Figure 1

Study flow from database searches to inclusion oftrials. 40 trials were excluded after title and abstract screening because they obviously did not fulfill the inclusion criteria (not patients with COPD, patients with unstable COPD, short-acting bronchodilators, treatment duration < 4 weeks, no exacerbations ascertained). Reasons for exclusion for the 20 studies excluded after full text assessment are listed in Additional file 2.