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

During spontaneous breathing at rest, the alveolar pressure (P_alv)-flow (V̇) relation exhibits a prominent expiratory loop in many chronic obstructive pulmonary disease (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 that 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 P_alv on V̇ during expiration (A_exp). Before BD, A_exp was 360% greater in FL (n = 35) than in NFL (n = 25) patients (P < 0.001). After BD, A_exp was unchanged in NFL patients (ΔA_exp 0%, P = 0.882) and slightly decreased in FL patients who remained FL (n = 32, ΔA_exp -17%, P = 0.064). Three FL patients became NFL after BD, and their A_exp decreased markedly (ΔA_exp -61%), reaching values similar to those observed in NFL patients at baseline. In conclusion, the greater A_exp 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, A_exp can be used as a predictor of the presence of tEFL.NEW & NOTEWORTHY In stable chronic obstructive pulmonary disease (COPD) patients spontaneously breathing at rest, tidal expiratory flow limitation is the major determinant of the occurrence of expiratory looping in the plethysmographic flow-alveolar pressure diagram. In these patients the magnitude and the characteristics of the loop can be used as predictors of the presence of tidal expiratory flow limitation.