Inclusion Criteria:

• * CTEPH cohort:
• 1. Confirmed CTEPH based on having undergone PEA surgery.
• 2. Strongly suspected CTEPH based on right heart catheterization data showing mPAP\>26 and PCWP\<15, and imaging demonstrating chronic thromboembolic disease (either ventilation/perfusion scan, CT pulmonary angiography, or conventional pulmonary angiography).
• * PE cohort:
• 1. PE diagnosed by CT pulmonary angiography or ventilation/perfusion scan within the preceding 24 months.
• 2. No evidence of PH at the time of enrollment (based on absence of dyspnea, exertional chest pain, or exertional presyncope on history, lack of physical exam findings of PH or right heart failure, and lack of signs of PH on objective data including echocardiography and/or right heart catheterization).
• 3. No evidence of chronic thromboembolic disease at the time of enrollment (based on chronic appearing thrombus on CT pulmonary angiography or conventional pulmonary angiography, or based on prior high probability ventilation/perfusion scan showing a similar pattern of perfusion defects).
• * PAH cohort:
• 1. Diagnosis of group 1 PAH.
• 2. Prior normal or low probability ventilation/perfusion scan.
• 3. No reported history of prior VTE events.
• 4. If available, all prior evaluations for VTE (including CT pulmonary angiography and Doppler ultrasound exams) must also be negative.

Exclusion Criteria:

• * Under the age of 18