Electrical conduits within the inferior atrial region exhibit preferential roles in interatrial activation

Differences between conduction properties of interatrial conduits and their roles in initiation and maintenance of supraventricular arrhythmias remain unclear. Our objective was to determine details of interatrial activation in inferior atrial region and to correlate intra-atrial and interatrial activation patterns with the site of origin of atrial ectopic activation. In 9 dogs, basket-catheters carrying 64 electrodes were deployed into both the right atrium (RA) and left atrium (LA). A 10-electrode catheter was inserted into the coronary sinus (CS). Activation patterns of the RA, LA, and CS were compared during pacing in the CS, in RA inferoparaseptum posterior to Eustachian ridge-tendon of Todaro (TT), and in inferior RA near the CS ostium (anterior to TT). We found that pacing in proximal and middle CS resulted in a RA breakthrough invariably at the CS ostium, consistent with conduction through a CS-RA connection. Meanwhile, LA breakthrough emerged in inferoposterior region (inferior to mitral annulus), suggesting conduction through a CS-LA connection. While pacing in distal CS, LA breakthrough shifted to middle posterolateral wall. Whereas, the RA was activated by the LA directly through the septum. During pacing in RA inferoparaseptum posterior to TT, the LA was activated directly through the septum at 22 +/- 4 ms. Whereas, during pacing anterior to TT, the LA was activated through both the CS and the septum while earliest activation was delayed by 38 +/- 5 ms. In conclusion, both the interatrial septum and CS musculature form electrical conduits in inferior atrial region in canine. Differences in activation properties between the conduits in inferior interatrial region result in selective interatrial activation patterns during ectopic activation.