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Figure 2 An example of an abrupt response of inappropriate sinus tachycardia to radiofrequency ablation (Patient 7). Shown are leads V1, I and II, an electrogram recorded in the right atrium (RA) near the ablation site, a recording from the ablation catheter in the high lateral right atrium (HRA) and lead III. The cycle length during a 2-µg/min infusion of isoproterenol was 400 ms, and the first five applications of radiofrequency energy at the site of earliest endocardial activation (–35 ms) had only either a temporary or no effect on the cycle length. Approximately 2 s after the onset of the sixth application of radiofrequency energy (RF), there was an abrupt and permanent 42% increase in cycle length to 570 ms. In this patient, the P wave morphology after radiofrequency ablation of the inappropriate sinus tachycardia did not change.