Influence of the electric axis of stimulation on the induced transmembrane potentials in ellipsoidal bidomain heart.

Abstract

This theoretical study was provoked by and designed to interpret, complement and extend the implications of recent experimental observations by Wikswo and Lin (PACE, 21:940, 1998) on the epicardial surface of rabbit hearts. Using a macroscopic bidomain representation of the cardiac structure and the finite element method, we model the response of the heart to uniform electric fields applied under different angles. To overcome intra- and interspecies differences in the geometric and structural characteristics of the cardiac muscle, the analysis is conducted for an idealized ellipsoidal heart. Although idealized, this heart model incorporates important structural features, i.e., fiber curvature, transmural fiber rotation, and unequal anisotropy for the intra- and extracellular domains. This study shows that regions of maximum polarization of opposite sign may develop along an axis, significantly deviating from the axis of the applied electric field. The polarization evoked inside the ventricular wall seems to be a major contributor to this phenomenon. Nonperiodic structural inhomogeneities on multicellular level (endocardial "trabeculation" in our model) result in local unaligned polarization dipoles weakening the magnitude of the global polarization dipole and reducing its deviation from the axis of stimulation. Our results might be helpful in improving current understanding of defibrillation mechanisms.