## Electrophysiology of AVNRT **Key Point:** AVNRT (the most common type of SVT) depends on the presence of **dual AV nodal pathways**—typically a slow pathway (α) and a fast pathway (β)—with different conduction velocities and refractory periods. ### Dual Pathway Mechanism 1. **Slow pathway (α):** longer refractory period, slower conduction (~0.04 m/s) 2. **Fast pathway (β):** shorter refractory period, faster conduction (~0.06 m/s) 3. **Reentrant circuit:** impulse travels down fast pathway, up slow pathway (or vice versa) **High-Yield:** The difference in refractory periods allows unidirectional block and reentry: - An atrial premature beat may block in the fast pathway (still refractory) but conduct slowly through the slow pathway - By the time the impulse exits the slow pathway, the fast pathway has recovered - The impulse re-enters the fast pathway retrogradely, completing the circuit ### Typical vs. Atypical AVNRT | Feature | Typical (90%) | Atypical (10%) | |---------|---------------|----------------| | **Antegrade conduction** | Fast pathway | Slow pathway | | **Retrograde conduction** | Slow pathway | Fast pathway | | **RP interval** | Short | Long | | **PR interval during SVT** | Long | Short | **Mnemonic:** **SLOW-FAST** = Slow pathway conducts antegrade (down), Fast pathway conducts retrograde (up) in typical AVNRT. **Clinical Pearl:** AVNRT is NOT dependent on an accessory pathway (unlike WPW/AVRT). It is purely a nodal phenomenon caused by dual pathways within the AV node itself. [cite:Harrison 21e Ch 226]
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