In some situations, conditions exist that permit an impulse to re-enter a previously excited region and to establish a self sustaining re-entry circuit. The presence of an AV nodal bypass tract (arrow) that allows an impulse to travel in a retrograde direction from the ventricles to the atria is one such example.
Re-entry is the mechanism responsible for the paroxysmal supraventricular tachycardias that occur in patients with and without ventricular pre-excitation, for atrial flutter and fibrillation, for many cases of ventricular tachycardia and for ventricular fibrillation.