A 35-year-old man with a history of recurrent palpitations is admitted for electrophysiological study. During the study, atrial pacing at progressively faster rates is performed. At a cycle length of 400 ms, dual AV nodal physiology is demonstrated: a sudden increase in the AH interval from 60 ms to 120 ms occurs with a 10 ms decrease in pacing cycle length. Spontaneous reentrant tachycardia is induced. The arrhythmia is regular at 140 bpm with a narrow QRS complex. The VA interval (measured from the onset of ventricular depolarization to the earliest atrial activation) is 80 ms. What is the most likely diagnosis?

Explanation

## Electrophysiological Diagnosis of AVNRT This case demonstrates **AVNRT in the slow-fast form** (also called "common" AVNRT), the most frequent variant (~80% of AVNRT cases). ### Key Electrophysiological Findings **High-Yield: Dual AV Nodal Physiology** — The sudden jump in AH interval (from 60 to 120 ms with only 10 ms decrease in pacing cycle length) is the **hallmark of dual AV nodal pathways**: - **Fast pathway**: conducts rapidly but has a longer refractory period - **Slow pathway**: conducts slowly but has a shorter refractory period At faster pacing rates, the fast pathway becomes refractory, and conduction shifts to the slow pathway, causing the sudden AH prolongation ("jump"). ### Mechanism of Slow-Fast AVNRT **Key Point:** In slow-fast AVNRT: 1. **Anterograde conduction**: via the **slow pathway** (long AH interval) 2. **Retrograde conduction**: via the **fast pathway** (short VA interval) 3. **Reentrant circuit**: entirely within or around the AV node ### VA Interval Interpretation **Clinical Pearl:** The VA interval of 80 ms is **short**, consistent with retrograde conduction via the fast pathway. In slow-fast AVNRT, the VA interval is typically **≤100 ms** (short RP tachycardia). ### ECG Characteristics of Slow-Fast AVNRT | Feature | Slow-Fast AVNRT | |---------|------------------| | **RP interval** | Short (P buried in QRS or early ST) | | **VA interval (EP)** | ≤100 ms (short) | | **AH interval (EP)** | Prolonged (slow pathway conduction) | | **HV interval** | Normal | | **Dual AV nodal physiology** | Present (jump in AH) | | **Tachycardia rate** | 130–250 bpm | **Mnemonic: SLOW-FAST = AVNRT** - **SLOW** pathway conducts anterograde (long AH) - **FAST** pathway conducts retrograde (short VA) - Result: short RP interval on ECG ### Why Dual AV Nodal Physiology Is Diagnostic The **sudden AH jump** during atrial pacing is pathognomonic for dual AV nodal pathways and is the gold standard for diagnosing AVNRT. This finding is NOT seen in AVRT (which requires an accessory pathway outside the AV node). ### Induction of Tachycardia During atrial pacing: 1. Fast pathway becomes refractory 2. Conduction shifts to slow pathway 3. Slow pathway conducts the atrial impulse to the ventricle (long AH) 4. Fast pathway recovers and conducts retrograde (short VA) 5. Reentrant circuit is established → tachycardia [cite:Harrison 21e Ch 226; Zipes & Jalife Cardiac Electrophysiology 6e]