Electrophysiological mechanisms of typical atrial flutter after atrial switch operation : critical role of the septal corridor

Background Peritricuspid, or cavotricuspid isthmus (CTI)-dependent, atrial flutter is the most common arrhythmia in patients with dextro-transposition of the great arteries (D-TGA) treated with atrial switch surgery, but its underlying mechanisms remain poorly understood. Purpose It was hypothesized that high-resolution characterization of the atrial substrate in this population might provide insight into the mechanisms underlying this common arrhythmia. Methods Twenty consecutive patients with prior atrial switch surgery referred for atrial flutter ablation were included. All underwent transbaffle puncture and ultra-high-density mapping. Anatomical barriers, activation patterns, conduction velocities and low voltage areas were assessed. Results In all cases of peritricuspid atrial flutter, surgical incisions defined the posterior boundary of the circuit. The narrowest segment of the reentry was consistently located within a septal corridor bounded posteriorly by the baffle incision and anteriorly by the tricuspid annulus, with different activation patterns in the region of the coronary sinus ostium (Figure 1). Slow conduction (<30 cm/s) was identified in this septal corridor in 92% of peritricuspid flutters and in 85% of patients during sinus or paced rhythm. Apart from surgical incisions, low-voltage areas were limited and frequently co-localized with regions of slow conduction. No patient exhibited slow conduction or low voltage at the CTI. The septal corridor was also involved in 60% of non-peritricuspid reentrant atrial tachycardias. In the last two patients included, programmed atrial stimulation with a fixed S2 (delivered 30 ms above the atrial effective refractory period) was performed. In the first patient, S2 mapping accentuated conduction delay within the septal corridor (Figure 2A). In the second patient, the S2 beat created a functional unidirectional block (Figure 2B), preceding the induction of typical atrial flutter. Conclusions In patients with D-TGA corrected by atrial switch, surgical incisions define posterior boundaries and create a narrow septal corridor, or "baffle-tricuspid isthmus", characterized by slow conduction and a propensity for functional block. These anatomical and electrophysiological features likely underlie the high prevalence of peritricuspid reentry in this population, representing a ubiquitous critical substrate for arrhythmia initiation and maintenance. Systematic CTI ablation should therefore be considered in these patients.