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Figure 2 Intracardiac recordings showing concealed premature atrial contraction (PAC), which eventually managed to activate the atrium and initiate AF. Differently from what we observed distally in the vein, ostial initiations appeared to have a more longitudinal activation sequence. Note simultaneous activation in T1, T2, T3, T6, and T7. Bipole T4 does not record any PVP, but appeared to correspond to the take off of a branch. See Figure 1 for abbreviations.