Beat-to-beat 3D activation patterns in the heart: Modeling the activation-refractoriness relationships

The temporal relationship between the repolarization process of a cardiac cell and the instant it is reactivated, is known to affect the cell\´s action potential and the activation of neighboring cells. This effect is insignificant in a normal activation cycle. It becomes significant as the cell is reactivated early in the cycle, specifically during the relative refractory period. This may happen within ischemie areas, wherein all the activation parameters change drastically. Since the myocardial tissue is three dimensional (3D), the propagation patterns generated under such pathological conditions may be very complex, e.g. conduction block, early reactivation, reentrant activity. The proposed model describes the electrical activation in the simplified 3D geometry of left ventricular (LV) myocardium by assigning the elements in the 3D space with appropriate characteristics, and a set of rules by which each element may affect its 26 neighbors. An "ischemie" area is characterized by small changes of the parameters in the border area, and larger ones in its central zone. Normal activation is initiated in the "Purkinje" elements, which evenly cover the endocardium, and propagates radially in orderly fashion, except within the "ischemie" area, in which it moves slowly and generates "Late Potentials." Transient premature beats are generated when the "normal" model is paced at a high rate; when an ectopie element is initiated, it may generate transient reentry loops through the LV. Rapid simulations of multiple beats are routinely carried out The model thus serves as a tool for studying complex activation patterns due to combinations of conditions like slowed propagation, ectopie activity and different pacing rates.