Copyright ©ERS Journals Ltd 1998 European Respiratory Journal

Bronchodilating agents are commonly used in patients with chronic obstructive pulmonary disease (COPD) to reduce airway obstruction [1]. Although many COPD pati-ents may obtain substantial increase in airflow after in-halation of bronchodilators [2], one-third of them do not show significant acute spirometric changes [3]. It is a common tenet, however, that improvements in symptoms and exercise capacity may occur even in the absence of spirometric improvement [4]. Such findings are frequent in patients with relatively severe COPD (forced expiratory volume in one second (FEV1) <50 % pred), who often ex-hibit expiratory flow limitation (EFL) during resting brea-thing, i.e. their tidal expiratory flow is maximal under the prevailing condition [5]. In the presence of EFL, the expiratory flow can increas