Sinoatrial and Atrioventricular Blocks: Who First Described Them and How? [Retrospectroscope].

The relationship among cardiac pacemakers is characterized by the fact that one pacemaker is usually dominant and all the others are subsidiary. The sinoatrial node acts as the dominant pacemaker, and all other potential pacemaker tissues are discharged by a conducted impulse before their respective diastolic depolarizations attain threshold. These pacemakers are called subsidiary to emphasize the fact that, under normal circumstances, they are engaged in conducting impulses, but, under abnormal circumstances, they may become actual pacemakers.