The ventricle of a functionally univentricular heart is dilated, hypertrophic, and hypocontractile, certainly after the completion of a Fontan circulation (reviewed in Reference). It is difficult, however, to determine the reasons for these abnormal ventricular conditions. The abnormal size of the ventricle, along with the less than adequate function, can be due to the congenital malformation itself, the previous surgical conditions, or the very abnormal working conditions of the ventricle during the various stages of palliation. Moreover, because of the varying morphology of the pumping ventricle in patients with a functionally univentricular arrangement, it is difficult to assess size, shape, hypertrophy, stress, strain, contraction and relaxation, both in cross sectional and serial follow-up.