Modes of Initiation of Two Types of Atrial Reentry in a Patient with Typical Atrial Flutter: Isthmus‐dependent Micro‐reentry versus Macro‐reentry

We studied the modes of initiation of two types of atria] reentrant tachycardias (i.e., microreentry isthmus tachycardia and counterclockwise atrial flutter) in a 39‐year‐old male with typical atrial flutter. Rapid atrial pacing from proximal coronary sinus at a cycle length of 220 msec initiated micro‐reentry isthmus tachycardia (non‐sustained), while rapid atrial pacing at a cycle length of 210 msec initiated sustained atrial flutter circulating counterclockwise around the tricuspid annulus. It was suggested that initiation of the counterclockwise atrial flutter was associated with a pacing‐induced conduction block in the entire width of the isthmus, whereas initiation of the micro‐reentry isthmus tachycardia was associated with a pacing‐induced conduction block in a limited segment of the isthmus (i.e., partial isthmus block).