The three-dimensional geometry and anisotropic properties of the heart give rise to nonhomogeneous distributions of stress, strain, electrical activation and repolarization. In this article we review the ventricular geometry and myofiber architecture of the heart, and the experimental and modeling studies of three-dimensional cardiac mechanics and electrophysiology. The development of a three-dimensional finite element model of the rabbit ventricular geometry and fiber architecture is described in detail. Finally, we review the experimental results, from the level of the cell to the intact organ, which motivate the development of coupled three-dimensional models of cardiac electromechanics and mechanoelectric feedback.

Abstract-The analysis of surface-activation patterns and measurements of conduction velocity in ventricular myocardium is complicated by the fact that the electrical wavefront has a complex 3D shape and can approach the heart surface at various angles. Recent theoretical studies suggest that the optical upstroke is sensitive to the subsurface orientation of the wavefront. Our goal here was to (1) establish the quantitative relationship between optical upstroke morphology and subsurface wavefront orientation using computer modeling and (2) test theoretical predictions experimentally in isolated coronary-perfused swine right ventricular preparations. We show in numerical simulations that by suitable placement of linear epicardial stimulating electrodes, the angle of wavefronts with respect to the heart surface can be controlled. Using this method, we developed theoretical predictions of the optical upstroke shape dependence on . We determined that the level V F * at which the rate of rise of the optical upstroke reaches the maximum linearly depends on . A similar relationship was found in simulations with epicardial point stimulation. The optical mapping data were in good agreement with theory. Plane waves propagating parallel to myocardial fibers produced upstrokes with V F *Ͻ0.5, consistent with theoretical predictions for Ͼ0. Similarly, we obtained good agreement with theory for plane waves propagating in a direction perpendicular to fibers (V F *Ͼ0.5 when Ͻ0). Finally, during epicardial point stimulation, we discovered characteristic saddle-shaped V F * maps that were in excellent agreement with theoretically predicted changes in during wavefront expansion. Our findings should allow for improved interpretation of the results of optical mapping of intact heart preparations. Key Words: optical action potential Ⅲ conduction velocity Ⅲ optical mapping Ⅲ voltage-sensitive dye P atterns of electrical activation within the ventricular wall are determined in large part by the specific 3D organization of myocardial fibers. Histological studies of intact hearts from many species have shown conclusively that myocardial fiber orientation rotates significantly across the heart wall. 1 The total angle of rotation across the myocardial wall can reach 180°. 2 Because of the complex transmural organization of myocardial fibers, an excitation wavefront typically assumes a complex 3D profile that, depending on the mode of stimulation, can approach the myocardial surface at a variety of different angles. [3][4][5][6] For this reason, obtaining accurate quantitative information about the wavefront orientation with respect to the surface is quite important for the interpretation of surface recordings, particularly with regard to measurements of conduction velocity.In a recent modeling study, 7 we discovered that the upstroke morphology of optical action potentials (OAPs) was sensitive to the local subsurface orientation of the excitation wavefront. This suggests that analysis of OAP upstroke morphology may prove useful in determini...

To determine regional stress and strain distributions in rabbit ventricular myocardium, an anatomically detailed finite element model was used to solve the equations of stress equilibrium during passive filling of the left ventricle. Computations were conducted on a scalable parallel processing computer and performance was found to scale well with the number of processors used, so that stimulations previously requiring approximately 60 min were completed in just over 5 min. Epicardial strains from the model analysis showed good agreement (RMSE = 0.007332) with experimental measurements when material properties were chosen such that cross fiber strain was more heterogeneous than fiber strain, which is also consistent with experimental observations in other species.

Abstract-Studies have characterized conduction velocity in the right and left bundle branches (RBB, LBB) of normal and genetically engineered mice. However, no information is available on the action potential characteristics of the specialized conduction system (SCS). We have used microelectrode techniques to characterize action potential properties of the murine SCS, as well as epicardial and endocardial muscle preparations for comparison. In the RBB, action potential duration at 50%, 70%, and 90% repolarization (APD 50,70,90