Hierarchy of supraventricular pacemakers.

Definitive subsidiary atrial pacemakers (SAPs) exist within the crista terminalis and in the atrial free wall tissue at the junction of the inferior vena cava and the inferior right atrium. These pacemakers are capable of maintaining cardiac rhythm at a rate intermediate between that of the sinoatrial (SA) nodal and atrioventricular (AV) junctional pacemakers upon default of the normal SA nodal cells. The conventional concept of a single (junctional) escape mechanism needs to be updated by recognition of these SAP sites, which are under comprehensive autonomic regulation. A great variety of clinically important cardiac syndromes require understanding of the SAPs, not only for diagnostic significance but also for therapeutic consideration. SAPs differ from the SA pacemakers in that they possess intrinsically slower spontaneous rates and are less stable. There is early competition among them for instantaneous dominance. They are more responsive to both sympathetic and parasympathetic control, and they are initially more susceptible to underdrive stimulation by another pacemaker (whether artificial or natural) than are pacemakers within the SA node.