Functional Conduction Block During Extrastimulus Mapping Masks Deceleration Zones and Reveals Additional VT Substrate

BACKGROUND: Extrastimulus pacing may identify ventricular tachycardia substrate by exposing decremental conduction, a proarrhythmic myocardial property. Functional conduction block may also occur during extrastimulus pacing but its relevance as a marker of arrhythmogenicity is unknown. OBJECTIVES: This study sought to establish the differences in ventricular tachycardia substrate identification between steady-state (S1) and single extrastimulus (S2) pacing using isochronal late activation mapping deceleration zones (DZs). METHODS: S1 and S2 (ventricular effective refractory period +20 ms) maps were collected during right ventricular pacing. DZs were identified for all maps. Annotation delta (ΔS1S2), the difference in last deflection between S1 and S2, was calculated to identify regions demonstrating decremental conduction and functional conduction block. Changes in DZ location and ΔS1S2 were analyzed to identify altered substrate behavior evoked by the extrastimulus. RESULTS: Eleven patients (age 66 ± 10 years, 9 male) were included. DZ location was significantly different between S1 and S2 maps. Extrastimulus pacing revealed 5 DZs not identified during steady-state pacing, but functional conduction block masked 4 DZs on the S2 maps that were present on their corresponding S1 map. Regions of significant ΔS1S2 colocalized to the primary DZ in 19 of 22 maps, with no significant difference between the proportion of positive and negative delta observed (P = 0.898). CONCLUSIONS: Extrastimulus pacing may change the location of DZs identified on isochronal late activation mapping maps and can reveal additional substrate; functional block can mask DZs on S2 maps. Analysis of both S1 and S2 maps facilitates more comprehensive substrate characterization, and regions demonstrating significant ΔS1S2 may represent important ablation targets.