This study introduced a new method for the quantification of the synchronization (S) and the causal verse of activation (S12) in couples of atrial electrograms recorded during atrial fibrillation (AF). The synchronization indexes S and S12 relied on the measure of the propagation delays between coupled activation times in two atrial signals and on the characterization of their dispersion by Shannon-Entropy (SE). S and S12 were validated both on simulated activation time series and endocavitary signals in patients. In simulation, S and S12 were equal to 1 for propagation of one single wavefront in a fully excitable tissue, while they decreased for reentries in partially excitable tissue (S = 0.70 ± 0.05, S12 - 0.66 ± 0.05) and multiple wavelet propagation (S = 0.46 ± 0.06, S12 = 0.39 ± 0.08). In patients S, was equal to 1 during atrial flutter (AFl) and decreased with increasing complexity of AF (AF1: S = 0.76 ± 0.05; AF2: S = 0.56 ± 0.06; AF3: S = 0.39 ± 0.03). Moreover S12 evidenced the preservation of a correct activation sequence during AFl and AF1 (S12 = S) and its loss during AF2 (S12 = 0.41 ± 0.12 < S) and AF3 (S12 = 0.26 ± 0.03 < S). As indirect markers of the electrophysiological properties of atrial tissue, indexes S and S12 may provide a new insight in understanding the mechanisms initiating and maintaining AF and support new clinical treatments for its interruption.