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Figure 3 Tracking of dynamic changes in isthmus conduction immediately after termination of clockwise flutter by RF delivery in the isthmus. Top panel (A), pacing from the low lateral RA was initiated 1.5 s after flutter termination and the initial eight paced P waves show a terminal positivity similar to the flutter morphology, but which changes thereafter (star) to a negative deflection. The paced P wave morphology later changed (star) to the same characteristic morphology seen immediately after flutter termination during additional RF applications, as shown in the bottom panel (B), and intracardiac mapping confirmed the achievement of complete isthmus block. This indicates recovery of transient isthmus conduction block after flutter termination despite the continuing delivery of RF energy at the same site. In both panels, arrows indicate the pacing stimulus.