Normally, the AV node is the only pathway that allows the wave of depolarization to conduct from the atria to the ventricles.
However, anomalous bands of tissue( accessory pathways or bypass tracts) may exist and form an additional conduction pathway. The conductive properties of these bypass tracts differ from those Av of the node in that they can conduct at extremely rapid rates without developing conduction block(i.e., they do not manifest the decremental conduction property of normal conduction tissue). The bypass tracts may conduct in one direction only or may be bidirectional.

These properties provide the substrate for macroreentrant arrhythmias using the bypass tract as one limb of the reentrant circuit and the AV node as the other. The WPW syndrome represents the most common accessory pathway and forms direct connection from the ventricular myocardium(Fig, 10-7). Other path ways are less frequent and may connect the atrial tissue directly to the His-Purkinje system(Lown-Ganong Levine or atrionodal pathway). The majority of patients with bypass tracts have otherwise anatomically normal hearts, although there is an increased incidence of accessory pathways in patients with Ebstein’s anomaly of the right side of the heart. When the bypass tract in WPW symdrome conducts in an antegrade fashion, preexcitation of the ventricles may occur; that is, an atrial depolarization will conduct to the ventricle more rapid down the AV node and results in a short PR interval(

The extent of preexcitation is determined by the conductive properties of the path ways as Av nodal conduction of the ventricle is preexcited through conduction down the bypass tract. With a”manifest” accessory pathway the preexcitation can be identified on the surface ECG as a slurring of the initial portion of the QRS complex The delta of WPW syndrome.