Inhomogeneity of action potential waveshape assists frequency entrainment of cardiac pacemaker cells

The Role of the Hyperpolarization-Activated Inward Current$I_rm f$in Arrhythmogenesis: A Computer Model Study

Atrial fibrillation is the most common cardiac arrhythmia. Structural cardiac defects such as fibrosis and gap junction remodeling lead to a reduced cellular electrical coupling and are known to promote atrial fibrillation. It has been observed that the expression of the hyperpolarization-activated current If is increased under pathological conditions. Recent experimental data indicate a possible contribution of If to arrhythmogenesis. In this paper, the role of If in action potential propagation in normal and in pathological tissue is investigated by means of computer simulations. The effect of diffuse fibrosis and gap junction remodeling is simulated by reducing cellular coupling nonuniformly. As expected, the conduction velocity decreases when cellular coupling is reduced. In the presence of If the conduction velocity increases both in normal and in pathological tissue. In our simulations, ectopic activity is present in regions with high expression of If and is facilitated by cellular uncoupling. We conclude that an increased If may facilitate propagation of the action potential. Hence, If may prevent conduction slowing and block. Overexpression of If may lead to ectopic activity, especially when cellular coupling is reduced under pathological conditions.

A comparison of 1-D models of cardiac pacemaker heterogeneity

In this paper, we investigate the role of sinoatrial node (SAN) cellular heterogeneity in two key aspects of normal cardiac pacemaker function: frequency entrainment of the SAN, and propagation of excitation into the atrial tissue. Using detailed ionic models of electrical activity in SAN and atrial myocytes, we have formulated a number of one-dimensional models of SAN heterogeneity based on discrete-region (in which central and peripheral SAN type cell are separated into discrete regions), gradient and mosaic models of SAN organization. Each of the different models were assessed on their ability to achieve frequency entrainment of the SAN and activation of the adjoining atrial tissue in the presence of both uniform and linearly increasing conductivity profiles. Simulation results suggest that the gradient model of SAN heterogeneity, in which cells display a smooth variation in membrane properties from the center to the periphery of the SAN, produces action potential waveshapes and a site of earliest activation consistent with experimental observations in the intact SAN. The gradient model also achieves frequency entrainment of the SAN more easily than other models of SAN heterogeneity. Based on these results, we conclude that the gradient model of SAN heterogeneity, in the presence of a uniform conductivity profile, is the most likely model of SAN organization.