Sildenafil is a phosphodiesterase-5 enzyme inhibitor with multiple uses. When dosed three times daily at 5 mg per dose, it is used for the treatment of pulmonary arterial hypertension. Decreased work of breathing indicate the patient is experiencing the intended effect of the drug. As needed dosing of sildenafil is used for erectile dysfunction. Improved hypercapnia and decreased systolic blood pressure are not the intended effects of sildenafil.