Systemic venous pressures, which are approximately 5 mmHg in healthy controls at rest and remains unchanged during exercise, are higher in Fontan patients. In Fontan patients, the absence of a right ventricle for pressure generation, some degree of congestion is required in order to force transpulmonary flow. However, at rest pressures above 20 mmHg are rarely seen as such pressures would lead to complications (oedema, pleural effusions and ascites). Interestingly, pressure change observed during exercise in normals and Fontan patients is quite similar. In healthy controls, an increase in mean pulmonary artery pressure from 15 mmHg at rest to 30 mmHg during exercise. In Fontan patients, mean right atrial pressure increased from 15 mmHg at rest to 25 mmHg during exercise.