Extensive PPI mapping confirmed the exact macroreentrant circuit: The PPI-TCL in the entire CS, including the CS ostium and the inferolateral exit to the LA with fractionated signals, was <10msec Figure 2 .

Moreover, the PPI at several positions in the LA and RA (see exemplary white crosses in Figure 3) excluded a perimitral, a roof-dependant, and a biatrial atrial flutter utilizing only the LA and right atrial septum or the LA and the tricuspid valve.

The true circuit was a peri-coronary sinus flutter with an activation sequence proximal to distal in the coronary sinus musculature. LA excitation begins inferolateral and proceeds endocardially through the inferior LA back to the CS ostium. Figure 3