 Allowed medication There will be a 3-week run-in period during which patients on will receive open label ipratropium Atrovent MDI 20μg puff, 4 puffs QID and prn salbutamol Ventolin MDI 100μgpuff, 2 puffs every 3-4 hours PRN. Ipratropium will be allowed only during the run-in and washout periods. Ipratropium will be stopped twelve hours before study visits Visit 3, 4, and 6. 