Antegrade atrioventricular conduction through an accessory pathway

Schematic representation of atrioventricular (AV) conduction in the presence of an accessory pathway (AP) with antegrade conduction.
(A) The normal pacemaker is in the sinoatrial (SA) node at the junction of the superior vena cava and the right atrium (shown in green). The SA node activates the right and left atria. Ventricular activation results from the impulse traversing the AV node, the specialized infranodal conducting system (His bundle and bundle and fascicular branches, shown in red), thereby activating the ventricular myocardium (shown in yellow). In addition, however, the presence of an AP which conducts in the antegrade direction causes early activation (preexcitation) of a portion of the left ventricle (shown in blue). The degree of preexcitation depends upon the time required to conduct through the right and left atria, the AP, and the ventricular myocardium as compared to conduction through the normal pathways.
(B) The inset on the right shows the electrocardiogram (ECG) timing of these events. The net effect is a QRS complex that is a fusion of ventricular preexcitation (blue) and normal excitation (yellow). The combination of preexcitation and normal excitation leads to a short PR interval, a delta wave, and prolongation of the QRS duration. This ECG pattern is referred to as the Wolff-Parkinson-White pattern.