Group 1: idiopathic, HIV, meds for weight loss, connective tissue disease, liver disease, congenital heart dz, schistosomiasis
Group 2: Related to left heart failure
Group 3: Related to severe copd, ILD, OSA
Group 4: Related to chronic thrombo-embolic disease
Group 5: Mixed - including sarcoid, p vera, essential thrombocytosis, other