In this example, the impulse is conducted through both the accessory pathway (the bypass tract) and the AV node. The ECG reveals a short PR interval and a slurred early portion of the QRS complex (arrow). This is referred to as a delta wave and is caused by the pre-excitation of part of the right ventricle via the bypass tract. Thereafter, the QRS complex appears more normal and reflects activation of the ventricles via the AV node and normal conduction pathway. Thus, the QRS complex represents a “fusion” of activation via the accessory pathway and the AV node.