Mechanisms of reentry in cardiac arrhythmias

Schematic representation of possible reentrant circuits. The thick black arrow represents the circulating impulse; thin black lines represent advancing wavefronts in completely refractory tissue; speckled areas are partially refractory tissue; white areas are fully excitable tissue.
A is the original model of circus movement around a fixed obstacle. There is a fully excitable gap, and the length and location of the circuit are fixed.
B represents circus movement around 2 fixed anatomic obstacles. A fully excitable gap is present.
C represents rapidly conducting bundles forming closed loops that serve as preferential circuits through which the impulse may travel.
D is the leading circle type of reentry which does not require an anatomic obstacle. Instead, the impulse propagates around a functionally refractory core and among neighboring fibers that have different electrophysiologic properties. Since the refractoriness of the core is variable, the circuit size changes but will be the smallest possible circuit that can continue to propagate an impulse. Functional circuits tend to be small, rapid, and unstable.
E represents reentry around a fixed anatomic obstacle, but a fully excitable gap is absent.
F demonstrates an area of slowed conduction (hatched lines) between anatomic boundaries, while in
G all areas of slowed conduction neighbor an anatomic obstacle.
H represents anisotropic reentry. There are differences in the conduction of a single impulse in various fibers as a result of differences in their orientation.