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Figure 2 Pacing at a proximal site in the reentry circuit where an isolated potential (IP) is present is shown. From the top of the figure are 10-ms time lines, 12 surface electrogram leads from I to V6 and intracardiac, bipolar recordings from the distal and proximal electrodes of the mapping catheter (A-BI-D and A-BI-P) and the right ventricular apex (RVA). Sustained monomorphic ventricular tachycardia is present with a cycle length (CL) of 490 ms. The last three stimuli (S) of a stimulus train at a CL of 450 ms are shown. Pacing accelerates the QRS complexes to the paced CL of 450 ms without altering their morphology as compared with that of the QRS complexes during tachycardia (entrainment with concealed fusion). The postpacing interval (PPI) is 510 ms. The S-QRS complex interval is 300 ms, which is less than 70% and more than 50% of the tachycardia CL and therefore consistent with proximal site. The electrogram recorded from the distal electrode pair of mapping catheter has an IP (arrowhead) with amplitude of 0.1 mV and duration of 10 ms. The main component of the electrogram (arrows) is also abnormal and is 90 ms in duration with an amplitude of 0.4 mV.
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