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