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©2013 UpToDate ® Print Email AV conduction through an open accessory pathway Compared to normal conduction in the preceding diagram, the accessory pathway (AP) is now open. As a result, ventricular activation results from both early activation (preexcitation) of the free wall of the left ventricle (shown in blue) and from normal activation (shown in yellow). The degree of unopposed 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. The inset on the right shows the ECG timing of these events. The net effect is a QRS complex that is a fusion of ventricular preexcitation (blue) and normal excitation (yellow). Early activation throught the AP (V P ) occurs at about the same time as His bundle depolarization (H). This leads to a shorter PR interval, a small delta wave (arrow), and some prolongation of the QRS duration.