Pressure loss from flow energy dissipation: relevance to Fontan-type modifications

Pressure loss from flow energy dissipation may impair cardiac performance when a heart with a single ventricle must support the circulation. Therefore, the goal of this study was to use a simple description of fluid motion to provide insight into flow energetics relevant to Fontan-type procedures. Our findings indicate that when either the cross-sectional area or the axial direction of flow changes "abruptly," disturbances are set up within the fluid that lead to dissipation of available energy. The theoretical pressure losses associated with these flow disturbances were described by relating the initial and final average velocities of the streams to anatomical features within the fluid's connection pathway causing obstruction to flow, e.g., the ratio of diameters characterizing a change in the cross-sectional area and/or the angle governing an alteration in the axial direction of the flow. Significant pressure losses were found in situations in which the "magnitude" of the fluid's velocity is suddenly changed as flow enters or leaves a large chamber or when the "direction" of the fluid's velocity is acutely altered as flow negotiates a sharp bend in a vessel or conduit. We found the Bernoulli equation to be inaccurate when predicting the corresponding changes in pressure under these conditions. In view of these findings, we discuss operative strategies aimed at avoiding pressure losses, thus aiding univentricular heart function by conserving flow energy.