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Figure 2 Left, an example of a labile P wave change during RF delivery at a gap site in the isthmus; the negative P wave in leads II, III and aVF changes two beats later to a biphasic P wave with a terminal positivity most prominent in lead II, which persists only for five beats and recovers its previous negative morphology despite continuing RF delivery at the same site with persisting isthmus conduction. Right, stable achievement of the identical P wave change by RF delivery at a nearby site, which was confirmed to be associated with intracardiac evidence of complete isthmus block. The achievement of block results in a terminal positivity of the paced P wave evident in leads II, III, aVF and, less so, in V6 (with the reverse changes as a result of recovery). In both panels, P wave changes are denoted by stars. Time markers are 1 s apart.