Periods of Highly Organized Activation During VF Background: Little is known about long-duration ventricular fibrillation (LDVF), lasting 1-10 minutes when resuscitation is still possible. Methods and Results: To determine global left ventricle (LV) endocardial activation during LDVF, 6 canines (9.5 ± 0.8 kg) received a 64-electrode basket catheter in the LV, a right ventricular (RV) catheter, and a 12-lead electrocardiogram (ECG). Activation sequences of 15 successive cycles after initiation and after 1, 2, 3, 5, 7, and 10 minutes of LDVF were determined. Early during VF, LV endocardial activation was complex and present throughout most (78.0 ± 9.7%) of each cycle consistent with reentry. After 3-7 minutes of LDVF in 5 animals, endocardial activation became highly synchronized and present for only a small percentage of each cycle (18.2 ± 7.7%), indicating that LV endocardial reentry was no longer present. During this synchronization, activations arose focally in Purkinje fibers and spread as large wavefronts to excite the Purkinje system followed by the subendocardial working myocardium. During this synchronization, the ECG continued to appear irregular, consistent with VF, and LV cycle length (183 ± 29 ms) was significantly different than RV cycle length (144 ± 14 ms) and significantly different than the LV cycle length when synchronization was not present (130 ± 11 ms). Conclusion: After 3-7 minutes of LDVF, a highly organized, synchronous, focal LV endocardial activation pattern frequently occurs that is not consistent with reentry but is consistent with triggered activity or abnormal automaticity in Purkinje fibers. The ECG continues to appear irregular during this period, partially because of differences in LV and RV cycle lengths. © 2010 Wiley Periodicals, Inc.