Upper turnaround point in a reentry circuit of the idiopathic left posterior fascicular ventricular tachycardia.

Background: The anatomic extent of the reentry circuit in idiopathic left posterior fascicular ventricular tachycardia (LPF‐VT) is yet to be fully elucidated. We hypothesized that entrainment mapping could be used to delineate the reentry circuit of an LPF‐VT, especially including the upper turnaround point. Methods: Twenty‐three consecutive LPF‐VT patients (mean age, 29 ± 9 years, 18 males) were included. We performed overdrive pacing with entrainment attempts at the left bundle branch (LBB) and the left His bundle (HB) region. Results: Overdrive pacing from the LBB region showed concealed fusion in all 23 patients (post‐pacing interval [PPI], 322.1 ± 64.3 ms; tachycardia cycle length [TCL], 319.0 ± 61.6 ms; PPI‐TCL, 3.1 ± 4.6 ms) with a long stimulus‐to‐QRS interval (287.9 ± 58.0 ms, approximately 90% of the TCL). Pacing from the same LBB region at a slightly faster pacing rate showed manifest fusion with antidromic conduction to the LBB and minimal in‐and‐out time to the LBB potential (PPI‐TCL, 21.3 ± 13.7 ms). Overdrive pacing from the left HB region showed manifest fusion with a long PPI‐TCL (53.9 ± 22.5 ms). Conclusions: Our pacing study results suggest that the upper turnaround point in a reentry circuit of the LPF‐VT may extend to the proximal His‐Purkinje conduction system near the LBB region but below the left HB region. The LPF may constitute the retrograde limb of the reentry circuit. [ABSTRACT FROM AUTHOR]