A 28-year-old woman with a history of palpitations since childhood presents with a regular narrow-complex tachycardia at 180 bpm. An ECG shows a short PR interval and a delta wave. Regarding the electrophysiological properties of accessory pathways in Wolff-Parkinson-White (WPW) syndrome, all of the following are true EXCEPT:

Explanation

## Electrophysiology of Accessory Pathways in WPW Syndrome ### Key Properties of Accessory Pathways **Key Point:** Accessory pathways are abnormal conduction tissues that bypass the AV node, allowing both antegrade (atrium → ventricle) and retrograde (ventricle → atrium) conduction. **High-Yield:** The refractory period of an accessory pathway is typically **shorter** than that of the AV node. This is critical because a short refractory period allows rapid ventricular conduction during atrial fibrillation, risking degeneration to VF. ### Mechanisms of SVT in WPW | SVT Type | Mechanism | Frequency in WPW | ECG Features | |----------|-----------|------------------|---------------| | **Orthodromic AVNRT** | Antegrade via AP, retrograde via AV node | ~90% | Narrow QRS, RP interval short | | **Antidromic AVNRT** | Antegrade via AV node, retrograde via AP | ~10% | Wide QRS (pre-excited) | | **Atrial fibrillation** | Rapid conduction via AP | Variable | Irregularly irregular, wide QRS | **Clinical Pearl:** In WPW, orthodromic AVNRT (not AVNRT as a standalone entity) is the most common SVT mechanism, occurring in ~90% of cases. The question's distractor states "orthodromic atrioventricular reentrant tachycardia (AVNRT)" — this is a **terminology error**. AVNRT (atrioventricular nodal reentrant tachycardia) is a distinct arrhythmia that occurs in patients WITHOUT accessory pathways; it uses dual AV nodal pathways. In WPW, the correct term is **orthodromic AVNRT** or **orthodromic reentrant tachycardia**, not "AVNRT." **Warning:** Do not confuse: - **AVNRT** = reentry within the AV node itself (no accessory pathway needed) - **AVNRT in WPW** = reentry using the accessory pathway + AV node (orthodromic or antidromic) ### Why the Correct Answer is Wrong Option 2 conflates two distinct arrhythmias. AVNRT (atrioventricular nodal reentrant tachycardia) is the most common SVT in the general population, but it does NOT occur in WPW syndrome because it requires dual AV nodal pathways, not an accessory pathway. In WPW, orthodromic reentrant tachycardia (using the AP for antegrade conduction) is the most common SVT mechanism. **Mnemonic:** **WOWPW** — WPW has **Orthodromic** (90%) and **antidromic** (10%) reentrant tachycardias; **AVNRT** is a different entity (dual AV node pathways).