The N + 1 difference: a new measure for entrainment mapping
Kyoko Soejima, MDa,
William G. Stevenson, MD, FACCa,
William H. Maisel, MDa,
Etienne Delacretaz, MDa,
Corinna B. Brunckhorst, MDa,
Kristin E. Ellison, MDa and
Peter L. Friedman, MD, PhD, FACCa
a Cardiovascular Division, Department of Internal Medicine, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts, USA
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Figure 1 A schematic of a figure-eight re-entry circuit is shown. The black area represents inexcitable scar tissue. Specific sites in the circuits are indicated by numbers. The sites between 10 and 1 are in the central common pathway, with 1 being an exit and 10 being an entrance. Sites 3, 6 and 9 are in the inner loop, and sites 24, 27 and 30 are in the outer loop of the circuit. A bystander pathway containing site 35 is connected to the central common pathway. The QRS onset occurs after the excitation wave exits from the common pathway at site 1. The borderline between the normal tissue and the infarct tissue is shown by the dotted line. Pacing at sites to the right of the dotted line produces wavefronts that alter activation remote from the circuit, producing QRS fusion.
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Figure 2 Pacing from an outer loop site in the computer simulation is shown. (A) Capturing stimuli produce wavefronts that propagate directly away from the circuit altering the QRS complex and producing QRS fusion. The S-QRSn is the short interval from the stimulus to the fused QRS after the stimulus. (B) S-QRSn+1 is the interval required for the stimulated wavefront to travel orthodromically through the circuit to the exit (arrow) to produce QRSn+1. (C) The stimulated orthodromic wavefront continues through the circuit, returning to the stimulus site. The PPI is the revolution time around the circuit (423 ms). (D) The S-QRSn+1 interval is 369 ms, equaling the Egn+1-QRSn+2 and also Egn+2 - QRSn+3 interval. Thus, the N + 1 difference is 0. The PPI of 423 ms equals the tachycardia cycle length. See text for further discussion. Eg = electrogram; PPI = postpacing interval; S = stimulus.
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Figure 3 Pacing from a bystander dead-end pathway in the computer simulation of the same circuit as in Figure 2 is shown. (A) The S-QRSn interval is produced by conduction of the stimulated wavefront from the pacing site to the circuit exit (arrow). Entrainment occurs with concealed fusion. X is the conduction time from the pacing site to the circuit common pathway, and y is the conduction time from the junction of the bystander pathway and circuit to the circuit exit, which produces the onset of the QRS. (B) The wavefront continues through the circuit, returning to the exit to produce QRSn+1. S-QRSn+1 is the sum of the S-QRSn (x + y) plus the TCL. (C) The PPI is the sum of the conduction times from the pacing site to the circuit, through the circuit and back to the pacing site. (D) The S-QRSn+1 interval is 484 ms and is longer than the Egn+1-QRSn+2 and Egn+2-QRSn+3 interval, which is 434 ms. The N + 1 difference is 484 – 434 = 50 ms. The PPI-TCL difference is 473 – 423 = 50 ms. See text for discussion. Eg = electrogram; PPI = postpacing interval; S = stimulus; TCL = tachycardia cycle length; VT = ventricular tachycardia.
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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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Figure 5 Graphs of the PPI-TCL difference (abscissa) versus the N + 1 difference (ordinate) are shown for patients with VT (A) and atrial flutter (B). In each panel, the N + 1 difference calculated using a QRS complex or P-wave in the surface ECG is indicated by open circles; the N + 1 differences calculated using an intracardiac Eg are indicated with black squares. ECG = electrocardiogram; PPI = postpacing interval; TCL = tachycardia cycle length; VT = ventricular tachycardia.
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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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