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Figure 2 Entrainment of ventricular tachycardia (VT) at a site with a double potential is shown. From the top are surface electrocardiogram leads (I, II, III), and bipolar intracardiac electrograms recorded from the distal (Abl D) and proximal (Abl P) electrode pairs of the ablation catheter; VT with a cycle length of 410 ms is present. Pacing at a cycle length of 390 ms entrains tachycardia without altering the QRS morphology (concealed fusion). Recordings at the pacing site show two potentials marked with an asterisk and arrowhead, respectively. During entrainment, the potential marked with the asterisk is present and accelerated to the pacing cycle length. The tissue generating this potential is not directly depolarized by the pacing stimulus and is, therefore, designated as far-field (FFP). The local potential (indicated by an arrowhead) is not discernable during pacing, consistent with direct capture, but reappears after the last stimulus. The true post-pacing interval (PPI) (TPPI), which is measured from the last stimulus to the local potential, is 425 ms. The false PPI (FPPI), which is measured from the last stimulus to the FFP, is 210 ms, or, if measured to the next FFP electrogram one cycle later, would be 620 ms. Radiofrequency ablation at this site abolished tachycardia. Time lines are 20 ms.
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