Bronchodilators increase airway instability in cystic fibrosis

Introduction The maximal expiratory flow-volume (MEFV) curve is a plot of maximal expiratory flow (VEmax) against lung volume (1). The VEmax, at all but the largest lung volumes, is considered fixed and cannot be exceeded by further increasing transpulmonary pressure (2-4). However, supramaximal expiratory flow transients may develop during maximal voluntary ventilation and when forced expiration is initiated from less than total lung capacity (5, 6). This phenomenon is explained by unequal time constants in different parts of the lung, which empty at different rates. It was shown that the rapidly expired volume originates in the suddenly compressed downstream segment of the anatomic dead space (6). By superimposing a partial expiratory flow-volume (PEFV) curve onto a MEFV curve, the partial curve can be divided into 2 parts (figure 1). The first is the "overshooting" flow transient with its end point where the PEFV tracing returns to the MEFV curve. The volume expired in this portion comes in part from the parenchyma but is augmented by air from large airways that were dilated by the preceding inspiration. The more distensible these airways, the larger this volume of airway contribution (VAC). In the second portion of the forced expiration, which is characterized by the identical course of the MEFV and the PEFV curves, the whole volume originates in the parenchyma. Consequently, any increase in airway compliance (distensibility), e.g., by bronchial smooth muscle relaxation, should result in an increased contribution of the airway-related volume to the forced expiration. Increased expiratory volumes and flow rates, measured by routine pulmonary function tests (PFT) after bronchodilator medication, may represent such extra volume expired from a more compliant dead space. On the other hand, decreased bronchomotor tone reduces the ability of the airways to withstand dynamic compression during forced expiration (7-9), which should result in deSUMMARY Supramaximal flow transients of partial expiratory flow-volume curves are caused by a rapidly emptying compartment. By superimposing a maximal and a series of partial expiratory