- ongoing therapy with parenteral (intravenous or subcutaneous) pulmonary vasodilators such as IV/SQ prostacyclin analogs (Epoprostenol - Flolan or Treprostinil - Remodulin or PGE1 - Alprostadil) or IV phosphodiesterase 5 inhibitors (sildenafil - Revatio) at the time of randomization. In addition, initiation of therapy with these two classes of parenteral medications during the first 24 hours of study drug initiation is not permitted and will be considered a protocol deviation. The risk of systemic hypotension is high during the first 24 hours of study-drug (milrinone) infusion and hence parenteral administration of other pulmonary vasodilators is avoided to minimize risk of hypotension.