Sildenafil is used for pulmonary arterial hypertension (PAH). This is sort of like high blood pressure in the lungs. Tell your doc that you have shortness of breath, chest pains, nausea, dizziness and fatigue when you exercise and he should prescribe it for the intended use.
Just as a side note, Viagra's first intended uses were for lowering blood pressure and altitude sickness. Since then, it has become very popular in the mile high club.