Nature and extent of the burden and risks associated with participation, benefit and group relatedness: All investigations, except blood analysis, are non-invasive and free of risk. The burden for the patients mainly consists of the time that is consumed by the visits to the clinic. At these visits time will be consumed by: history taking and physical investigation (15 minutes); quality of life score (15 minutes); laboratory tests (6 times venopuncture, total amount of blood withdrawn approximately 90ml). Cardiopulmonary exercise testing (1hour), echocardiography (15 minutes) and CMR (45 minutes) are part of regular medical care. Adverse effects from losartan are usually limited and consist of dizziness due to hypotension, renal impairment, hyperkalemia and liver impairment. We expect no change or an increase in RV function in the intervention group compared to the control group over the two-year follow up period, which would be a great benefit for this young study population.