The calculated sample size to reach a power of 0.80 is 83 patients in each arm. Patients with complex CHD and exercise intolerance are at increased risk for premature death and severe cardiac complications including arrhythmia needing treatment, heart failure and circulatory failure, pulmonary hemorrhage, pulmonary embolism and endocarditis. Hence, the likelihood of major adverse cardiac events during the study phase is considerably high. However, there is no evidence of functional worsening by low-level exercise. Smaller studies with patients with congenital heart disease and/or pulmonary hypertension did not report safety issues. Although sudden cardiac death is one of the leading modes of death in this population, it is extremely rare that sudden death occurs during exercise. Cardiac patients who are at specific risk for exercise-induced arrhythmia are not included into the study (i.e. patients with hypertrophic obstructive cardiomyopathy).