Background-Interaction between wave fronts in the right and left atrium may be important for maintenance of atrial fibrillation, but little is known about electrophysiological properties and preferential routes of transseptal conduction. Methods and Results-Eighteen patients (age 44Ϯ12 years) without structural heart disease underwent right atrial electroanatomic mapping during pacing from the distal coronary sinus (CS) or the posterior left atrium. During distal CS pacing, 9 patients demonstrated a single transseptal breakthrough near the CS os, 1 patient in the high right atrium near the presumed insertion of Bachmann's bundle and 1 patient near the fossa ovalis. The mean activation time from stimulus to CS os was 48Ϯ15 ms compared with 86Ϯ15 ms to Bachmann's bundle insertion (PϽ0.01) and 59Ϯ23 ms to the fossa ovalis (PϭNS and PϽ0.01, respectively). During left atrial pacing, the earliest right atrial activation was near Bachmann's bundle in 5 and near the fossa ovalis in 4 patients. The activation time from stimulus to CS os was 70Ϯ15 ms compared with 47Ϯ16 ms to Bachmann's bundle (PϽ0.01) and 59Ϯ25 ms to the fossa ovalis (PϭNS). Whereas the total septal activation time was not significantly different during CS pacing compared with left atrial pacing (41Ϯ16 versus 33Ϯ17 ms), the total right atrial activation time was longer during CS pacing (117Ϯ49 versus 79Ϯ15 ms; PϽ0.05). Conclusions-Three distinct sites of early right atrial activation may be demonstrated during left atrial pacing. These sites are in accord with anatomic muscle bundles and may have relevance for maintenance of atrial flutter or fibrillation.

Left septal AT ablation is safe and effective. There was no consistent P-wave morphology associated with this particular type of AT. This arrhythmia appears to be resistant to adenosine and moderately responsive to calcium antagonists.

Anatomic barriers, along with statistical properties of conduction and refractoriness during atrial fibrillation, may explain the remarkably stereotypical pattern of endocardial activation during the initiation of atrial flutter via fibrillation and the rarity of degeneration of flutter to fibrillation once it stabilizes.

Use of the P-wave morphology on the 62-lead surface ECG in patients with normal biatrial anatomy allows separation of the origin of ectopic RA impulse formation into one of 17 different endocardial segments with an approximated area size of 3.5 cm2. This database of paced P-wave integral maps provides a versatile clinical tool to perform detailed noninvasive localization of right-sided atrial tachycardia before radiofrequency catheter ablation.