An investigation of the importance of myocardial anisotropy in finite-element modeling of the heart: methodology and application to the estimation of defibrillation efficacy

Finite-element (FE) modeling has been widely used in studies of bioelectric phenomena of tissues, including ventricular defibrillation. Most FE models, whether built from anatomical atlases or subject-specific tomographic images, treat the myocardium as an isotropic tissue. However, myocardium has been experimentally shown to have significant anisotropy in its resistivities, although myocardial fiber directions are difficult to measure on a subject-specific basis. In this paper, we: (1) propose a method to incorporate a widely known myocardial fiber direction model to a specific individual and (2) assess the effects of myocardial anisotropy on myocardial voltage gradients computed for a study of implantable defibrillators. The thoracic FE model was built from CT images of a young pig, and the myocardial fiber structures were incorporated via elastic mapping. Our results demonstrate a good mapping of geometry between the source and target hearts with an average root-mean-square error of less than 2.3 mm and a mapped fiber pattern similar to those known to exist in vivo. With the mapped fiber information, we showed that the estimated minimal myocardial voltage gradient over 80% of the myocardium differs by less than 10% between using an isotropic and anisotropic myocardial models. Thus, myocardial anisotropy is expected to have only a small effect on estimates of defibrillation threshold obtained from computed voltage gradients. On the other hand, anisotropy may be essential if defibrillation efficacy is analyzed by transmembrane voltage of the myocardial cells.