Eccentric activation of the coronary sinus during typical atrial flutter: What is the mechanism?

Abstract

A 79-year-old man with a history of ventricular septal defect underwent catheter ablation of drug refractory paroxysmal atrial flutter (AFL). The 12-lead electrocardiogram during tachycardia showed a biphasic, predominantly positive flutter wave with an initial negative component in leads II, III, and aVF, and positive flutter waves in leads V1 through V6 (Fig. 1, A), consistent with typical counterclockwise (CCW) AFL [1]. A decapolar catheter was advanced with its ♯17 and ♯18 poles in the proximal coronary sinus (CS), and a deflectable duodecapolar Halo catheter was placed parallel to the tricuspid annulus (TA), across the inferior vena cava (IVC)-TA isthmus, with its tip at the CS ostium (Fig. 1, B and C). Although we did not perform contrast CS venography to confirm the location of the CS ostium, we believe that the CS ostium is located near the 5 o'clock position of the mitral annulus as estimated by the shape of the CS catheter projected in the left anterior oblique fluoroscopic view, consistent with the location of the ♯17 and ♯18 poles of the CS catheter. The atrial activation sequence along the TA during ongoing tachycardia, combined with entrainment pacing at the IVC-TA isthmus, and 10 o'clock and 2 o'clock positions of the TA with a pacing cycle length of 220 ms confirmed the diagnosis of typical CCW AFL. We did not perform entrainment pacing from the CS. During entrainment pacing from any site along the TA, all atrial deflections in CS recordings immediately after the last pacing stimulus were captured with the activation sequence similar to that during the AFL and a spikeatrial interval shorter than AFL cycle length, consistent with an orthodromic capture. A line of radiofrequency energy delivery blocked conduction across the IVC-TA isthmus, terminated AFL, and restored sinus rhythm. Differential pacing from the low septal and lateral right atrial (RA) region confirmed the successful creation of bidirectional isthmus block. It is noteworthy that, during ongoing AFL before ablation, (1) a latency of low RA conduction, consistent with conduction across the septal isthmus, was observed along the distal Halo catheter (Fig. 1, D), and (2) recording of atrial activation along the CS, instead of being proximal to distal, was centrifugal from CS poles 9 to 10 (Fig. 1, D). What is the underlying electrophysiological mechanism of atypical atrial activation during typical CCW AFL?