Tidal Expiratory Flow Limitation at Rest as a Functional Marker of Pulmonary Emphysema in Moderate-to-Severe COPD

Background: Tidal expiratory flow limitation (EFL) is a step of paramount importance in the functional decline in COPD. Among mechanisms contributing to EFL, loss of airway-parenchymal interdependence could mostly be involved. Aim: To assess if EFL is a functional marker more frequently linked to prevalent pulmonary emphysema rather than to prevalent chronic bronchiolitis in COPD patients with moderate-to-severe airflow obstruction. Methods: Forty consecutive stable COPD patients with FEV₁ between 59 and 30% of predicted were functionally evaluated by measuring spirometry, maximal flow-volume curve and lung diffusion capacity (DL_CO) and coefficient of diffusion (K_CO). EFL was assessed by the negative expiratory pressure (NEP) method both in sitting and supine position. Chronic dyspnea was also scored by modified Medical Research Council (mMRC) scale. Results: In sitting position 13 patients (33%) were flow limited (FL) and 27 were non-flow limited (NFL). Only FEV₁/FVC, FEV₁ and MEF_25–75% were different between FL and NFL patients (p < 0.01). In supine position, however, among NFL patients in sitting position those who developed EFL, had significantly lower values of DL_CO and K_CO (p < 0.05) and higher mMRC score (p < 0.01), but similar values of FEV₁ as compared to those who did not have EFL. Conclusions: In COPD EFL in sitting position is highly dependent by the severity of airflow obstruction. In contrast, the occurrence of EFL in supine position is associated with worse DL_CO and K_CO and greater chronic dyspnea, reflecting a prevalent emphysematous phenotype in moderate-to-severe COPD patients.