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Figure 3 Morphologic variations of clockwise isthmus-dependent atrial flutter on 12-lead surface electrocardiogram: (A) Type 1: F+(n) or F+ in II, III, aVF, and V6; F– in V1, narrow F-wave/distinct isoelectric segment. (B) Type 2: f–/F+9(n) or F+(n) inferiorly and V6, isoelectric in V1, with a broad F-wave and no distinct isoelectric segment. Abbreviations as in Figure 1.