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Figure 3 Another example of preexisting partial cavotricuspid isthmus block. Left panel: A sequentially reconstituted withdrawal map in a patient before ablation during counterclockwise atrial flutter (AF) from the tricuspid valve (TV) edge to the inferior vena cava (IVC) edge. Double potentials (DPs) are noted from midisthmus to the IVC edge (bottom three panels). Middle panel: The first radiofrequency (RF) application at a single potential with a far-field second potential near the TV (star) terminated AF without isthmus block. Right panel: Further withdrawal mapping during low lateral atrial pacing revealed a fractionated electrogram near the previous ablation site (right upper panel). A second RF application at this gap site resulted in widely separated DPs (right bottom panel) and eliminated isthmus conduction. Scale bars indicate 0.1 mV and 100 ms. ABL = bipolar electrogram from ablation catheter; V = ventricular electrogram.