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