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Figure 1 Electrogram recordings before (left) and after (right) verapamil administration. The electrograms from the decapolar catheter at the right atrial free wall are designated E1 to E10; those from the coronary sinus CS1 to CS4 and the monophasic action potential recording MAP. The mean fibrillatory interval shortened at each recording site. The signal quality in E1 was inadequate for measurement of fibrillatory intervals. This recording site was excluded from the analysis.