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Figure 2 Heart rate turbulence onset (TO) after spontaneous atrial premature beats in 15-min periods during the 1 h preceding atrial fibrillation (AF) episode(s). In the subgroup of patients (n = 15; 17 recordings) with at least one atrial ectopic beat in each 15-min period during the hour preceding an AF episode, TO is significantly less negative during the 0- to 15-min period before AF than during the 45- to 60-min period preceding AF.