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Figure 2 Lead II and bipolar electrogram recordings (A), endocardial electrode positions (B) and body surface potential maps (C) of a CCW typical atrial Fl episode obtained in patient 17. Nine of the 11 electrograms are numbered in accordance to the temporal sequence of their individual activation times. The encircled numbers in panels A and B relate these nine electrograms with their anatomical origin in an adapted endocardial diagram of the exposed right atrium previously published by Anderson and Becker (22). The remaining two electrograms were recorded at the proximal (prox) and distal (dis) coronary sinus. The endocardial diagram depicts the location of the superior (SVC) and inferior vena cava (IVC), crista terminalis (CT), fossa ovalis (FO) and the coronary sinus os (CSO) and features the CCW direction of impulse propagation in the macroreentrant circuit. The encircled numbers in panels B and C relate the time at which each of the nine potential maps are displayed with the time of local activation at the numerically coded endocardial sites in the right atrial diagram. The actual time instant is depicted below every map and marked by the vertical bar in the scalar ECG tracing. This ECG tracing was obtained at lead position C11 (right lower anterior chest). Isopotential contours of positive and negative voltages are indicated by solid (gray area in the map) and dashed lines, respectively (see Fig. 1 for further explanation). The potential maps show a stable pattern with superiorly directed forces during the first 48 ms of the Fl wave while caudocranial activation of the right atrial septum (sites 1 to 3) and proximal-to-distal CS activation is taking place concurrently. After a rapid reversal in the potential map pattern, the remaining part of the Fl wave (56 to 148 ms) demonstrates again pattern stability with inferiorly oriented forces while craniocaudal excitation of the free wall and lateral isthmus is occurring (sites 5 to 9). Note, the complex multipolar pattern (50 ms) upon the onset of right free wall activation (site 4). The anatomic diagram of the right atrium is reproduced with permission of Gower Medical Publishing.