Figure 2.

Schematic diagram illustrating the mechanism of resetting in reentrant arrhythmias. Panel A represents a circular reentrant circuit. Most of it is surrounded by a black circumferencial barrier, that is, interrupted on its right side. The black area inside the circuit represents unexcitable tissue. The white area around the circuit would be the remaining of the heart (the cardiac chamber). Inside the circuit electrical activation is taking place, represented by the blue arrowhead, that is followed by a tail of refractoriness (blue area). So all the blue area is not excitable at this precise moment. The tissue in the circuit that is excitable, usually referred to as excitable gap, is depicted in light gray. As the activation inside the circuit proceeds, it is expected that it will be confined in the circuit only as long as a barrier exists, so it will exit to the surrounding myocardium as soon as there is no barrier (exit site, “exit” in the figure). If activation wavefronts generated outside the circuit approach it, they would activate the tissue inside the circuit if there is no barrier and if the tissue is excitable. So the entry site would be the closest site to the external activation wavefront that is not surrounded by barrier and that is excitable. Considering the situation in panel B, where the wavefront is generated geometrically closer to the upper boundary of the barrier, the entry site would be its most superior end, as represented in panel A (“entry”). Panel B represents the situation several milliseconds later. An extrastimulus has been delivered at a site away from the circuit. The corresponding wavefront has reached the reentrant circuit at a time the tissue was excitable (small red arrow). Activation inside the circuit proceeds both in the direction of activation during tachycardia (orthodromic wavefront, “ortho” in the figure) and in the opposite direction (antidromic wavefront, “antidromic” in the figure). Since the antidromic wavefront collides with the activation wavefront inside the circuit, the final result is that the activation jumps from site 1 to site 2, thus shortcircuiting the circuit (see text for further explanations).