Study to Assess Acute Vasodilation Response of Inhaled Nitric Oxide

This study will be divided into 2 parts: a Pilot Phase followed by Pivotal Phase. * The Pilot Phase will enroll up to 20 subjects. The information derived from the Pilot Phase will evaluate safety, and assess the vasoreactivity test response rate. * Approximately 150 subjects will be enrolled in the Pivotal Phase of the study, to gather further data on weaned successes from parenteral PGI. The Investigator and study staff involved with the wean and patient management will be blinded in this phase of the study to the vasoreactivity results.