Influences of parenchymal tethering on the reopening of closed pulmonary airways.

We investigated the influence of parenchymal tethering on the reopening of collapsed pulmonary airways. Reopening experiments were performed with freshly excised canine lobes placed in a vacuum chamber with pleural pressure (Ppl) set by vacuum pressure. Noncartilaginous 2- to 3-mm airways were collapsed by suction and remained collapsed on subsequent atmospheric pressure equalization. The airway was reopened by constant-flow insufflation, and peak pressure (Ppeak) needed to reopen the collapsed airway was measured. Yield pressure needed to begin axial meniscus motion decreased markedly at Ppl = -7.5 cmH2O, indicating a possible change in airway-meniscus configuration from compliant collapse to meniscus occlusion, thus promoting onset of reopening. Two distinct types of reopening behavior were observed: unstable low-frequency fluttering phenomenon characteristic of small magnitudes of Ppl in which airway tended to recollapse after being reopened and stable reopening phenomena at larger magnitudes of Ppl in which airway remained patent after it was reopened. Stable reopening was always observed at Ppl < or = -7.0 cmH2O. Effective transmural pressure (=Ppeak - Ppl) required to reopen airway and subsequent postreopening airway pressure, reflecting airway and collateral resistance, decreased with increasing magnitudes of Ppl due to increased influence of parenchymal tethering. However, at Ppl < -8.0 cmH2O, an increase in lung volume did not result in a reduction of effective transmural pressure, possibly indicating full airway distension and influence of airway wall hoop stress.