A 35-year-old man with known Wolff-Parkinson-White syndrome presents with palpitations. ECG during the acute episode shows a wide-complex tachycardia at 280 bpm with a left bundle branch block pattern. A colleague suggests the arrhythmia is atrial fibrillation with accessory pathway conduction. Which feature best distinguishes atrial fibrillation with accessory pathway conduction from orthodromic AVRT in this patient?

Explanation

## Atrial Fibrillation with AP Conduction vs. Orthodromic AVRT ### Critical Distinction: Rhythm Regularity **Key Point:** The single most reliable discriminator is RR interval regularity. Orthodromic AVRT is perfectly regular; atrial fibrillation with AP conduction is irregularly irregular. ### Comparison Table | Feature | Orthodromic AVRT | AF with AP Conduction | |---------|------------------|----------------------| | **RR Interval** | Regular (fixed cycle length) | Irregularly irregular | | **P Waves** | Visible (retrograde) | Absent (fibrillatory waves) | | **QRS Morphology** | Constant during tachycardia | May vary (fusion beats possible) | | **Heart Rate** | 140–300 bpm (usually 180–250) | 200–350+ bpm (very rapid) | | **Adenosine Response** | Abrupt termination | May slow briefly; AF persists | | **Baseline ECG** | Delta wave + short PR | Delta wave + short PR | ### Pathophysiological Basis **High-Yield:** In orthodromic AVRT, the reentry circuit is fixed (AP → ventricle → AV node → atrium → AP), producing a regular rhythm. In AF with AP conduction, the atrial fibrillation is inherently irregular, and the AP conducts a random subset of atrial impulses, preserving the irregularity. **Clinical Pearl:** Atrial fibrillation with AP conduction is a medical emergency because the AP bypasses the AV node's rate-limiting function, allowing dangerously rapid ventricular rates (>300 bpm). This can degenerate into ventricular fibrillation. Adenosine and AV nodal blockers are contraindicated; treatment is procainamide or ibutilide followed by DC cardioversion if unstable. ### Why Regularity is the Key Orthodomic AVRT: - Fixed reentry pathway → predictable, constant cycle length - RR intervals are identical beat-to-beat AF with AP conduction: - Chaotic atrial activity → variable AV nodal and AP conduction - RR intervals vary randomly **Mnemonic: AF = Irregularly Irregular; AVRT = Regular as a metronome** [cite:Harrison 21e Ch 226]