Inclusion Criteria:  Adults aged 18 or older. Diagnosed with idiopathic, heritable, simple congenital heart defect, or drug- or toxin-associated pulmonary arterial hypertension (PAH) according to World Health Organization consensus recommendations. Stable PAH-specific medication regimen for three months prior to enrollment. Subjects with only a single diuretic adjustment in the prior three months will be included. Adjustments in IV prostacyclin for side effect management are allowed. FEV1> or = 60% predicted and no more than mild abnormalities on lung imaging WHO Functional Class II-IV Ambulatory  Exclusion Criteria:  Prohibited from normal activity due to wheelchair bound status, bed bound status, reliance on a cane/walker, activity-limiting angina, activity-limiting osteoarthritis, or other condition that limits activity Pregnancy Diagnosis of PAH etiology other than idiopathic, heritable, simple congenital heart defect, or associated with drugs or toxins Drug and toxin associated PAH patients with active drug use Prior diagnosis of cirrhosis Malignancy eGFR by MDRD <60mL/min Known allergy to l-carnitine supplements