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Figure 4 Upper panel, Simultaneous surface ECG leads II and III and BC and CS recordings in a patient with counterclockwise AF. Splines A and B were located in the anterolateral region; spline C in the septal region; spline D in the posterior region; spline E in posterolateral region; and splines F through H in the lateral region of the RA. The BC was inserted from the jugular vein. Complete coverage of the reentry circuit is demonstrated. Lower panel, Animated maps. A, Isthmus conduction. B, First 30 ms of activation in the septal and posterior walls. C, After 100 ms, the activation reaches the roof of the RA. D, Craniocaudal activation of the lateral region. E, Locations of the BC splines versus anatomic structures of the RA. The proximal pole of the BC is located over the inferior vena cava–tricuspid valve isthmus.