Computer simulations and experiments in multiple mammalian species suggest that functional reentry is better explained by rotors or spiral waves (Figure 3D). This idea was first conceptualized by Russian scientists in the 1960s, and later popularized by Arthur Winfree to explain the reentry in all excitable media.213,214,215 The rotor is the organizing center of the reentrant excitation215; it spins at exceedingly high frequencies, radiating spiral wavefronts with outwardly decreasing curvature, forming an Archimedean spiral, and resulting in wave fragmentation in its periphery.216,217 Because CV decreases as the wavefront curvature becomes steeper toward the center tip, it follows that at that site (sometimes called the phase singularity [PS]) the curvature reaches a critical value, the velocity becomes zero, and the PS follows a circular trajectory.215,218 At each point the direction of propagation is perpendicular to the wavefront and the velocity increases toward the periphery. The PS is a unique point where the wavefront and the wavetail converge and velocity is zero, preventing the impulse from extending toward the center of the rotation. Instead, the PS becomes the rotor, circling around a small center of unexcited but excitable tissue.218 The concept of rotor can also be applicable to anatomical reentry in the atria; a pectinate muscle or the orifice of a PV can stabilize a reentrant rotor.156,219 Unlike leading-circle reentry, spiral-wave reentry is not determined by the wavelength, but rather by the source-sink relationship between the activation wavefront and the tissue that must be excited in front of it to maintain activity. The rotor concept has been applied to AF, and subsequent studies have confirmed its ability to account for the AF-suppressing actions of Na+ channel blockers.119