Effect of lung volume on collateral ventilation in the dog

We studied the effect of lung volume on resistance through collateral pathways (Rcoll) and small airways (Rsaw) before and after the injection of methacholine into obstructed segments of intact dogs. Before methacholine, Rcoll decreased 15.0 ± 4.9 (SE)% per cmH₂O increase in transpulmonary pressure (Ptp) and Rsaw decreased 5.1 ± 7.0 (SE)% per cmH₂O increase in Ptp. Following methacholine, lung inflation resulted in similar decreases in Rcoll and Rsaw. The fall in Rcoll was significantly greater than the fall in Rsaw. When pressure in an obstructed segment (Ps) was increased with constant Ptp (nonhomogeneous inflation), Rcoll fell approximately half as much for each cmH₂O increase in pressure compared to when Ptp was increased (homogeneous inflation). We conclude 1) that increases in lung volume have small effects on Rsaw so that there is a relative increase in flow through collateral channels serving obstructed portions of lung and 2) that Rcoll is a function of the size of the obstructed segment that increases more under homogeneous than nonhomogeneous conditions.