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Figure 4 (A) Entrainment of VT produced by pacing at an outer loop site in the circuit is shown. From the top of the panel are surface electrocardiogram leads (I, II, V1, V6), intracardiac Egs recorded from the ablation catheter, either unipolar high pass filtered (Abl 1 HP) or the distal bipole (Abl 1–2) and RVA and 10-ms time lines. Pacing entrains VT with QRS fusion. The TCL is 355 ms, and the PPI is 355 ms. Stimulus-QRSn+1 (arrow) is 370 ms and the Egn+2-QRSn+3 is 370 ms (arrow). The N + 1 difference equals the PPI-TCL difference; both are zero. Measurements using the RVA Eg as the fiducial point are also shown. The S-Vn+1 is 455 ms, and the Egn+2-Vn+3 is also 455 ms. (B) Entrainment of VT produced by pacing at a bystander site remote from the re-entry circuit is shown. Entrainment from this bystander site alters the QRS complexes indicating fusion. The TCL is 540 ms, and the PPI is 595 ms, yielding a difference of 55 ms. Stimulus-QRSn+1 is 615 ms and the Egn+2-QRSn+3 is 560 ms. The N + 1 difference is 615 – 560 = 55 ms. Measurements using the RV Eg as the fiducial point are also shown. The S-Vn+1 = 655 ms. Egn+2-Vn+3 = 600 ms. Thus, the N + 1 difference calculated from the RVA Eg = 55 ms. Eg = electrogram; PPI = postpacing interval; RVA = right ventricular apex; S = stimulus; TCL = tachycardia cycle length; VT = ventricular tachycardia.
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