Effects of activation pattern and active stress development on myocardial shear in a model with adaptive myofiber reorientation

Abstract

Pluijmert M, Bovendeerd PH, Kroon W, Prinzen FW, Delhaas T. Effects of activation pattern and active stress development on myocardial shear in a model with adaptive myofiber reorientation. Am J Physiol Heart Circ Physiol 306: H538 -H546, 2014. First published December 6, 2013 doi:10.1152/ajpheart.00571.2013.-It has been hypothesized that myofiber orientation adapts to achieve a preferred mechanical loading state in the myocardial tissue. Earlier studies tested this hypothesis in a combined model of left ventricular (LV) mechanics and remodeling of myofiber orientation in response to fiber cross-fiber shear, assuming synchronous timing of activation and uniaxial active stress development. Differences between computed and measured patterns of circumferential-radial shear strain Ecr were assumed to be caused by limitations in either the LV mechanics model or the myofiber reorientation model. Therefore, we extended the LV mechanics model with a physiological transmural and longitudinal gradient in activation pattern and with triaxial active stress development. We investigated the effects on myofiber reorientation, LV function, and deformation. The effect on the developed pattern of the transverse fiber angle ␣t,0 and the effect on global pump function were minor. Triaxial active stress development decreased amplitudes of Ecr towards values within the experimental range and resulted in a similar base-to-apex gradient during ejection in model computed and measured Ecr. The physiological pattern of mechanical activation resulted in better agreement between computed and measured strain in myofiber direction, especially during isovolumic contraction phase and first half of ejection. In addition, remodeling was favorable for LV pump and myofiber function. In conclusion, the outcome of the combined model of LV mechanics and remodeling of myofiber orientation is found to become more physiologic by extending the mechanics model with triaxial active stress development and physiological activation pattern. finite element; adaptation; cardiac deformation MATHEMATICAL MODELS OF CARDIAC mechanics can be used to relate abnormal cardiac deformation, as measured noninvasively by ultrasound or magnetic resonance tagging (MRT) to the underlying pathology. As a first step towards this application, one would require the models to be able to correctly predict deformation in the wall of the healthy human heart. Although most finite element (FE) models are able to correctly predict circumferential, longitudinal and radial strains (5,16,17,21,26,29), they are not capable to correctly predict the shearing pattern of the myocardium (6, 16, 29) as measured in experiments (2,20,23,28). Because myocardial shear is not kinematically coupled to the cavity volume, it would be a more sensitive measure to evaluate model results.FE modeling results from our group indicate a strong relation between myofiber orientation and strain distribution in the LV wall, in particular shear strain. However, experimental data show that shear strain varies little in...

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