Which is the commonest site of atrioventricular nodal reentry tachycardia (AVNRT) in the conduction system?

Explanation

## AVNRT: Pathway of Reentry **Key Point:** The slow pathway of the atrioventricular node is the commonest site of reentry in atrioventricular nodal reentry tachycardia (AVNRT), accounting for approximately 90% of AVNRT cases (typical AVNRT). ### Anatomical Organization of the AV Node The AV node contains two functionally and anatomically distinct pathways: | Pathway | Location | Conduction Speed | Refractory Period | AVNRT Role | |---------|----------|------------------|-------------------|------------| | Fast pathway | Anterior/superior | Fast | Long | Retrograde conduction in typical AVNRT | | Slow pathway | Posterior/inferior | Slow | Short | Anterograde conduction in typical AVNRT | ### Mechanism of Typical AVNRT (90% of cases) 1. **Anterograde conduction:** Sinus impulse travels down the slow pathway (longer conduction time) 2. **Retrograde conduction:** Impulse returns via the fast pathway (shorter refractory period allows recovery) 3. **Reentry circuit:** Impulse re-enters the slow pathway, perpetuating the tachycardia 4. **Result:** Narrow-complex tachycardia with P wave buried in or immediately after QRS ### Clinical Pearl In typical AVNRT, the slow pathway is the site of reentry because its longer refractory period allows the fast pathway to recover by the time the impulse returns, creating the necessary unidirectional block for reentry. Ablation of the slow pathway is the definitive treatment. **High-Yield:** Atypical AVNRT (10% of cases) involves reentry via fast pathway anterograde and slow pathway retrograde, but this is far less common and not the answer to "commonest site." ### Mnemonic **SLOW = Site of reentry in typical AVNRT (90%)**