Effect of laminar orthotropic myofiber architecture on regional stress andstrain in the canine left ventricle

Recent morphological studies have demonstrated a laminar (sheet) organizati on of ventricular myofibers. Multiaxial measurements of orthotropic myocard ial constitutive properties have not been reported, but regional distributi ons of three-dimensional diastolic and systolic strains relative to fiber a nd sheet axes have recently been measured in the dog heart by Takayama et a l. [30]. A three-dimensional finite-deformation, finite element model was u sed to investigate the effects of material orthotropy on regional mechanics in the canine left ventricular wall at end-diastole and end-systole. The p rolate spheroidal model incorporated measured transmural distributions of f iber and sheet angles at the base and apex. Compared with transverse isotro py, the orthotropic model of passive myocardial properties yielded improved agreement with measured end-diastolic strains when: (1) normal stiffness t ransverse to the muscle fibers was increased tangent to the sheets and decr eased normal to them; (2) shear coefficients were increased within sheet pl anes and decreased transverse to them. For end-systole, orthotropic passive properties had little effect, but three-dimensional systolic shear strain distributions were more accurately predicted by a model in which significan t active systolic stresses were developed in directions transverse to the m ean fiber axis as well as axial to them. Thus the ventricular laminar archi tecture may give rise to anisotropic material properties transverse to the fibers with greater resting stiffness within than between myocardial lamina e. There is also evidence that intact ventricular muscle develops significa nt transverse stress during systole, though it remains to be seen if active stress is also orthotropic with respect to the laminar architecture.