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Figure 4 Set of three integral maps of the initial (T1–T2), intermediate (T2–T3) and terminal CCW typical Fl wave (T3–T4) recorded in patient 17. The ECG was acquired at the E9 electrode position (left lower anterior chest) (see Fig. 1 for further explanation). The maps corresponding with initial and terminal Fl wave activation show a mirror image voltage distribution. It may be noted that the downward deflection of the positive Fl wave component in the ECG waveform demonstrates a steep-to-gradual voltage decline; the T3–T4 time period coincides with the initial part of this gradual voltage decline.