The SA node is considered the primary pacemaker of the heart. In<unk> , there are other locations within the<unk> besides the SA node that are responsible for each heartbeat. This is unusual because the SA node, AV node, bundle of His, bundle branches, and Purkinje fibers are the structures that have pacemaking capability. The<unk>  and ventricular muscle tissue do not have this capability. Originally, it was believed that the<unk> had different ectopic foci that were spontaneously depolarizing, each foci acting as a pacemaker for the heart. However, the<unk>  muscle tissue isn't able to spontaneously depolarize. Therefore, the mechanism is by extension of the self-depolarizing tissue between the SA and AV node. Over time, the extension of tissue becomes a tract with spontaneous depolarization capability. This allows for any location along the tract to spontaneously depolarize, hence different locations that are responsible for each heartbeat.
Wandering atrial pacemaker
Junctional ectopic tachycardia
AV nodal reentrant tachycardia
Ventricular escape beat
Atrioventricular reentrant tachycardia
Ectopic pacemaker
Accelerated idioventricular rhythm
Re-entry ventricular arrhythmia