A 28-year-old woman presents with recurrent episodes of palpitations and syncope since childhood. Electrophysiological study reveals an accessory pathway with anterograde conduction. Which is the most common site of the accessory pathway in patients with Wolff-Parkinson-White syndrome?

## Accessory Pathway Anatomy in WPW Syndrome **Key Point:** The left lateral wall of the mitral annulus is the most common site for accessory pathways in WPW syndrome, accounting for approximately 40–50% of all cases. ### Distribution of Accessory Pathways | Location | Frequency | Clinical Features | |---|---|---| | **Left lateral (mitral annulus)** | 40–50% | Most common; delta wave in lateral leads (I, aVL, V5–V6) | | Posteroseptal (coronary sinus) | 15–20% | Delta wave in inferior leads; risk of nodal injury during ablation | | Right free wall (tricuspid annulus) | 10–15% | Delta wave in right precordial leads | | Right anteroseptal | 5–10% | Least common; high risk of AV node damage during ablation | **High-Yield:** The location of the accessory pathway can be predicted by the **polarity of the delta wave** on the 12-lead ECG: - **Left lateral:** positive delta in I, aVL, V5–V6 - **Posteroseptal:** negative delta in aVR, positive in II, III, aVF - **Right free wall:** positive delta in aVR, V1–V2 - **Right anteroseptal:** biphasic or isoelectric delta in aVR **Clinical Pearl:** During radiofrequency ablation, the left lateral pathway is the safest to ablate because it is distant from the AV node and conduction system. In contrast, right anteroseptal pathways carry the highest risk of complete heart block due to proximity to the AV node. **Mnemonic:** LPRR — **L**eft lateral (most common), **P**osteroseptal, **R**ight free wall, **R**ight anteroseptal (least common). [cite:Harrison 21e Ch 233]