Epicardial suction: A new approach to mechanical testing of the passive ventricular wall

The lack of an appropriate three-dimensional constitutive relation for stre ss in passive ventricular myocardium currently limits the utility of existi ng mathematical models for experimental and clinical applications. Previous experiments used to estimate parameters in three-dimensional constitutive relations, such as biaxial testing of excised myocardial sheets or passive inflation of the isolated arrested heart, have not included significant tra nsverse shear deformation or in-plane compression. Therefore, a new approac h has been developed in which suction is applied locally to the ventricular epicardium to introduce a complex deformation in the region of interest wi th transmural variations in the magnitude and sign of nearly all six strain components. The resulting deformation is measured throughout the region of interest using magnetic resonance tagging. A nonlinear, three-dimensional, finite element model is used to predict these measurements at several suct ion pressures. Parameters defining the material properties of this model ar e optimized by comparing the measured and predicted myocardial deformations . We used this technique to estimate material parameters of the the intact passive canine left ventricular free wall using an exponential, transversel y isotropic constitutive relation. We tested two possible models of the hea rt wall first, that it was homogeneous myocardium, and second, that the myo cardium was covered with a thin epicardium with different material properti es. For both models, in agreement with previous studies, we found that myoc ardium was nonlinear and anisotropic with greater stiffness in the fiber di rection. We obtained closer agreement to previously published strain data f rom passive filling when the ventricular wall was modeled as having a separ ate, isotropic epicardium. These results suggest that epicardium may play a significant role in passive ventricular mechanics. [S0148-0731(00)00305-8] .