Wolff-Parkinson-White (WPW) syndrome is defined by a short PR interval on ECG (less than 120 milliseconds) with the presence of a delta wave (slurred segment between the P wave and QRS complex) and a history of palpitations/supraventricular tachycardia. Preexcitation (a delta wave on an ECG) exists in approximately 0.15 percent of the general population. Accessory pathways (or bypass tracts) are embryonic remnants between the atria and ventricles that are found in the atrioventricular groove.
Supraventricular tachycardia as a result of accessory pathways in children younger than six years old may be a temporary phenomenon. Subsequently, as the child develops, the accessory pathway may become nonfunctional (i.e., disappear). Therefore, it may be reasonable to hold off on catheter ablation in those children unless absolutely necessary.
Accessory pathways may be evident from a 12-lead ECG. This is called a manifest bypass tract, in which the presence of a delta wave is readily observed. In some instances (left lateral bypass tracts, for example), preexcitation is not evident except during AV nodal blockade (via carotid sinus massage and/or infusion of adenosine). This is called a latent bypass tract.
In other instances, the bypass tract conducts only in a retrograde direction (from the ventricle to the atrium). This is called a concealed bypass tract, where there is no risk for rapid conduction of atrial fibrillation down the bypass tract.