Tachycardia Circuit in Typical Atrial Flutter: The Role of a Posterolateral Line of Block in the Perpetuation of the Tachycardia

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The essential boundaries in typical atrial flutter (AF) are unknown. To examine the role of the tricuspid annulus (TA) and posterolateral line of block (LB) in maintaining AF, single extrastimuli were delivered during AF both around the LB and the TA in 29 patients. Single extrastimuli were delivered from the superior, middle, and inferior third of the anterior LB, superior, middle, and inferior third of the posterior LB, and the superior, lateral, inferior, and septal portions of the TA. The longest coupling interval (LCI) of single extrastimuli that reset AF and subsequent return cycle (RC) were analyzed. The resetting response showed two patterns (groups 1 and 2). The differences between the AF cycle length (AFCL) and the LCI (AFCL-LCI) at the superior, lateral, inferior, and septal portions of the TA were the shortest, and were significantly shorter than those at the other sites (P < 0.0001) in group 1. However, the AFCL-LCI at the superior, middle, and inferior third of the anterior LB, and the superior, lateral, inferior, and septal portions of the TA were the shortest, and were significantly shorter than those at the other sites (P < 0.0001) in group 2. The difference between the RC and the AFCL exhibited the same two patterns, similar to the AFCL-LCI. In group 1, a single extrastimulus produced an artificial conduction across the LB, but AF was not reset. Two types of reentry circuits exist in AF; one has its essential reentry circuit confined to the TA and thus the LB acts as a bystander, while the LB and the TA are essential boundaries in the other one.