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Figure 2 Morphologic variations of counterclockwise isthmus-dependent atrial flutter on 12-lead surface electrocardiogram: (A) Type 1: F– in II, III, aVF, and V6; F+ in V1. (B) Type 2: F–/f+ in II, III, aVF; F– or F–/f+ in V6, F+ in V1. (C) Type 3: f–/F+ in II, III, aVF, and V6; F+/f– in V1. Abbreviations as in Figure 1.