Distribution of active fiber stress at the beginning of ejection depends on left-ventricular shape

Left-ventricular shape is an important determinant of regional wall mechanics during passive filling. To examine the influence of left-ventricular shape for the ejection phase, the distribution of active fiber stress at the beginning of ejection was calculated in a finite element study. Hereto, finite element models were constructed with varying left-ventricular shapes, ranging from an elongated ellipsoid to a sphere, but keeping the initial cavity and wall volume constant. A realistic transmural gradient in fiber orientation was assumed. The passive myocardium was described by an incompressible hyperelastic material law with transverse isotropic symmetry along the muscle fiber directions. The activation of the left-ventricular wall was governed by the eikonal-diffusion equation. Active contraction was incorporated using a Hill-like model. For each left-ventricular shape, a simulation was performed in which passive filling was followed by isovolumic contraction. It was found that the transmural gradient of active fiber stress at the beginning of ejection steepens at the mid-height level when the left ventricle becomes more spherical, which was also obtained previously for end-diastolic passive fiber stress and strain.