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Figure 1 (A) A 4-s acquisition of an unfiltered wide bipolar electrogram from the coil electrodes placed in the left atrium and the right atrium. This signal was obtained just before a successful attempt to terminate atrial fibrillation (AF) through rapid burst atrial pacing. The high-amplitude activity represents the far-field ventricular activation, and the low-amplitude signal represents global bipolar interatrial activity. In this example, the AF signal is transiently very well organized ("flutterlike"), as evidenced by the regular 2:1 ventricular response to an atrial rhythm with a cycle length of 120 to 130 ms. (B) An unfiltered wide bipolar electrogram obtained from the same animal immediately before an unsuccessful attempt to terminate AF through rapid burst atrial pacing. The atrial activity and the ventricular response are grossly irregular, suggesting a more disorganized rhythm.