## Diagnosis: Atrioventricular Nodal Reentrant Tachycardia (AVNRT) ### Clinical Presentation and Key Features The patient is a middle-aged man with: - **Sudden-onset regular narrow-complex tachycardia** at 180 bpm - **Recurrent episodes** over months (paroxysmal SVT) - **Gradual slowing followed by abrupt termination** with vagal maneuver — pathognomonic for AVNRT - **P waves buried in the T wave** of the preceding beat (retrograde P waves) - **Normal structural heart** (negative troponin, no murmurs) ### Mechanism of AVNRT AVNRT is caused by **dual AV nodal pathways** with different conduction velocities and refractory periods: ```mermaid flowchart TD A[Sinus beat] --> B[AV node: fast and slow pathways]:::outcome B --> C{Premature atrial beat}:::decision C -->|Fast pathway refractory| D[Conduction via slow pathway]:::action D --> E[Slow conduction to ventricle]:::outcome E --> F[By the time ventricle depolarizes...]:::action F --> G[Fast pathway has recovered]:::outcome G --> H[Retrograde conduction via fast pathway]:::action H --> I[Re-enters atrium]:::outcome I --> J[Reentrant circuit: slow pathway anterograde, fast pathway retrograde]:::outcome J --> K[AVNRT: narrow QRS, retrograde P in T wave]:::outcome ``` ### P Wave Location: The Diagnostic Clue **High-Yield:** In AVNRT, retrograde P waves are **buried within or immediately after the QRS complex** or in the **early T wave** of the preceding beat. This is because: - The slow pathway conducts antegradely (takes time to reach the ventricle) - By the time the ventricle depolarizes, the fast pathway has recovered - Retrograde conduction via the fast pathway is nearly simultaneous with ventricular depolarization - Result: P wave is **superimposed on the QRS or T wave** and often invisible ### Vagal Maneuver Response: Diagnostic Gold Standard **Key Point:** The **gradual slowing followed by abrupt termination** is pathognomonic for AVNRT: 1. Vagal maneuvers increase AV nodal refractoriness 2. Conduction through the slow pathway slows progressively (HR decreases gradually) 3. Eventually, the slow pathway becomes too refractory to conduct 4. The reentrant circuit is disrupted → **abrupt termination** This is distinct from AVRT, which terminates **abruptly** without a gradual slowing phase. ### Comparison: AVNRT vs. AVRT vs. Atrial Flutter | Feature | AVNRT | AVRT | Atrial Flutter | |---------|-------|------|----------------| | **Mechanism** | Dual AV nodal pathways | Accessory pathway + AV node | Atrial macro-reentry | | **Age** | 40–60 years (common) | Childhood/young adult | Any age | | **Rate** | 140–250 bpm | 150–250 bpm | Atrial 250–350 bpm; ventricular depends on AV conduction | | **P wave** | Buried in QRS/T wave | Retrograde, after QRS | Sawtooth flutter waves | | **Vagal response** | Gradual slowing → abrupt termination | Abrupt termination | Increased AV block, slowing only | | **Accessory pathway** | No | Yes (WPW) | No | | **Adenosine response** | Terminates SVT | Terminates SVT | Increases AV block, reveals flutter waves | **Clinical Pearl:** AVNRT is the **most common SVT in the general population** (~60% of SVTs), while AVRT is more common in patients with pre-excitation (WPW syndrome). ### Management 1. **Acute termination:** Vagal maneuvers (Valsalva, carotid massage), then IV adenosine 6 mg rapid push 2. **Chronic prevention:** Beta-blockers (first-line), calcium channel blockers (verapamil, diltiazem), or flecainide 3. **Definitive cure:** Radiofrequency ablation of the slow AV nodal pathway (>95% success, low risk of complete AV block <1%) **Warning:** Do NOT use adenosine in patients with severe asthma or COPD (can cause bronchospasm); use verapamil instead.
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