Wolff-Parkinson-White is characterized by the presence of an “accessory pathway” or a “bypass tract.” This connects the electrical system of the atria directly to the ventricles, allowing conduction to avoid passing through the atrioventricular node.
In normal individuals, when the sinus node creates an action potential, it must pass through the AV node to get to the ventricles. When an accessory pathway is present, the sinus node action potential can pass through the bypass tract before the AV node, resulting in the ventricles becoming rapidly depolarized. This is termed “pre-excitation” and results in a shortened PR interval on the ECG.
The typical ECG finding of WPW is a short PR interval and a “delta wave.“ A delta wave is slurring of the upstroke of the QRS complex. This occurs because the action potential from the sinoatrial node is able to conduct to the ventricles very quickly through the accessory pathway, and thus the QRS occurs immediately after the P wave, making the delta wave.
The combination of WPW and atrial fibrillation can potentially be fatal, especially if AV blocking agents are given (remember “ABCD” for adenosine or amiodarone, beta-blockers, calcium channel blockers and digoxin). The medical treatment is procainamide ― though electrical cardioversion is reasonable, especially with hemodynamic instability.
In patients with WPW and atrial fibrillation, the erratic atrial action potentials (occurring at 400-600 bpm) can conduct through the accessory pathway very quickly ― faster than through the AV node. Therefore, patients with WPW who develop atrial fibrillation have higher ventricular rates than those without WPW.
If an AV blocking agent is given, fewer atrial action potentials will pass through the AV node, and more will pass through the accessory pathway. This paradoxically increases the ventricular rate, potentially causing the fatal, hemodynamically unstable rhythm ventricular fibrillation. Procainamide or electrical cardioversion is recommended in these situations.
1. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
2. Surawicz B, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram. Circulation. 2009; doi:10.1161/CIRCULATIONAHA.108.191095.