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Figure 1 An example of a step-wise increase in cycle length in response to radiofrequency ablation of inappropriate sinus tachycardia (patient 27, second procedure). Shown are leads I, II and III at several stages of the procedure. (A) The baseline sinus cycle length (CL) was 600 ms. (B) During infusion of isoproterenol (iso) at a rate of 1 µg/min, the cycle length shortened to 440 ms, and the P waves in lead III were larger in amplitude. The isoproterenol infusion was continued during the radiofrequency procedure. (C) After several applications of radiofrequency energy at the site of earliest endocardial activation (–35 ms) in the high lateral right atrium, the cycle length increased to 510 ms, and there was a change in P wave morphology in all three leads. (D) After additional applications of radiofrequency energy at a new site of earliest endocardial activation (–25 ms) approximately 2 cm caudal to the original ablation site, the cycle length increased to 580 ms, again with a change in P wave morphology. The 32% increase in cycle length during isoproterenol infusion was considered to be an adequate endpoint for the procedure. (E) After discontinuation of the isoproterenol infusion, the resting cycle length was 780 ms, representing a 30% increase compared with the baseline cycle length. Note that after discontinuation of the isoproterenol infusion, the P wave morphology again changed.