Adult PAH patients on a stable dose of an approved PAH medication will undergo the following baseline assessments: cardiac magnetic resonance imaging (MRI), right heart catheterization (RHC), echocardiogram, 6-minute walk test (6-MWT), plasma NT-ProBNP (biomarkers of RV function) and serum catecholamine (measure of sympathetic activation), and quality of life. Patients will be randomized to carvedilol (3.125 mg bid and escalated to 9.375 mg bid, as tolerated, over 3 months) or placebo in a 1:1 fashion. After 6 months, testing is repeated and patients are crossed over to the alternate treatment. Testing is repeated at the end of the study (month 13).