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Figure 3 Tracings are ECG leads I, aVF, V₁, and intracardiac electrogram recorded from a mapping catheter, a catheter at the HBE, a catheter within the CS, and a catheter at the right ventricular apex (RVA) in a patient with focal atrial tachycardia originating from the non-coronary AS. (A) The earliest activation in the right atrium is shown at the His bundle region (RA-AS 1 to 2, RA-AS 3 to 4, RA-AS uni). Note that the earliest activation preceding the fixed atrial electrogram recorded from the CS 1 to 2 by 56 ms and a tiny His potential at this site. The tachycardia was unsuccessfully terminated by radiofrequency (RF) delivery at this site. (B) The earliest activation in the left atrium is located at the left anteroseptal region immediately opposite the His bundle in the RA (LA-AS 1 to 2, LA-AS 3 to 4, LA-AS uni). Note that the earliest activation preceded the fixed atrial electrogram recorded from the CS 1 to 2 by 54 ms and there was no His potential at this site. The tachycardia was also unsuccessfully terminated by RF delivery at this site. (C) The successful site with an earliest RA activation is shown at the non-coronary AS (non-coro AS 1 to 2, non-coro AS 3 to 4, non-coro AS uni). Note that the earliest activation preceded the fixed atrial electrogram recorded from the CS 1 to 2 by 68 ms and there was no His potential recording at this site. LA = left atrium; other abbreviations as in Figure 1.