Airway occlusion assessed by single breath N2 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 (PL-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 PL-V curve were simultaneously recorded in 40 healthy subjects and 43 COPD patients. Onset of phase IV identified CV in SBN test (CVSBN), whereas in the PL-V curve CV was identified by: a) deviation from the exponential fit (CVexp), and b) inflection point of the interpolating sigmoid function (CVsig). Results In the absence of phase IV, COPD patients exhibited a clearly discernible inflection in the PL-V curve. In the presence of phase IV, CVSBN and CVexp coincided (CVSBN/CVexp = 1.04 ± 0.04 SD), whereas CVsig was systematically larger (CVsig/CVexp = 2.1 ± 0.86). Conclusion The coincidence between CVSBN and CVexp, and the presence of the inflection in the absence of phase IV indicate that the deviation of the PL-V curve from the exponential fit reliably assesses CV.