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Figure 3 (A) Unipolar isopotential maps recorded during counterclockwise atrial fluter. The endocardium is opened, unfolded and flattened, resulting in stretching of the anterolateral right atrium (RA) (spread grid-lines) and compression of the posteroseptal wall (packed grid-lines). The positions at which bipolar electrograms have been reconstructed and displayed in B are labeled as a to e from superior to inferior along a line of block, compatible with the position of the crista terminalis (CT). Activation passes through the tricupsid annulus-inferior vena cava isthmus (frame 1) and turns toward the tricuspid annulus (TA) (arrow) around a line of block (possibly the eustachian ridge [ER]) (frame 2). The wave front the then begins to split (frame 3) with one front moving superiorly along the line of block while the second front moves slowly along the ER. The first wave front turns at the superior vena cava (SVC) (frame 4) and then passes inferiorly along the line of block (frames 5 and 6). The second front passes slowly through a break in the line of block (frames 4 to 6). The first and second wave fronts then fuse (frame 7), pass around the inferior vena cava (IVC) to enter (frame 8) and pass through the IS (frame 1). (B) The surface electrocardiographic lead II, an electrogram from a catheter at the coronary sinus os and reconstructed bipolar electrograms from points a to e on the isopotential maps in A. The numbers 1 to 8 represent the points at which the isopotential maps have been displayed in A. The electrograms from the line of block show double potentials apart from those recorded at the break in the line of block, which are long and fractionated, representing the slow activation at this point.