ANISOTROPY, FIBER CURVATURE, AND BATH LOADING EFFECTS ON ACTIVATION IN THIN AND THICK CARDIAC TISSUE PREPARATIONS - SIMULATIONS IN A 3-DIMENSIONAL BIDOMAIN MODEL

Introduction: A modeling study is presented to explore the effects of tissue conductivity, fiber orientation, and presence of an adjoining e xtracellular volume conductor on electrical conduction in cardiac musc le. Simulated results are compared with those of classical in vitro ex periments on superfused thin layer preparations acid on whole hearts. Methods and Results: The tissue is modeled as a three-dimensional bido main block adjoining an isotropic bath. In the thin layer model, the f ibers are assumed parallel. In the thick block model, fiber rotation, curvature, and tipping are incorporated. Results from the thin layer m odel explain experimental observations that the rate of rise of the en tire action potential up-stroke is faster and the magnitude of the ext racellular potential is smaller across fibers than along fibers in a u niformly propagating front. The simulation identified that this behavi or only arises in tissue with unequal anisotropy in the two spaces and adjoining an extracellular bath. Simulated conduction and potential d istributions in the thick block model are shown to well approximate ex perimental maps. The potentials are sensitive to changes in the fiber orientations. A slight 5 degrees tipping of intramural fibers out of t he planes parallel to the epicardium and endocardium will lead to an a symmetry of the magnitudes of the positive regions. In addition, the i ntroduction of fiber curvature leads to more realistic isochrone and e xtracellular potential distributions. The orientation of the central n egative region of the extracellular potential is shown to be determine d by the average of the fiber direction at the plane of pacing and the plane of recording. Conclusions: The simulations demonstrate the sens itivity of spread of activation and potential time courses and distrib utions to the underlying electrical properties in both thick and thin slabs. The bidomain model is shown to be a useful representation of ca rdiac tissue for interpreting experimental data of activation.