Preexcitation of the ventricles is caused by a structural defect within the heart. The ventricles are depolarised earlier than normal because conduction has not been slowed by the atrioventricular node.
Preexcitation syndromes describe the presence of an additional, accessory pathway which bypasses the normal conduction route. The most significant of these syndromes is Wolff-Parkinson-White syndrome, in which the accessory pathway is called the Bundle of Kent.
Wolff-Parkinson-White syndrome is arrhythmogenic. This means that it creates a predisposition to rhythm abnormalities. Commonly associated arrhythmias include atrial fibrillation and atrioventricular reentrant tachycardia.
ECG criteria are a short PR interval (less than 0.12 seconds), a delta wave (a slurred upstroke in the QRS complex representing early ventricular activation), and possible ST segment and T wave changes.
The ECG trace below starts with normal ventricular conduction. The last two beats exhibit ventricular preexcitation, showing a delta wave in the QRS complex (shaded) and changes to the T wave (arrowed).