Patients with IPAH fall into two categories: those who respond to acute exposure to vasodilators, and those who don't. Nonresponders (about 95 percent of IPAH patients) have varying prognoses. The few who do respond improve with long-term therapy and have an excellent prognosis. This disparity in outcomes has fueled speculation that vasodilator-responsive and nonresponsive IPAH are distinct diseases.
This observational study of 14 patients with IPAH provides the strongest evidence to date that vasodilator-responsive IPAH is characterized by immediately reversible vasoconstriction, whereas nonresponsive IPAH involves irreversible vascular obliteration. Therefore, despite sharing the baseline hemodynamic profile of IPAH, responders and nonresponders represent different vascular phenotypes and perhaps different diseases.