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Figure 4 Isopotential maps with the lateral wall of the right atrium (RA) removed so that the virtual endocardium shows the septum (Sept), tricuspid annulus-inferior vena cava (IVC) isthmus (IS) and tricuspid annulus (TA) in detail and the reader is looking in toward the TA. The possible position of the eustachian ridge is drawn onto the virtual endocardium as a yellow line. The atrial flutter wave front enters and passes through the IS (frame 1) and encounters a line of block at the posterior IS. The wave front turns toward the TA (arrow), passes up over this line of block and back down the posterior aspect of the line of block (frames 2 and 3). The wave front exits the posterior RA (frames 3 to 4). The posterolateral RA activates before the TA as does the anterior RA (frames 4 to 6). Activation then progresses from both areas toward the TA where the wave front(s) extinguish. The anterolateral wave front then passes into the anterior IS and through the IS to repeat the circuit (frame 6 to 1). Ant - anterior; cs = coronary sinus; His = His bundle; SVC = superior vena cava.