BACKGROUND-: During persistent atrial fibrillation (AF), waves with a focal spread of activation are frequently observed. The origin of these waves and their relevance for the persistence of AF are unknown. METHODS AND RESULTS-: In 24 patients with longstanding persistent AF and structural heart disease, high-density mapping of the right and left atria was performed during cardiac surgery. In a reference group of 25 patients, AF was induced by rapid pacing. For data analysis, a mapping algorithm was developed that separated the fibrillatory process into its individual wavelets and identified waves with a focal origin. During persistent AF, the incidence of focal fibrillation waves in the right atrium was almost 4-fold higher than during acute AF (median, 0.46 versus 0.12 per cycle per 1 cm (25th to 75th percentile, 0.40 to 0.77 and 0.01 to 0.27; P<0.0001). They were widely distributed over both atria and were recorded at 46±18% of all electrodes. A large majority (90.5%) occurred as single events. Repetitive focal activity (>3) happened in only 0.8%. The coupling interval was not more than 11 ms shorter than the average AF cycle length (P=0.04), and they were not preceded by a long interval. Unipolar electrograms at the site of origin showed small but clear R waves. These data favor epicardial breakthrough rather than a cellular focal mechanism as the underlying mechanism. Often, conduction from a site of epicardial breakthrough was blocked in 1 or more directions. This generated separate multiple wave fronts propagating in different directions over the epicardium. CONCLUSIONS-: Focal fibrillation waves are due to epicardial breakthrough of waves propagating in deeper layers of the atrial wall. In patients with longstanding AF, the frequency of epicardial breakthroughs was 4 times higher than during acute AF. Because they provide a constant source of independent fibrillation waves originating over the entire epicardial surface, they offer an adequate explanation for the high persistence of AF in patients with structural heart disease.

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Keywords atrial fibrillation, conduction, mapping, reentry, remodeling
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de Groot, N., Houben, R.P.M., Smeets, J.L., Boersma, H., Schotten, U., Schalij, M.J., … Allessie, M.A.. (2010). Electropathological substrate of longstanding persistent atrial fibrillation in patients with structural heart disease: Epicardial breakthrough. Circulation (Baltimore), 122(17), 1674–1682. doi:10.1161/CIRCULATIONAHA.109.910901