Airway occlusion assessed by single breath N test and lung P-V curve in healthy subjects and COPD patients.

Purpose: To determine whether the analysis of the slow expiratory transpulmonary pressure-volume (P-V) curve provides an alternative to the single-breath nitrogen test (SBN) for the assessment of the closing volume (CV).

Methods: SBN test and slow deflation P-V curve were simultaneously recorded in 40 healthy subjects and 43 COPD patients. Onset of phase IV identified CV in SBN test (CV), whereas in the P-V curve CV was identified by: a) deviation from the exponential fit (CV), and b) inflection point of the interpolating sigmoid function (CV).

Results: In the absence of phase IV, COPD patients exhibited a clearly discernible inflection in the P-V curve. In the presence of phase IV, CV and CV coincided (CV/CV=1.04±0.04 SD), whereas CV was systematically larger (CV/CV=2.1±0.86).

Conclusion: The coincidence between CV and CV, and the presence of the inflection in the absence of phase IV indicate that the deviation of the P-V curve from the exponential fit reliably assesses CV.