Figure 11.

Constant and progressive fusion in the absence of entrainment and reentry. Both panels show a 12-lead ECG at the end of an atrial pacing train introduced during a “focal ventricular tachycardia” simulated by ventricular pacing in the VVI mode at a constant cycle length of 560 ms in a patient with ventricular preexcitation. In the left panel atrial pacing is performed at a cycle length of 520 ms and in the right panel at 480 ms. Note that there is constant fusion at each paced cycle length, not unexpected in preexcitation. As the pacing rate increases (right panel) there is a subtle change in the degree of fusion, as observed particularly in III, aVF, and V1. In both panels the “ventricular tachycardia” resumes after pacing is stopped. So fusion is progressive and the second criterion of entrainment met. Obviously the QRS morphology with atrial pacing at the same rate in the absence of “ventricular tachycardia” was identical, so fusion did not look at all as an intermediate morphology between tachycardia and fully paced beat. This emphasizes the importance to demonstrate that fusion is due in part to the tachycardia morphology. See text for further discussion. Courtesy of Dr. J. Garcia Fernandez.