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Figure 4 Activation maps of the left superior pulmonary vein during pulmonary vein (PV) distal pacing and during coronary sinus (CS) distal pacing from a representative episode. Letters A to H identify basket splines. An anatomic grid is shown in an apical view, with the distal electrodes in the center and proximal electrodes on the periphery. Lines with arrows indicate activation wave fronts. Thin lines represent isochrones at 10-ms intervals. (Left map) During PV distal pacing from bipoles A 1–2 (star), the earliest activation site around the PV-left atrium junction (i.e., the exit breakthrough point) is B 7–8. (Right map) During CS distal pacing, the entrance breakthrough point is A or H 7–8.