This study plans to learn more about activity levels in children with pulmonary hypertension. Pulmonary hypertension is a condition where the pressure in the lungs is higher than normal. This can affect the person's heart. The purpose of this study is to see if measuring activity in children with pulmonary hypertension and comparing it to activity in children without pulmonary hypertension can give their doctor helpful information on how they are feeling and how their treatment is working.

Inclusion criteria for children with PAH:

1. Ages 0-6 years at the time of consent
2. Current diagnosis of pulmonary hypertension in World Health Organization (WHO) Diagnostic Group 1 as per established clinical criteria including prior catheterization meeting Group 1 criteria
3. Panama Functional Class II-IIIa
4. Must have been receiving an approved oral endothelin receptor antagonist, calcium channel blocker, phosphodiesterase 5 inhibitor, prostanoid and/or soluble guanylate cyclase stimulator for at least 30 days prior to consent, and has been at the current stable dose, other than weight-based adjustments, for at least 30 days prior to consent
5. On stable doses of other medical therapy for 14 days prior to enrollment visit with no dose adjustments, additions, or discontinuations (exception diuretics and anticoagulants; OTC/cold/seasonal allergy medications).

Exclusion criteria for children with PAH:

1. Diagnosis of congenital diaphragmatic hernia, or a chronic lung disease, such as bronchopulmonary dysplasia, or interstitial lung disease

2. Any bone (e.g., osteogenesis imperfecta, ankle, knee, or hip injuries), neuromuscular (e.g., muscular dystrophy), or other pathology that may limit activity (e.g., arthritis)

3. Down syndrome

4. Use of any medications known to limit activity (e.g., sedative)

5. Active infection (may re-screen for enrollment once resolved)

6. Any other cardiovascular, liver, renal, hematologic, gastrointestinal, immunologic, endocrine, metabolic, or central nervous system disease or condition that, in the opinion of the Investigator, may adversely affect the safety of the participant or interfere with the interpretation of study assessments

   Examples:

   1. Current diagnosis of uncontrolled sleep apnea as defined by their physician
   2. Severe renal insufficiency as defined by the requirement for dialysis at screening
   3. Moderate to severe hepatic dysfunction defined as elevated aspartate aminotransferase (AST) or alanine aminotransferase (ALT) liver function tests, greater than or equal to three times the upper limit of normal at screening

7. Actively listed for transplantation

8. Patient and/or legal guardian has/have an unstable psychiatric condition or is/are mentally incapable of understanding the objectives, nature, or consequences of the trial, or has any condition in which the Investigator's opinion would constitute an unacceptable risk to the participant's safety

Inclusion criteria for control children:

1. Ages 0-6 years at the time of consent
2. In good general health as evidenced by medical history reported by parent/legal guardian during screening and/or available medical records

Exclusion criteria for control children:

1. Known diagnosis of asthma (controlled or uncontrolled)
2. Inability to exercise normally either due to an intellectual disability (e.g., Down Syndrome), or a physical disability which could impede activities of daily living.
3. Child and/or legal guardian has/have an unstable psychiatric condition or is/are mentally incapable of understanding the objectives, nature, or consequences of the trial, or has any condition in which the Investigator's opinion would constitute an unacceptable risk to the participant's safety

PH patients of PIs seen in cardiology clinics, and a community sample of healthy control participants