Principles of Cardiac Electric Propagation and Their Implications for Re-entrant Arrhythmias

The study of clinical electrophysiology essentially comprises examining how electric excitation develops and spreads through the millions of cells that constitute the heart. Given the enormous number of cells in a human heart, there is an extremely large number of possible ways that the heart can behave. We encounter rhythms across the spectrum from the organized and orderly behavior of sinus rhythm through repetitive continuous excitation (via reentry) in structurally defined circuits like atrial flutter and, finally, the complex, dynamic, and disorganized behavior of fibrillation. Despite these myriad possibilities, one can apply a basic understanding of the principles of propagation to predict how cardiac tissue will behave under varied circumstances and in response to various manipulations. In this article, we review the principles of propagation and how these can be used to understand reentry of all degrees of complexity. We use these principles to explain the mechanisms by which antiarrhythmic medications and ablation can terminate and prevent reentry. This article is not intended to be an exhaustive description of the physiology of cardiac propagation, rather, it is meant to capture the essence of propagation with sufficient detail to provide an intuitive feel for the interplay of the physiological features relevant to propagation. The figures and videos used in this article were created using a computational model of cardiac propagation (VisibleEP LLC, Colchester, VT). It is a hybrid between a physics-based and cellular automaton model. The model incorporates the fundamental features of propagation without modeling individual ion channels.1 The model manifests several relevant emergent properties, for example, electrotonic interactions, restitution of action potential duration, and conduction velocity as well as source–sink balance–dependent propagation. ### Cell Excitation A cell becomes excited when the balance of inward and outward currents passes a critical point after which inward currents exceed outward and an action potential ensues. …