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Figure 3 Virtual electrograms and superimposed isopotential maps of a reentrant ventricular tachycardia (VT) that had a cycle length of 670 ms. The location of the distal electrode of a conventional catheter was determined with a locator signal, and the green locator line extends from the center of the balloon to the distal electrode of the conventional catheter. (A) Identification of the entire diastolic component of the reentry circuit. Shown are leads I, V1, V6 and aVF, along with the virtual and actual electrograms recorded at site 3, as depicted by the green locator signal in the left ventricular reconstruction. Note that the virtual electrogram faithfully reproduces the fractionated endocardial electrogram and the diastolic potentials recorded at site 3 with the conventional catheter. The instantaneous isopotential maps at the five time points depicted by the vertical lines through the electrograms have been superimposed on the left ventricular reconstruction to depict the path of the VT wavefront during diastole and at the onset of the QRS complex. (B) Isopotential maps during concealed entrainment with varying stimulus-QRS intervals. Shown are leads I, V1, V6 and aVF. Pacing was performed at a cycle length of 630 ms during VT at the two sites designated by the green locator signal, and, at both sites, the QRS complexes were identical to the QRS complexes during VT. In the top panel, the stimulus-QRS interval is 520 ms, and the path traveled by the wavefront (blue arrow) corresponds to the path of activation during the undisturbed VT depicted in (A). In the bottom panel, the stimulus-QRS interval is 190 ms, and, as expected, the wavefront originates at a site closer to the exit site of the VT and also follows the same activation path (blue arrow) as during VT. Successful ablation was achieved at the location depicted in the bottom figure.