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Figure 1 A 45° left anterior oblique fluoroscopic view of catheter positions during atrial flutter ablation (ABL) is shown. A duo-decapolar halo catheter is positioned around the tricuspid annulus and along the cavo-tricuspid isthmus. Electrode pairs of the halo catheter are labeled E1 to E9. Note that E1 and E2 are positioned just lateral to the intended ABL line. A quadripolar electrode catheter is positioned within the coronary sinus (CS) for pacing. The ABL catheter is positioned at the intended ABL line. The recordings displayed in Figures 2 through 5 were obtained with the catheters positioned as shown in this figure.