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Figure 5 Tracings are ECG leads I, II, V₁, and intracardiac electrogram recorded from a mapping catheter, a catheter in the RA, a catheter at the HBE, a catheter within the CS, and a catheter at the RVA in a patient with focal atrial tachycardia originating from the non-coronary aortic sinus. (A) Mapping in the RA during tachycardia demonstrates that the earliest right atrial activation is found at the HBE (RA-AS 1 to 2, RA-AS 3 to 4, RA-AS uni). Note that: 1) the earliest activation preceded the fixed atrial electrogram recorded from the coronary sinus by 67 ms and a discrete His potential at this site; and 2) the amplitude change of His activation during the tachycardia indicates an unstable catheter position at the HBE. (B) Mapping in the AS demonstrates the earliest atrial activation in the non-coronary AS (non-coro AS 1 to 2, non-coro AS 3 to 4, non-coro AS uni) with the same activation time compared to the earliest right atrial activation. Note that: 1) the earliest activation preceded the fixed atrial electrogram recorded from the coronary sinus by 67 ms; and 2) no His activation is found at the successful site in the non-coronary AS. Abbreviations as in preceding figures.