Inclusion Criteria:

• Male or female, age 3-18 years
• Diagnosis of PAH due to IPAH, connective tissue disease, or congenital heart disease
• Pulmonary arterial hypertension confirmed by a cardiac catheterization, with mPAP > 25 mmHg at rest, PCWP or LVEDP ≤ 15 mmHg, and PVR ≥ 240 dynesseccm-5 (ie, ≥ 3.0 unitsm2)
• NYHA/WHO functional Class II, III, or IV
• Have had a 6-minute walk test (6-MWT) performed within the 3 months preceding the screening visit (if  8 years of age)
• Have had cardiopulmonary exercise testing (CPET) performed within the 3 months preceding the screening visit (if  8 years of age and > 130 cms in height. CPET testing will be conducted on NYHA/WHO Class IV patients at the discretion of the Principal Investigator)
• If receiving therapy for PAH, on a stable dose and regimen for at least 3 months prior to the screening visit
• If receiving digoxin, diuretic, or oxygen therapy, on a stable dose and regimen for at least 30 days preceding the screening visit
• Must demonstrate the ability to appropriately use the nebulizer device as part of the screening process

Exclusion Criteria:

• Prior use of prostacyclins or prostacyclin analogues within 3 months prior to screening
• Portal hypertension or clinically relevant hepatic disease
• Eisenmenger syndrome with resting SpO2 <88% on room air
• Clinically relevant asthma or other chronic lung disease, such as bronchopulmonary dysplasia, cystic fibrosis, or repaired or unrepaired congenital diaphragmatic hernia
• Diagnosis or current evidence of active lung infection or clinically relevant hemoptysis
• Left-sided heart disease, including valvular disease or heart failure
• Initiation or dosage adjustment of PAH-specific therapy within 3 months prior to screening
• Initiation of treatment with digoxin, diuretics, anticoagulation, or oxygen therapy within 30 days prior to screening