Effects of rotational myocardial anisotropy in forward potential computations with equivalent heart dipoles

Rotating fibers in the heart lead to a myocardium of inhomogeneous anisotropic conductivity. Besides affecting the activation isochrones, this anisotropy modifies the equivalent dipoles used in calculating extracardiac potentials, rendering them oblique rather than normal to the activation wavefront due to an added axial dipole component oriented along the fibers. Herein, however, consequences of the assumption usually made in forward potential calculations that the equivalent dipoles act in a myocardium that is homogeneous and isotropic are examined. A layered inner block representing the heart was placed inside an outer block representing an isotropic volume conductor. Fiber direction in the inner block rotated uniformly from layer to layer. Current dipoles of different orientations were placed in the inner block and the potentials calculated everywhere. Effects of the anisotropy of the inner block were gauged by computing an equivalent dipole that best fit the outer block surface potentials. For volume conductor conductivities close to that of the torso, the anisotropy diminished dipoles oriented along the fibers. Since the intraventricular blood masses in the heart also diminish such dipoles, these reductions of the axial component may explain the success of heart model simulations that ignore this component.