The diagnosis of WPW typically occurs via ECG. The pathognomonic ECG findings in WPW are the delta wave, characterized by a slurred upstroke in the QRS complex and a short PR interval <120 ms (Figure 1). Depolarization of the ventricles via the accessory pathway contributes to QRS durations longer than 120 ms. The location and refractory period of the accessory pathway may diminish the prominence of the delta wave, making the diagnosis more challenging in some cases. ECG findings associated with a subtle WPW pattern include left-axis deviation, abnormal Q waves in leads V5 and V6, ST-segment depression, and T-wave changes. An intermittent WPW pattern on ECG (ie, a delta wave present on every other QRS complex) is considered low risk for ventricular arrhythmia.

 Figure 1.