Nat. Rev. Cardiol. doi:10.1038/nrcardio.2017.37 Figure 1 Main features of leading circle versus spiral wave concepts of re-entry Figure 1 | Main features of leading circle versus spiral wave concepts of re-entry. a | The leading circle concept of re-entry. Re-entry establishes itself at the periphery of a zone with the smallest dimension that can maintain re-entry. This shortest path length is defined by the distance travelled in one refractory period, known as the wavelength, and is equal to the product of refractory period and conduction velocity. According to the leading circle concept, shorter paths are impossible because the re-entering impulse would encounter refractory tissue and be extinguished, and longer paths would have a longer cycle length and be overdriven by the impulse recirculating over the shortest path equal to the wavelength. b | The spiral wave concept of re-entry. Upper panel: schematic representation. The leading edge of the spiral wavefront (solid red line) revolves around an excitable, but nonexcited, core (grey). The curvature of the active wavefront is greatest near the core and decreases further outward. Where the curvature is great (for example, point X), the ratio of the number of activated cells to cells that are being activated (source to sink) is relatively small. The inward current moving from each cell in the source to activate the sink is relatively large, decreasing the amount of current available to maintain the action potential (AP) plateau and produce conduction and, therefore, reducing AP duration and conduction velocity (represented schematically by the black arrows). Further away from the core (for example, at point Y), the curvature is less, the source–sink relationship is less critical, and AP duration and conduction velocity are greater. The dashed red line is the trailing edge of the spiral wave, marking the location of tissue refractoriness behind the activation wavefront. The phase singularity (PS) point is located at the inner tip of the spiral wavefront adjacent to the core. The arrows emanating from the spiral wavefront represent conduction velocity vectors. Lower panel: snapshot of transmembrane potential during spiral-wave re-entry induced by an extrastimulus in a simulated 2D sheet of human atrial cells with realistic ion-current properties19,91. The dashed white line is at the leading edge of the spiral wavefront. Nattel, S. et al. (2017) Demystifying rotors and their place in clinical translation of atrial fibrillation mechanisms Nat. Rev. Cardiol. doi:10.1038/nrcardio.2017.37