Diagram: AV nodal reentrant tachycardia

The AV node often has dual pathways; in this diagram the alpha pathway is fast, but has a long refractory period; the beta pathway is conducts more slowly, but recovers faster.In sinus rhythm the faster alpha pathway is used and accounts for the normal PR interval. When a PAC occurs, however, the impulse may find the alpha pathway refractory, but able to traverse the beta pathway. When the premature impulse reaches the intersection of the two pathways, alpha may be recovered and allow retrograde activation of the atria; this may enable a reentrant tachycardia to develop, as illustrated in the diagram