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Intra‐Isthmus Reentry: Another Form of Typical Atrial Flutter?

After the early work of Puech in the late 1950s, later studies have confirmed that typical atrial flutter is a macroreentrant arrhythmia that is confined to the right atrium.1-3 It has uniformly been accepted that it was due to a macroreentry circuit for which the cavotricuspid isthmus (CTI) was an obligatory route, that was limited by an anterior border formed by the tricuspid annulus and a posterior border by the crista terminalis or the sinus venosus, and within which the rotation was either clockwise or counterclockwise.4-7 More recently, a variant of the latter was described in which a short cut within the circuit occurred at various height across the crista terminalis, termed lower loop reentry, and was still classified within the typical form of atrial flutter owing to the necessary use of the CTI.8,9 In this issue of the Journal, Yang et al.11 propose another type of circuit encompassing the CTI, that often mimics the most common typical form of atrial flutter and may even explain some of the recurrences observed after an apparently successful initial ablation procedure. After an initial paper on the same topic, Yang et al. report here another 14 out of 384 patients (≈4%) referred for atrial flutter ablation over a 5-year period.10,11 Most patients were male and 10 had a surface ECG resembling counterclockwise typical atrial flutter whereas another 2 suggest clockwise rotation within the same circuit and in the last 2 flutter waves were not interpretable. Interestingly, the majority of these patients had a previous history of successful or failed CTI ablation. Classical multielectrode activation and entrainment mapping, was performed in all cases whereas electroanatomical mapping which was available in 8 cases, showed a reentrant tachycardia mechanism. Yet the originality of this paper was that, despite its surface ECG aspect, in the vast majority of the cases (12/14, 86%) the anteroinferior CTI was not part of the circuit while its septal part was within it. In addition, in 3 cases this part of the CTI showed the longest postpacing interval suggesting the presence of complete CTI block during ongoing flutter. Spontaneous changes of atrial activation from clockwise to counterclockwise as well as mixed forms were observed during ongoing reentry without any change in tachycardia cycle length (TCL). Although there is no doubt that the septal CTI is part of the circuit, some questions still remain unanswered.