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Figure 4 Pacing at an exit site 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 (M-DIS and M-PR) and the right ventricular apex (RVA). Sustained monomorphic ventricular tachycardia (VT) is present with a cycle length (CL) of 420 ms. The last three stimuli (S1) of a stimulus train at a CL of 400 ms are shown. Pacing accelerates the QRS complexes to the paced CL of 400 ms without alternating their morphology as compared with that of the QRS complexes during tachycardia (entrainment with concealed fusion). After the last stimulus, the next electrogram recorded at the pacing site is an IP, which occurs after 420 ms. The postpacing interval (PPI) therefore matches the VTCL consistent with a reentry circuit site. The S-QRS complex interval is 80 ms, which is less than 30% of the VTCL and therefore consistent with an exit site. The electrogram recorded from the distal electrode pair of the mapping catheter contains an IP with an amplitude of 0.2 mV and duration of 30 ms (arrowheads). The main component of the electrogram (arrows) is also abnormal with a duration of 160 ms and an amplitude of 0.3 mV. All measurements are in milliseconds.