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Figure 2 Recordings during coronary sinus (CS) pacing before (left and middle) and after (right) complete isthmus block are shown. Displayed are leads II, III and V₁, electrograms recorded by the ablation catheter (Abl), electrograms recorded at E9 through E1 of the halo catheter, an electrogram recorded in the CS and the stimulus (St) channel. The arrows in the electrograms recorded by the Abl point to the components of the double potentials (DPs). The Abl was positioned at exactly the same site in all three panels. (Left) After several applications of radiofrequency energy along the ablation line, the interval separating the two components of DPs (DP_1-2) is 61 ms, and there is incomplete block. (Middle) After an additional application of radiofrequency energy, the DP_1-2 interval increases to 96 ms, but isthmus block is still incomplete. (Right) After a final application of radiofrequency energy, the DP_1-2 interval lengthens to 124 ms, and now there is complete block, based on both the atrial activation sequence in E1 to E9 and a change in the initial polarity of E1 and E2 from positive to negative. Note that when the DP_1-2 interval was 96 ms, the segment separating the two components of the DP was not isoelectric, providing further evidence that there was a persistent gap in the ablation line. On the transition to complete block, the segment with the DP became isoelectric.