In atrioventricular nodal reentrant tachycardia (AVNRT), which conduction pattern is most common and defines the 'fast-slow' form?

Explanation

## AVNRT Conduction Pathways **Key Point:** The 'fast-slow' AVNRT is the most common form (~90% of cases) and involves anterograde conduction through the slow pathway and retrograde conduction through the fast pathway. ### Dual AV Nodal Pathways The AV node contains two functionally distinct pathways: | Pathway | Conduction Speed | Refractory Period | Location | |---------|------------------|-------------------|----------| | **Fast pathway (α)** | Rapid (~400 mm/s) | Long | Anterolateral | | **Slow pathway (β)** | Slow (~200 mm/s) | Short | Posterolateral | ### Fast-Slow AVNRT Mechanism 1. **Anterograde conduction:** Impulse travels down the slow pathway (longer PR interval) 2. **Retrograde conduction:** Impulse returns up the fast pathway (short RP interval) 3. **Result:** Reentry circuit established within the AV node **High-Yield:** In fast-slow AVNRT, the RP interval is very short (<100 ms), making the P wave appear buried in or immediately after the QRS complex. This is pathognomonic for AVNRT. ### ECG Features of Fast-Slow AVNRT ```mermaid flowchart TD A[Sinus rhythm with dual pathways]:::outcome --> B{Premature atrial beat}:::decision B -->|Blocks fast pathway| C[Conduction via slow pathway]:::action C --> D[Prolonged PR interval]:::outcome D --> E[Slow pathway recovers]:::decision E -->|Retrograde via fast pathway| F[Reentry circuit]:::action F --> G[Fast-slow AVNRT]:::outcome G --> H[Short RP interval, P in QRS]:::outcome ``` **Clinical Pearl:** The 'slow-fast' variant (anterograde fast, retrograde slow) accounts for ~10% of AVNRT cases and produces a long RP interval, mimicking atrial tachycardia on ECG. **Mnemonic:** **Fast-Slow = Slow down, Fast back = Short RP = P buried in QRS**