As mentioned earlier, transpulmonary flow in Fontan patients is dependent on the presence of low PVR and normal behavior of PVR during exercise in order to be able to increase cardiac output. An increase in PVR during exercise would cause a more pronounced underfilling of the systemic ventricle with subsequently less increase or even decrease in stroke volume. Both pressure and flow measurements have been shown feasible in patients with congenital heart disease at rest and during exercise.