Comparison of Inhaled Nitric Oxide and Oxygen in Participants Reactivity During Acute Pulmonary Vasodilator Testing

Inclusion Criteria: 1. Must have any one of these three disease categories: * Idiopathic Pulmonary Arterial Hypertension * Mean pulmonary arterial pressure (PAPm) \> 25 millimeters of mercury (mmHg) at rest, pulmonary capillary wedge pressure (PCWP) ≤ 15 mmHg, and PVRI\> 3 u•m\^2 or diagnosed clinically with no previous catheterization * Congenital heart disease (CHD) with pulmonary hypertension repaired and unrepaired * PAPm \> 25 mmHg at rest and PVRI\> 3 u•m\^2 or diagnosed clinically with no previous catheterization * Cardiomyopathy * PAPm \> 25 mmHg at rest and Pulmonary vascular resistance index (PVRI)\> 3 u•m\^2 or diagnosed clinically with no previous catheterization 2. Scheduled to undergo right heart catheterization to assess pulmonary vasoreactivity by acute pulmonary vasodilation testing. 3. Male or female, ages 4 weeks to 18 years, inclusive 4. Signed informed consent/assent Exclusion Criteria: 1. Focal pulmonary infiltrates on chest radiograph. 2. Diagnosed with severe obstructive or restrictive pulmonary disease that is significantly contributing to the patient's pulmonary hypertension. 3. Received treatment with nitric oxide for inhalation within 30 days prior to study initiation, are on other investigational medications, nitroglycerin, sodium nitroprusside, sildenafil, other Phosphodiesterase type 5 (PDE-5) inhibitors, or prostacyclin 4. Pregnant \[urine human chorionic gonadotropin positive (HCG +)\] 5. Baseline Pulmonary capillary wedge pressure (PCWP) \> 20 mmHg