The volume conductor effects of anisotropic muscle on body surface potentials using an eccentric spheres model

The purpose of this paper is to determine the volume conductor effects of muscle anisotropy on body surface potentials using an eccentric spheres model with a uniform double layer source configuration. Previous eccentric spheres work assumed that cardiac muscle anisotropy was small and that skeletal muscle effects could be accounted for by boundary extension, i.e., by scaling the conductivities and dimensions. However, in this paper, anisotropy for both the myocardium and the skeletal muscle is explicitly incorporated into the eccentric spheres volume conductor model. The anisotropy is treated as having uniform orthogonal components in the radial and tangential directions for both the skeletal muscle and myocardium. The solution for Laplace's equation is written in a series expansion of appropriate basis functions for each region. In the isotropic regions spherical harmonics with integer radial dependence and Legendre polynomial azimuthal dependence are utilized. For the anisotropic regions, Legendre polynomials are still appropriate for the azimuthal dependence, but noninteger powers of radial dependence are required. The approximate representation for anisotropy, i.e., the boundary extension method for the skeletal muscle and a scaled homogeneous conductivity without boundary extension for the myocardium are compared with explicit representations for the two regions. Two basic conclusions are drawn from the results. First, the treatment of skeletal muscle anisotropy by the boundary extension method is a valid and useful simplification which yields errors of 2% for the peak body surface potential. The second conclusion drawn from this study is that myocardial anisotropy has a significant effect on the magnitude of body surface potentials.(ABSTRACT TRUNCATED AT 250 WORDS)