In pulmonary arterial hypertension ( PAH ) , adding oral sildenafil to intravenous epoprostenol improved 6-minute walk distance ( 6MWD ) and hemodynamics and delayed time to clinical worsening in a 16-week randomized , placebo-controlled trial ( Pulmonary Arterial Hypertension Combination Study of Epoprostenol and Sildenafil [ PACES-1 ] ) .