Optimizing ventricular fibers : uniform strain or stress, but not ATP consumption, leads to high efficiency

The aim of this work is to investigate the influence of fiber orientation in the left ventricular (LV) wall on the ejection fraction, efficiency and the heterogeneity of the distributions of developed fiber stress, strain and ATP consumption. A finite element model of LV mechanics was used with active properties of the cardiac muscle described by the Huxley-type cross-bridge model. The computed variances of sarcomere length (VarSL), developed stress (VarDS) and ATP consumption (VarATP) have several minima at different transmural courses of helix fiber angle. We identified only one region in the used design space with high ejection fraction, high efficiency of the LV and relatively small VarSL, VarDS, and VarATP. This region corresponds to the physiological distribution of the helix fiber angle in the LV wall. Tra! nsmural fiber angle can be predicted by minimizing VarSL and VarDS, but not VarATP. If VarATP was minimized then the transverse fiber angle was considerably underestimated. The results suggest that ATP consumption distribution is not regulating the fiber orientation in the heart.