The essence of a Fontan circuit is the creation of the Fontan “neoportal system,” which allows for oxygenation at near normal levels, but at the cost of a chronic state of systemic venous congestion and decreased cardiac output. The heart, while still the engine of the circuit, cannot compensate for this major flow restriction: the ventricle has lost control of the output and venous congestion. Systolic and diastolic ventricular dysfunction may aggravate the hemodynamic burden. The abnormal hemodynamics affects organs outside the heart and may lead to liver cirrhosis/malignancy, protein-losing enteropathy, or plastic bronchitis. The chronic low flow state causes an increase of pulmonary (and systemic) vascular resistance and ventricular filling pressures, causing failure of the Fontan to be progressive with increasing functional impairment.