The bi-syncytial model of the heart leads to the result that the volume density of cardiac current source moment is proportional to the gradient of the macroscopic transmembrane action potential distribution. If the anisotropy ratios of inner and outer domains (syncytia) of the myocardium are equal, then the volume distribution of cardiac sources can be replaced by an appropriate double layer on the heart surface. For an intramyocardial electrode there is an additional term proportional to the transmembrane potential. The double layer source distribution (heart surface source model) provides a basis for calculating heart surface potentials from cardiac sources.

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