The aim of the study is to investigate the effect of tiotropium from different devices on a panel of small (IOS, MBNW, DLCO, FVC) and large airway (FEV1, PEF) responses in patients with mild-moderate COPD. Comparisons will be made between Tiotropium Handihaler 18 micrograms once daily and Tiotropium Respimat 5 micrograms once daily