Double loop ventricular tachycardia activation patterns with single loop mechanisms: Asymmetric entrainment responses during "pseudo-figure-of-eight" reentry.

Background: The classical paradigm of scar-related reentrant ventricular tachycardia (VT) features a circuit with a double loop figure-of-eight (F8) activation pattern.

Objective: The purpose of this study was to interrogate VT circuits with F8 activation patterns by entrainment mapping to differentiate an active loop from a passive loop.

Methods: Sixty VT circuits with >90% of tachycardia cycle length delineated in high resolution were retrospectively analyzed in 55 patients (nonischemic 49%). A pseudo-F8 VT circuit was defined as a double loop activation pattern driven by a single loop mechanism with a passive loop that yields a long postpacing interval (postpacing interval - tachycardia cycle length ≥ 30 ms).

Results: Single loop activation patterns were observed in 33% (n = 20). Of 40 circuits with F8 patterns by activation mapping, 20 were studied with entrainment mapping, where a passive loop was identified by a long postpacing interval in 50%. In 6 circuits where entrainment mapping was performed from both outer loop regions, all demonstrated asymmetric responses to entrainment, confirming a single loop mechanism. Entrainment from both lateral margins of the common pathway (n = 7) demonstrated an asymmetric response in 29%. In all pseudo-F8 circuits (n = 10), the shorter loop functioned as the active loop and ablation targeting the active loop side of the isthmus resulted in VT termination with a single radiofrequency application.

Conclusion: In a selected cohort, single loop mechanisms are more prevalent than double loop reentry in reentrant human VT. Half of VT circuits with double loop activation patterns can be demonstrated to be sustained by a single active loop mechanism by entrainment mapping. Ablation targeting the shorter active loop resulted in rapid termination during radiofrequency application.