Wolff-Parkinson-White syndrome

Patients with WPW syndrome have an extra electrical pathway between the atria and the ventricles, known as an accessory pathway. The abnormal pathway directly connects the atria and ventricles and bypasses the AV node, which is the slowest conducting part of the heart's electrical system. As a result, the normal sinus impulse can travel down the normal pathway through the AV node, as well as the more rapidly conducting accessory pathway. This allows the impulse traveling through the accessory pathway to reach the ventricle earlier, causing what is termed "preexcitation."