d) Clinical and laboratory data at the time of initial six-minute walk test

We collected data on demographics, smoking status, co-morbidities, New York Heart Association (NYHA) functional class, N-terminal pro B-type natriuretic peptide (NT-proBNP), pulmonary function tests (PFTs), imaging studies (CXR and CT chest), transthoracic echocardiography with agitated saline contrast, and diagnostic RHC at the time of the initial 6MWT. We carefully recorded whether patients were on PAH-specific therapies at the time of the initial 6MWT. Patients could be receiving PAH-specific therapies at the time of the initial 6MWT if they could not walk immediately before the initiation of treatment or patients had started on PAH-specific therapies at outside practices or institutions.