Sildenafil is a phosphodiesterase-5 inhibitor that has been approved for the treatment of pulmonary arterial hypertension with orphan drug designation. Sildenafil modulates the nitric oxide (NO) pathway in the vessel wall. Since this pathway is impaired in pulmonary arteries of patients with pulmonary hypertension (PH) associated with chronic obstructive pulmonary disease (COPD), we hypothesized that sildenafil might improve pulmonary hemodynamics and increase exercise tolerance in this condition. However, in COPD sildenafil may also impair gas exchange due to the inhibition of pulmonary hypoxic vasoconstriction. The research project is aimed to evaluate these effects.