Rate-Dependent Pacemap Matching in Scar-Related Ventricular Tachycardia: Impact of “TR Fusion” Phenomenon

Background: The impact of varying rates of pacemapping (PM) rates on QRS morphology and PM score matching in patients with scar-related ventricular tachycardia (VT) has not been systematically assessed. Objectives: In this study, the authors sought to assess the variability in PM score matching at different pacing rates. Methods: During substrate mapping for VT ablation, PM was performed at cycle lengths (CLs) of 600 ms, 500 ms, 400 ms, 300 ms, and VT CL. PM scores were compared for the entire QRS, the first half (H1) of QRS, and the second half (H2) of QRS to examine the influence of the preceding T-wave superimposed into the onset of paced QRS complex (TR fusion). Results: A total of 269 PMs in 40 patients undergoing scar-related VT ablation were systematically analyzed. The PM score improved at rates closer to VT with a median difference of 6% (Q1-Q3: 4%-10%; range: 0%-33%) between the lowest and the highest PM scores at a given site. Greater slurring of the QRS onset was observed at faster-paced CL, corresponding to a superimposition of the preceding T-wave into QRS onset, with significant differences in H1 but not H2 of the QRS complex. At faster PM rates, 32% of overall sites developed pseudo delta wave and 69% of endocardial pacing sites fulfilled epicardial criteria. Conclusions: The rate of pacemapping can significantly alter morphologic score matching, with the most optimal match observed closest to VT CL. The onset of QRS complex morphology is influenced by superimposition of the preceding T-wave at faster rates, resulting in an underrecognized TR fusion phenomenon that may confound epicardial electrocardiographic criteria predicated upon the initial QRS slope and vector.