Wolff-Parkinson-White Syndrome: ECG Presentation
Classic ECG Diagnostic Triad
WPW syndrome is diagnosed on ECG by three pathognomonic features: PR interval <120 ms, delta wave (slurred upstroke of the QRS), and widened QRS complex >120 ms. 1
The Three Essential Components
Short PR interval (<120 ms) occurs because the accessory pathway bypasses the AV node, conducting the electrical impulse prematurely to the ventricles 1, 2
Delta wave represents the slurred, slow upstroke at the initial portion of the QRS complex, reflecting early ventricular activation through the accessory pathway before normal AV nodal conduction arrives 1, 2
Widened QRS complex (>120 ms) results from fusion of two wavefronts—one through the accessory pathway and one through the normal AV node-His-Purkinje system 1, 2
Secondary repolarization changes manifest as ST segment-T wave changes that are generally directed opposite (discordant) to the major delta wave and QRS complex 2
Critical Diagnostic Pitfalls
Left lateral accessory pathways may show minimal delta waves due to fusion with normal AV nodal conduction, potentially appearing as intermittent pre-excitation when actually continuously present 3
Septal Q waves can be present despite manifest pre-excitation, so physicians should not dismiss WPW in the presence of septal Q waves 4
Short PR interval alone (<0.12 seconds) without delta waves does not constitute WPW and may represent normal variant conduction, enhanced AV nodal conduction, or Lown-Ganong-Levine syndrome only if recurrent unexplained tachyarrhythmias occur 5, 3
Variable ECG Presentation
The degree of pre-excitation varies based on relative conduction through the AV node versus the accessory pathway, meaning the delta wave and QRS widening may be subtle or prominent 6
Intermittent pre-excitation (loss of delta wave on ambulatory monitoring or during exercise) indicates a longer accessory pathway refractory period and identifies low-risk pathways with 90% positive predictive value 3, 6
Associated Arrhythmias on ECG
Atrioventricular re-entry tachycardia (AVRT) is the most common arrhythmia, accounting for 95% of reentrant tachycardias in WPW patients, presenting as narrow or wide QRS complex tachycardia 3, 7
Pre-excited atrial fibrillation appears as an irregularly irregular wide QRS tachycardia with varying QRS morphology and is particularly dangerous when the shortest pre-excited R-R interval is <250 ms 3, 7
Risk Stratification Based on ECG Features
High-risk ECG features requiring immediate cardiology referral include:
Shortest pre-excited R-R interval <250 ms during atrial fibrillation is the strongest predictor of life-threatening events 3, 6
Abrupt loss of pre-excitation during exercise testing suggests a long anterograde refractory period and low risk of sudden death 5, 3
Intermittent loss of pre-excitation on resting ECG or ambulatory monitoring indicates low risk with 90% positive predictive value 3, 6
Mandatory Workup After ECG Diagnosis
Echocardiography is essential to rule out associated structural heart disease including Ebstein anomaly, hypertrophic cardiomyopathy, or glycogen storage cardiomyopathy (PRKAG2-related familial WPW) 5, 3
24-hour Holter monitoring detects paroxysmal arrhythmias and assesses for intermittent pre-excitation 3
Exercise ECG evaluates if pre-excitation disappears with exercise, suggesting low risk 5, 3
Electrophysiological study is the gold standard for risk stratification in both symptomatic and asymptomatic patients, identifying accessory pathway refractory period <240 ms, multiple pathways, or inducible sustained AVRT 3