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Figure 3 Voltage maps (A), isopotential maps (B), and unipolar electrograms along the re-entry circuit (C) in a patient with atypical flutter. (A) Normalized peak negative voltage (PNV) distribution of the right atrium (RA) in a posterolateral view. Note that the protected isthmus was found between 2 low voltage zone (LVZ) areas, where the normalized PNV was from 0% to 30%. (B) Activation wavefront of atypical flutter (clockwise upper loop re-entry flutter) passing through the protected isthmus identified by the voltage maps. CT = crista terminalis; IVC = inferior vena cava; SVC = superior vena cava. (C) Unipolar electrograms (Eg) within the isthmus exhibit fragmented electrograms (virtual 3 and 4), indicating there is slow conduction; whereas the unipolar electrograms outside the isthmus exhibit an rS pattern (virtual 1, 2, and 5).