Action potential propagation in inhomogeneous cardiac tissue: safety factor considerations and ionic mechanism

Heterogeneity of myocardial structure and membrane excitability is accentuated by pathology and remodeling. In this study, a detailed model of the ventricular myocyte in a multicellular fiber was used to compute a location-dependent quantitative measure of conduction (safety factor, SF) and to determine the kinetics and contribution of sodium current (I-Na) and L-type calcium current [I-Ca(L)] during conduction. We obtained the following results. 1) SF decreases sharply for propagation into regions of increased electrical load (tissue expansion, increased gap junction coupling, reduced excitability, hyperkalemia); it can be (1 locally (a value indicating conduction failure) and can recover beyond the transition region to resume propagation. 2) SF and propagation across inhomogeneities involve major contribution from I-Ca(L) 3) Modulating I-Na Or I-Ca(L) (by blocking agents or calcium overload) can cause unidirectional block in the inhomogeneous region. 4) Structural inhomogeneity causes local augmentation of I-Ca(L) and suppression of I-Na in a feedback fashion. 5) Propagation across regions of suppressed I-Na is achieved via a I-Ca(L)-dependent mechanism. 6) Reduced intercellular coupling can effectively compensate for reduced SF caused by tissue expansion but not by reduced membrane excitability.