## Distinguishing AVNRT from AVRT ### Key Diagnostic Feature **Key Point:** The presence of an accessory pathway (AP) is the pathophysiological hallmark that separates AVRT from AVNRT. AVRT requires a bypass tract; AVNRT does not. ### Mechanism-Based Comparison | Feature | AVNRT | AVRT | |---------|-------|------| | **Anatomical substrate** | Dual AV nodal pathways (slow & fast) | Accessory pathway (Kent bundle, Mahaim, nodofascicular) | | **Electrophysiology finding** | No AP on EPS; dual nodal physiology | **AP visible on EPS** (earliest retrograde activation outside AV node) | | **RP interval** | Short (atypical: long) | Variable; depends on AP location | | **Retrograde P location** | Buried in QRS or early ST | Can be anywhere from QRS to mid-ST | | **Adenosine response** | Terminates via AV block | Terminates via AV block | ### Why EPS Finding is the Gold Standard **High-Yield:** Electrophysiology study directly visualizes the accessory pathway in AVRT through: - Earliest retrograde atrial activation site outside the AV node - Ability to demonstrate pre-excitation during sinus rhythm (if WPW syndrome) - Conduction over the AP during tachycardia **Clinical Pearl:** While ECG features (RP interval, P wave location) are useful clues, they overlap significantly between AVNRT and AVRT. Only EPS can definitively prove the presence or absence of an accessory pathway. ### Mnemonic **AVRT = Accessory pathway Required for Tachycardia** — the AP is the defining anatomical difference. ### When RP Interval Alone Is Misleading **Warning:** Atypical AVNRT (slow-fast conduction over slow pathway anterogradely, fast pathway retrogradely) can mimic AVRT with a long RP interval. Both can present with RP > PR. This is why EPS is the definitive discriminator, not ECG morphology alone.
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