The goal of this clinical trial is to learn if a specific type of exercise training (high intensity interval training) can improve exercise capacity in people with a congenital heart defect that required the creation of a new connection between the right ventricle and pulmonary artery. This includes people with a truncus arteriosus, pulmonary atresia with a ventricular septal defect or severe tetralogy of Fallot. This study focuses on people aged 12 to 45 years. The main questions it aims to answer are: * Can a 12-week home-based high intensity interval exercise training program increase the exercise capacity? * Can factors that predict whether or not the exercise training program can increase the exercise capacity in specific people be identified? Researchers will compare the results from the intervention group to the control group. Participants will be assigned to one of these two groups at inclusion. The control group will also receive the intervention, after the control period. Participants will: * Participate in a 12-week home-based exercise training program (3x30 minutes a week, digitally supervised); * Attend 2 or 3 study visits (which partially is standard care) (2 visits for the intervention group, 3 visits for the control group); * Each study visit includes: echocardiography, magnetic resonance imaging (MRI) of the heart, cardiopulmonary exercise testing (CPET), blood and feces sampling, and questionnaires on quality of life and physical activity.