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Figure 6 (A) Entrainment from a site in an atrial flutter circuit is shown. From the top of the panel are 10-ms time lines, surface electrocardiogram leads (I, II, V1, V5), bipolar recordings from the HRA and a distal pair of electrodes of the ablation catheter (Abl 1–2). The PPI is 235 ms and equals the TCL. Using the flutter wave as a fiducial point, the interval from the last stimulus to the second following F wave (S-Fn+1) is 340 ms. The interval from the local Eg of the ablation catheter to the second after the F wave (Egn+2-Fn+3) is 340 ms. Thus, the N + 1 difference is 0. Using the HRA electrogram as a reference, the interval from the last stimulus to the second after the A in the HRA (S-An+1) is 310 ms. The interval from the local Eg of the ablation catheter to the second following A in the HRA (Egn+1-An+2) is also 310 ms. Thus, the N + 1 difference is also 0. The intervals using the flutter wave as a reference are shown with dashed lines, and the intervals using the HRA Eg as a reference are shown with dotted lines. (B) Entrainment from a bystander site is shown. Entrainment occurs without definite P wave fusion although it is difficult to assess due to the low P-wave amplitude. The TCL is 240 ms, and the PPI is 300 ms, so the PPI-TCL difference is 60 ms, consistent with a bystander site. Stimulus-An+1 is 380 ms. The interval from the local Eg of the ablation catheter to the second after the Eg in the HRA (Egn+1-An+2) is 320 ms. Thus, the N + 1 difference using the HRA Eg as the fiducial point is 60 ms. Using the P-wave as the fiducial point, the S-Fn+1 = 375 ms and the Egn+1-Fn+2 = 315 ms, yielding the N + 1 difference of 60 ms. The intervals using the flutter wave as a reference are shown with dashed lines, and the intervals using the HRA Eg as a reference are shown with dotted lines. Eg = electrogram; HRA = high right atrium; PPI = postpacing interval; TCL = tachycardia cycle length.
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