Although there is still no unequivocal proof, several authors indicated that PVR appears to be the major determinant of cardiac output in Fontan patients at rest and during exercise. A low PVR is even more important in these patients as they do not have the possibility for adapting to a higher afterload, whereas in patients with a subpulmonary ventricle, increased afterload will be countered by RV hypertrophy. It has been suggested that Fontan patients having low PVR remain stable for many decades, whereas a high PVR appears to be poor prognostic factor.