This study looks at kids with chronic venous thromboembolic disease, which means they have blood clots in their veins or lungs. The study checks how their heart and lungs work and involves tests like MRIs (a type of scan) and exercise tests. Kids aged 8-21 can join if they had a clot or had surgery and were told to rest. The study takes place in Dallas and involves visits at week 12 and month 12. Participants need to fill out questionnaires and give blood samples.

• Participants visit study sites over a 3-day period at week 12 and month 12.
• Tests include exercise, lung and heart scans, and blood samples.
• Participants must be willing to follow the study rules and be healthy enough to do so.

To join, kids need to be healthy enough, speak English, and be okay with following study instructions. Kids cannot join if they have certain long-term illnesses, are pregnant, or have conditions that make exercise difficult. This study helps doctors understand how to treat kids with these types of blood clots better.