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Figure 5 Intra-atrial variability in cycle length and activation sequence. (A) Counterclockwise isthmus-dependent atrial flutter (IDAFL) (patient in Figs. 2 and 4A) with mean cycle length (CL) 231 ms and standard deviation (SD): low lateral right atrium (RA) (Halo 3) 4.58 ms, septal RA (Halo 8) 4.37 ms, mid coronary sinus (CS3) 3.32 ms, and distal CS (CSD) 4.36 ms. Spatially, SD of activation time differences (CSD to Halo 3) was 3.32 ms. (B) Non-isthmus-dependent atrial flutter (NIDAFL) (patient in Figs. 3 and 4B). Mapping confirmed lateral RA re-entry with mean CL 303 ms and greater CL SD: low lateral RA (Halo 2) 6.23 ms, septal RA (Halo 8) 8.57 ms, proximal CS (CSP) 7.00 ms, and distal CS (CSD) 5.46 ms. Spatially, activation time (CSD to Halo 2) had SD 10.15 ms (Table 2).