An anatomically defined re-entry circuit also exists in patients with ventricular pre-excitation via an AV nodal bypass tract. It is responsible for the paroxysmal supraventricular tachycardias that occur in these patients (the Wolff-Parkinson –White Syndrome). Re-entry is possible because of differences in the electrophysiologic properties of the AV node and the bypass tract.
In the example illustrated in this figure, the bypass tract is located on the lateral aspect of the right atrium (arrow). A non-premature sinus beat will conduct more rapidly through the bypass tract than through the AV node to “pre-excite” the ventricles and cause the delta wave on the electrocardiogram. It will then fuse with the impulse reaching the ventricles via the normal AV nodal pathway and His-Purkinje system to cause the remainder of the QRS complex. The two wave fronts will collide and extinguish themselves somewhere in the right ventricle.