Mechanisms of Posterior Fascicular Tachycardia

During electrophysiology study, mapping was performed using a 20 pole multielectrode catheter (MEC) via a retrograde aortic approach (Figure 1C). The baseline A–H and H–V intervals were 57 and 55 ms, respectively (Figure 1B, left). A wide QRS tachycardia identical to the clinical tachycardia was observed with cycle length of 320 ms, H–V interval of −30 ms (Figure 1B, right). This was consistent with a diagnosis of fascicular ventricular tachycardia (FVT) with origin from the left posterior fascicle (LPF). Three distinct potentials could be recorded from the MEC during FVT: A) sharp inflection, high frequency potentials (P1) activated from midproximal to distal MEC; B) presystolic LPF potentials (P2) activated from distal to proximal MEC; and C) left-septal ventricular (V) potentials (Figure 1B, right). Right ventricular programmed stimulation (S1S2, 400/300 ms) was used to induce FVT (Figure 2A). During S1 stimulation, P1 was recorded with a stable S1–P1 interval (S1–P1=255 ms) and the same conduction sequence as during tachycardia. With delivery of an extrastimulus, the S2–P1 interval increased to 320 ms, followed by P2 and V potentials, and induction of sustained FVT. Additional pacing maneuvers were performed during tachycardia.