Tidal expiratory flow-limitation induces expiratory looping of the alveolar pressure-flow relation in COPD patients

During spontaneous breathing at rest the alveolar pressure (Palv) - flow (V̇) relation exhibits a prominent expiratory loop in many COPD patients. Among the possible determinants of the loop, tidal expiratory flow-limitation (tEFL) may be the main responsible. To compare the characteristics of the expiratory loop in COPD patients with flow-limitation (FL) and without flow-limitation (NFL), tEFL was assessed with the negative expiratory pressure technique in stable, mild to very severe COPD patients, undergoing body-plethysmography before and after bronchodilation (BD), an intervention which is able to reduce mechanical heterogeneity, recruitment/derecruitment and gas trapping, but rarely abolishes tEFL. The magnitude of the expiratory loop was indexed by the integral of Palv on V̇ during expiration (Aexp). Before BD, Aexp was 360% greater in FL (n=35) than in NFL (n=25) patients (P<0.001). After BD, Aexp was unchanged in NFL patients (ΔAexp 0%, P=0.882) and slightly decreased in FL patients who remained FL (n=32, ΔAexp -17%, P=0.064). Three FL patients became NFL after BD, and their Aexp decreased markedly (ΔAexp -61%), reaching values similar to those observed in NFL patients at baseline. In conclusion, the greater Aexp measured in FL relative to NFL COPD patients, its relative invariance after BD when flow-limitation persists, and its fall when flow-limitation is abolished indicate that tEFL is a major determinant of the magnitude of the expiratory loop. Furthermore, Aexp can be used as a predictor of the presence of tEFL.