Analysis of Excitation Conduction in the Heart and the Reconstruction of Electrocardiogram Using Ionic -channel Models

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The normal excitation and conduction in the heart are maintained by the coordination between the dynamics of ionic conductance of each cell and the electrical coupling between cells.(1) Using a spatially-discrete model of conduction of excitation, we examined functional roles of these two factors. The model is one-dimensional and is composed of five kinds of myocardial cells. We first determined quantitatively the coupling coefficients from the apparent space constants, then found the conditions of cell number and the couping coefficients for the pacemaker activities of SA-node. We also investigated the very slow conduction in AV-node and revealed that the weak coupling coefficients between cells in AV-node has the strongest effect.(2) A digital computer model was constructed for the simulation of the electrocardiogram during ventricular activation and repolarization. The part of the ventricular septum and the left ventricula free wall of the heart were represented by a two dimensional array of 730 units. Ionic current models were used to determine the spatial distribution of the electric activities of these units at each instant of time during simulated cardiac cycle. In order to reconstruct the electrocardiogram, the model was expanded three dimensionally with equipotential assumption along the third axis and then the surface potentials were calculated using solid angle method. ECG for normal subject was well reconstructed. ECGs for hyper-and hypokalemia were also reconstructed by modifying the related ionic current dynamics. We used Euler-method for numerical integration through this project. However, in order to accomodate the improvement of ionic current model, we adopted GEAR method which has a power to solve so-called "stiff" differential equation.