What is pre-excitation in Wolff-Parkinson-White (WPW) syndrome?

Pre-excitation in Wolff-Parkinson-White Syndrome

Pre-excitation in WPW syndrome refers to an ECG pattern reflecting the presence of a manifest accessory pathway connecting the atrium to the ventricle, characterized by a short PR interval, delta wave, and widened QRS complex, resulting from ventricular activation that bypasses the normal AV nodal delay. 1, 2

Electrophysiological Mechanism

Pre-excitation occurs due to the following mechanism:

• An accessory pathway (Kent bundle) creates a direct muscular connection between the atria and ventricles, bypassing the AV node 2
• This pathway typically shows rapid, non-decremental conduction (similar to His-Purkinje system) 2
• During normal sinus rhythm, the electrical impulse travels simultaneously through:
  • The normal AV nodal-His Purkinje system (which has physiologic delay)
  • The accessory pathway (which has no physiologic delay) 1

ECG Characteristics

The classic pre-excitation pattern on ECG includes:

• Short PR interval (<120 ms) - due to rapid conduction through the accessory pathway
• Delta wave - slurring of the initial upstroke of the QRS complex
• Widened QRS complex (>120 ms) - resulting from fusion of normal and accessory pathway conduction 1, 2

The degree of pre-excitation depends on the relative contribution of ventricular activation from:

• Normal AV nodal/His-Purkinje conduction
• Conduction through the manifest accessory pathway 1

Clinical Significance

The importance of recognizing pre-excitation lies in its association with:

1. Tachyarrhythmias: Pre-excitation creates the substrate for various arrhythmias:

   • Orthodromic AVRT (most common - 95% of reentrant tachycardias in WPW) 1
   • Antidromic AVRT (less common)
   • Pre-excited atrial fibrillation (potentially life-threatening) 3

2. Risk stratification: The degree and stability of pre-excitation helps determine risk:

   • Intermittent pre-excitation (abrupt loss of delta wave) generally indicates lower risk 1
   • Persistent pre-excitation with shortest pre-excited R-R interval ≤250 ms during atrial fibrillation indicates high risk 1, 2

3. Diagnostic classification:

   • Pre-excitation pattern: ECG finding only
   • Asymptomatic pre-excitation: Pre-excitation ECG pattern without symptoms or documented SVT
   • WPW syndrome: Pre-excitation plus documented SVT or symptoms consistent with SVT 1

Important Clinical Considerations

• Pre-excitation can be intermittent or subtle in some patients with pathways capable of anterograde conduction 1
• Pre-excitation can be enhanced or revealed by maneuvers that slow AV nodal conduction (vagal maneuvers, adenosine)
• In drug-treated patients, propafenone reduces conduction and increases the effective refractory period of the accessory pathway 4
• Sudden cardiac death risk is approximately 0.15-0.39% over 3-10 years in WPW patients 1
• High-risk features include shortest pre-excited R-R interval <250 ms during AF, history of symptomatic tachycardia, multiple accessory pathways, and Ebstein's anomaly 1

Management Implications

Understanding pre-excitation is crucial for proper management:

• Avoid AV nodal blocking agents (calcium channel blockers, beta-blockers, digoxin) in patients with pre-excited atrial fibrillation, as they can enhance conduction through the accessory pathway and potentially precipitate ventricular fibrillation 1
• Catheter ablation is the definitive treatment for symptomatic patients with pre-excitation, with success rates exceeding 95% 2
• Risk stratification should guide management decisions, particularly in asymptomatic patients with incidentally discovered pre-excitation 1, 2

Pre-excitation in WPW syndrome represents a critical electrophysiological finding that requires careful assessment and appropriate management to prevent potentially life-threatening arrhythmias.