An integrated overview of AV node physiology

The atrioventricular (AV) node generates half of the AV delay needed for blood pumping and filters atrial impulses that could otherwise induce life-threatening ventricular arrhythmias. It is also a pacemaker and a key target in the treatment of cardiac arrhythmias. The special roles of the AV node primarily arise from its slow conduction, long refractory period, and cellular automaticity. However, efforts to establish the dynamics of these properties and their interaction led to many controversies. In fact, the AV node's behavior is so complex that it seems to escape broadly applicable rules. This review summarizes progresses made in resolving these issues and in integrating the multiple roles of the AV node within a common functional model. Presented evidence shows that the rate-dependent conduction and refractory properties of the AV node can be reliably characterized and reconciled from nodal responses to S₁ S₂ S₃ protocols. It also supports the concept that dual pathways constitute a feature of the normal AV node and account for its overall conduction and refractory properties. In this model, the posterior extension and compact node provide the core of the slow and fast pathway, respectively. The transitional tissues and lower nodal bundle provide a common proximal and distal pathway, respectively. These pathways would also support bidirectional conduction. The dual pathway involvement can also be extended to widely variable AV nodal responses, such as Wenckebach cycles, hysteresis, and ventricular response to atrial fibrillation. In brief, the intricate AV nodal behavior may obey a limited set of accessible and definable rules.