Abstract
Tissue microstructure, in particular the alignment of myocytes (fibre direction) and their lateral organisation into sheets, is fundamental to cardiac function. We studied the effect of microstructure on contraction in a computational model of rat left ventricular electromechanics. Different fibre models, globally rule-based or locally optimised to DT-MRI data, were compared, in order to understand whether a subject-specific fibre model would enhance the predictive power of our model with respect to the global ones. We also studied the impact of sheets on ventricular deformation by comparing: (a) a transversely isotropic versus an orthotropic material law and (b) a linear model with a bimodal model of sheet transmural variation. We estimated ejection fraction, wall thickening and base-to-apex shortening and compared them with measures from cine-MRI. We also evaluated Lagrangian strains as local metrics of cardiac deformation. Our results show that the subject-specific fibre model provides little improvement in the metric predictions with respect to global fibre models while material orthotropy allows closer agreement with measures than transverse isotropy. Nonetheless, the impact of sheets in our model is smaller than that of fibres. We conclude that further investigation of the modelling of sheet dynamics is necessary to fully understand the impact of tissue structure on cardiac deformation.
Highlights
Cardiac tissue is arranged into a complex architecture
We present the results of the series of electromechanical simulations we performed in order to (a) compare sixteen globally defined fibre models (SnxRy for x~0:5,1,2,3 and y~50,60,70,80) against one subject-specific fibre model (Karadag), and (b) evaluate the effect of introducing sheets by comparing transverse isotropy to orthotropy and with linear and bimodal sheet models
Global measures The global measures considered in this study are 1) ejection fraction EF, 2) wall thickening WT, and 3) shortening in the longaxis direction SL
Summary
Cardiac tissue is arranged into a complex architecture. Due to their elongated shape, myocytes are highly anisotropic and their local orientation in cardiac tissue is usually called the fibre direction [1]. The extracellular connective tissue matrix that surrounds myocytes groups them into layers 4-6 cells thick [2], separated by cleavage planes, forming a branching laminar architecture Such layers are commonly referred to as sheets. The importance of local myocyte orientation in cardiac function, both in health and disease, has promoted considerable effort in recent years towards subjectspecific modelling of cardiac electromechanics [8], [9], [10]. Pathological conditions such as dyssynchrony [11] have been investigated, with realistic ventricular to whole-organ geometries, combined with fibre orientations obtained from imaging [10]. The results are promising, but it is an open question as to whether current electromechanical models are becoming a useful support tool in clinical decision-making
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