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Figure 5 An interval separating the two components of double potentials (DP_1-2) of 94 ms was recorded along the ablation (Abl) line in the cavo-tricuspid isthmus during coronary sinus (CS) pacing, after several applications of radiofrequency energy. The DP_1-2 interval is between 90 and 110 ms, which is equally likely when block is incomplete or complete. In this case, the fractionated, low-amplitude electrogram within the DP, along with the predominantly positive polarity of the second component of the DP, suggests that block is incomplete. The presence of incomplete block is confirmed by the atrial activation sequence in E1 to E8 and by the positive initial polarity of E1 and E2. Without moving the ablation catheter, an additional application of radiofrequency energy was delivered, and the DP_1-2 interval increased to 132 ms, a value always associated with complete isthmus block. Complete block was confirmed by the atrial activation sequence in the halo catheter and by a change to negative initial polarity of E1 and E2. St = stimulus channel.