Pre-excitation

Definition

Early activation of part of the ventricular muscle through an accessory pathway (classically a bundle of Kent) that conducts the atrial impulse around the AV node. The classic triad is a short PR interval (under 120 ms), a slurred initial QRS upstroke (delta wave), and a widened QRS (over 120 ms). The QRS blends early activation through the pathway with normal activation through the AV node and His-Purkinje system, and how much muscle each route activates can change, so the degree of pre-excitation varies and can even become hard to see. The ECG finding alone is called the WPW pattern; when it is accompanied by symptoms such as tachyarrhythmias it is termed Wolff-Parkinson-White syndrome.

Clinical noteUnlike the AV node, a typical accessory pathway (Kent bundle) does not slow its conduction as the atrial rate rises, so atrial fibrillation in a person with pre-excitation can be conducted to the ventricles very rapidly through the pathway (pre-excited atrial fibrillation) and can degenerate into ventricular fibrillation.