Reentry in the atrium.

: Studies with isolated atrial preparations of the rabbit showed that the occurrence of a single early premature beat may cause reentry not only in nodal tissue (SA node and AV node) but also in working myocardial tissue. In the SA node an early premature beat will cause a reentrant activation of the atrium only when the SA node is driven by an ectopic pacemaker. If the SA node is discharging spontaneously, no reentry could be demonstrated. In this situation the early impulse can not reach the center of the SA node because of a sinoatrial entrance block. Since the AV node fibers normally do not discharge spontaneously, an atrial premature beat may find an alternative route through the node and reenter the atrium. Such a reentrant beat or echo beat can start a tachycardia based on a circus movement of the impulse through the AV node. A supraventricular tachycardia can be started too by an early premature beat in the isolated left atrium, containing only working myocardial fibers and no slow conducting fibers as the nodal fibers are. By careful mapping the spread of activation during the premature beat and the subsequent beats of the tachycardia, a unidirectional block of the impulse of the premature beat was demonstrated. The impulse then turned around and invaded the blocked area retrogradely and reentered the area where it originated. This circus movement of the premature impulse was maintained during the subsequent tachycardial beats, showing that even in a small area of atrial muscle, containing no anatomical obstacle, a circus tachycardia can take place. To describe this kind of circus movement a new model (the "leading circle" concept) is introduced and briefly discussed.