A 32-year-old woman presents with recurrent episodes of palpitations lasting 30 minutes to 2 hours, associated with chest discomfort and dyspnea. ECG during an episode shows a regular narrow-complex tachycardia at 180 bpm with retrograde P waves. Which is the most common mechanism of supraventricular tachycardia in this patient?

## Most Common SVT Mechanism **Key Point:** AVNRT is the single most common form of SVT, accounting for 50–60% of all SVT cases in the general population. ### Clinical Features of AVNRT **High-Yield:** AVNRT is characterized by: - Regular narrow-complex tachycardia (usually 140–250 bpm) - Retrograde P waves (often buried in or immediately after the QRS, appearing as a pseudo-R' in V1 or pseudo-S in inferior leads) - Abrupt onset and termination - More common in women and middle-aged patients - No pre-excitation on baseline ECG ### Mechanism AVNRT involves a reentrant circuit within or around the AV node, typically due to dual AV nodal pathways (fast and slow): ```mermaid flowchart TD A[Dual AV nodal pathways present]:::outcome --> B{Premature atrial beat}:::decision B -->|Blocks fast pathway| C[Conduction via slow pathway]:::action C --> D[Fast pathway recovers]:::action D --> E[Retrograde conduction via fast pathway]:::action E --> F[Reentrant circuit established]:::outcome F --> G[Regular SVT with retrograde P waves]:::outcome ``` ### Comparison with Other SVT Types | Feature | AVNRT | AVRT | Atrial Flutter | Atrial Tachycardia | |---------|-------|------|-----------------|-------------------| | **Frequency** | 50–60% | 30–40% | 10–15% | 5–10% | | **P wave timing** | Retrograde, buried in QRS | Retrograde, after QRS | Sawtooth pattern | Before QRS | | **Pre-excitation** | Absent | May be present (WPW) | Absent | Absent | | **Mechanism** | Reentry within AV node | Reentry via accessory pathway | Atrial flutter waves | Ectopic focus or reentry | | **Gender predilection** | Female predominance | Equal | Slight male predominance | No clear predilection | **Clinical Pearl:** The retrograde P wave timing is the key diagnostic clue. In AVNRT, the P wave is typically inscribed at or very near the QRS complex (RP interval ≤70 ms), whereas in AVRT, the RP interval is usually longer (>70 ms) because the accessory pathway is located away from the AV node. ### Why AVNRT Is Most Common Dual AV nodal pathways are present in approximately 40% of the population, but only a fraction develop symptomatic AVNRT. The prevalence increases with age and is more common in women, making it the most frequently encountered SVT in clinical practice. **Warning:** Do not confuse AVNRT with AVRT (Wolff-Parkinson-White syndrome). AVRT requires an accessory pathway and shows pre-excitation (delta wave) on baseline ECG, which is absent in this case.