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Figure 4 A summary of the results of P wave tracking of dynamic changes in isthmus conduction as a result of RF ablation. After termination of flutter as a result of ablation, 12 of 15 patients had no block. Further ablation was required to produce stable isthmus block. Conduction recovery after the achievement of complete block across the isthmus occurred in 16 patients, all requiring further ablation. See text for details.