## Anatomical Basis of AVNRT ### The Reentrant Circuit Location **Key Point:** In atrioventricular nodal reentrant tachycardia (AVNRT), the reentrant circuit is entirely contained within or immediately around the AV node itself, not involving accessory pathways or other cardiac structures. ### Dual AV Nodal Pathways The AV node has two functionally distinct pathways: 1. **Fast (α) pathway** - Located anteriorly and superiorly - Shorter refractory period - Slower conduction velocity 2. **Slow (β) pathway** - Located posteriorly and inferiorly - Longer refractory period - Faster conduction velocity ### Mechanism of Reentry ```mermaid flowchart TD A[Sinus impulse reaches AV node]:::outcome --> B{Dual pathways present?}:::decision B -->|Normal sinus rhythm| C[Fast pathway conducts<br/>Slow pathway blocked<br/>by refractoriness]:::action B -->|AVNRT setup| D[Premature atrial beat<br/>finds slow pathway<br/>refractory]:::outcome D --> E[Impulse travels down slow pathway<br/>Fast pathway recovers]:::action E --> F[Retrograde conduction up<br/>fast pathway to atrium]:::action F --> G[Reentrant loop established<br/>within AV node]:::urgent G --> H[Regular narrow-complex<br/>tachycardia 150-250 bpm]:::outcome ``` **High-Yield:** The reentrant circuit involves both pathways but the anatomical site of the circuit is the **AV node and the perinodal tissue**. The impulse travels antegrade (downward) via one pathway and retrograde (upward) via the other, creating a closed loop. ### ECG Findings in AVNRT | Feature | Finding | |---------|----------| | Heart rate | 150–250 bpm (typically 180–200) | | QRS complex | Narrow (< 120 ms) | | P wave location | Retrograde, buried in or immediately after QRS or in ST segment | | RP interval | Short (< 70 ms) in typical AVNRT | | Regularity | Regular | **Clinical Pearl:** The retrograde P wave is often **not visible** because it is buried within the QRS complex or T wave, giving the appearance of a "pseudo-R' wave" in V1 or "pseudo-S wave" in the inferior leads. ### Why This Distinguishes AVNRT from Other Arrhythmias - ~~Accessory pathway arrhythmias (AVRT)~~ involve an anatomically separate pathway across the mitral or tricuspid annulus (e.g., in Wolff-Parkinson-White syndrome) - ~~Atrial flutter or fibrillation~~ originate in atrial tissue, not the AV node - ~~Sinus tachycardia~~ arises from the SA node and has visible upright P waves in the PR interval [cite:Harrison 21e Ch 230] 
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