Abstract
After creating a Fontan circuit, control of the circulation is shifted upstream from the ventricle to the newly created Fontan portal system. The goal of this review was to illustrate that the customary laws of biventricular cardiac output no longer apply and explain why standardized cardiac failure treatment regimens have little or no effect on a failing Fontan patient. A Fontan circulation is, in effect, a circulation in series regulated by the basic rules of any hydrodynamic circuit. We developed a formula that elucidates how flow through the critical bottleneck, and therefore through the whole circuit, is controlled. The critical bottleneck in a hydrodynamic model is the prime determinant of overall flow; other (less critical) bottlenecks may control local upstream congestion, but not overall flow. Once relieved, control of flow shifts to the next most significant bottleneck. The available options for improving flow in a hydrodynamic model are identical to those applicable to any dam: tackle the obstruction (the most impactful approach), push harder upstream (the easiest action) or pull/suck further downstream of the bottleneck (the least efficient strategy). In the early stages, the Fontan neo-portal circulation plays a pivotal role in the pathophysiology. The ventricle has little effect and has an impact only at a late stage. The Fontan formula in the present article stands as a valuable tool, aiding physicians in comprehending the pathophysiological and hydrodynamic intricacies of the Fontan circuit within the context of everyday clinical practice.