The effects of experimental airway obstruction on the distribution of ventilation

We studied various indices of ventilation distribution before and after airway occlusion (AO) was produced by insufflation of beads into bronchi of excised dog lobes. AO delayed multiple breath N2 washouts and markedly increased the slope of the alveolar plateau of the single breath N2 curves. To detect diffusion gradients, the HE/SF6 ratio was measured during expiration following a single breath of these gases. Before AO we found small gradients. However, after AO, we consistently found the initial gas HE rich and later samples SF6 rich. Absolute concentrations of both gases fell during expiration. Breathholding reversed both effects. Gas sampled from a catheter wedged into an airway of a normal lobe was consistently SF6 enriched. Thus, HE and SF6 must separate during inspiration, possibly by radial diffusion of HE, allowing SF6 to penetrate deeper into the alveoli. Diffusion equilibration across collateral channels is slow and the distribution of ventilation to the collaterally ventilated space is abnormal even though its tidal volume is normal