space must transmit stresses from the lung parenchyma to the artery and bronchus. To test how interstitial edema affects the interdependence between the artery and the bronchus, we measured orthogonal diameters ( Db2 , Db1 ) from tantulum bronchograms as a function of edema formation at a fixed transpulmonary pressure (Ptp). As lobe weight increased to three times normal, Db2 / Db1 decreased from 1.08 to 1.0 at a Ptp of 6 and 25 cm H2O. Cross-sectional area decreased only at a Ptp of 6 cm H2O. We conclude (1) that peribronchovascular interstitial (Pi) pressure became more uniform with edema present, causing the bronchus to assume a more circular shape, and (2) that it increased, causing bronchial cross-sectional area to decrease at a Ptp of 6 cm H2O. To determine whether Pi is uniform with height, in a separate group of lungs we measured Db1 as a function of height in two configurations: hilus-dependent, Db1 (d) and then inverted or hilus-nondependent, Db1 (nd). Db1 was unaffected by lobe inversion at 25, 10, 6 and 4 cm H2O Ptp. At a Ptp of 2 cm H2O, the difference Db1 (d) - Db1 (nd) was positive near the hilus, decreased to zero in the lung periphery, and increased with edema. This bronchial distortion due to lobe inversion was consistent with the effect of gravitational forces on lung parenchyma as modeled by a finite-element analysis, as was opposite to that predicted by a vertical hydrostatic gradient in Pi.