The N + 1 difference: a new measure for entrainment mapping

OBJECTIVES The purpose of this study was to develop and test a new entrainment mapping measurement, the N + 1 difference. BACKGROUND Entrainment mapping is useful for identifying re-entry circuit sites but is often limited by difficulty in assessing: 1) changes in QRS complexes or P-waves that indicate fusion, and 2) the postpacing interval (PPI) recorded directly from the stimulation site. METHODS In computer simulations of re-entry circuits, the interval from a stimulus that reset tachycardia to a timing reference during the second beat after the stimulus was compared with the timing of local activation at the site during tachycardia to define an interval designated the N + 1 difference. The N + 1 difference was compared with the PPI-tachycardia cycle length (TCL) difference in simulations and at 65 sites in 10 consecutive patients with ventricular tachycardia (VT) after myocardial infarction and at 45 sites in 10 consecutive patients with atrial flutter. RESULTS In simulations, the N + 1 difference was equal to the PPI-TCL difference. During mapping of VT and atrial flutter, the N + 1 difference correlated well with the PPI-TCL difference (r ≥ 0.91, p < 0.0001), identifying re-entry circuit sites with sensitivity of ≥86% and specificity of ≥90%. Accuracy was similar using either the surface electrocardiogram or an intracardiac electrogram (Eg) as the timing reference. CONCLUSIONS The N + 1 difference allows entrainment mapping to be used to identify re-entry circuit sites when it is difficult to evaluate Egs at the mapping site or fusion in the surface electrocardiogram.