Assessment of airway occlusion by single breath N2 test and deflation lung PL-V curve in healthy subjects and COPD patients

Cyclic airway closing and opening may induce lung injury in both normal and diseased lung. During tidal breathing this phenomenon occurs when closing volume (CV) exceeds the expiratory reserve volume. In humans, CV is currently assessed from the onset of phase IV of the single breath nitrogen test (CVSBN), but this method fails in severe chronic obstructive pulmonary disease (COPD) patients in whom phase IV can be absent, even if the amount of airway closure should be substantial. At present, no agreement exists whether an another potential indicator of airways closure, the inflection point on the transpulmonary pressure–volume curve (CVPL-V) can be used as a surrogate of CVSBN, when phase IV is not detectable. The comparison of CVSBN with CVPL-V assessed during the same slow deflation showed that both in healthy subjects and in COPD patients, when CVSBN was present, it coincided with CVPL-V. In the young subjects in whom CVSBN was absent, CVPL-V was not detectable, in accordance to the notion that in these subjects lung recoil can be high enough to prevent airway closure in the whole vital capacity range. On the other hand, in the COPD patients with no phase IV, CVPL-V was constantly present, as expected. These results suggest that measurement of CVPL-V is a reliable method for closing volume assessment in both normal and COPD subjects. In the latter group, the measurement of CVSBN can lead to heavily underestimate the extent of the airway collapse.