The mechanism of bidirectional VT: A simple “ping pong” in the His–Purkinje system

Both supraventricular or ventricular mechanisms of bidirectional VT (BVT) have been proposed in the past, involving either focal or reentrant mechanisms.(Levy 1989) A supraventricular mechanism with alternating LAFB and LPFB(Rosenbaum 1969)was largely excluded with the advent of intracardiac recordings, which failed to show a His bundle potential preceding the alternating QRS complexes during BVT. (Cohen 1973; Morris 1973) Postulated ventricular mechanisms have included a single focus in the proximal His bundle or bundle branches with alternating left fascicular block, or single or double foci in the distal HPS. In the single focus case, there is no obvious explanation for why fascicular or BBB should alternate during BVT. Usually, concealed retrograde conduction perpetuates block in the fascicle/bundle branch that initially develops conduction block. Even if a conduction gap prevents concealed retrograde conduction from perpetuating block, both fascicles/bundle branches would have to exhibit the same conduction gap phenomena, which seems highly unlikely. In the double foci case, if neither focus is protected by entrance block, the faster focus should overdrive the slower focus, producing monomorphic VT rather than BVT. If both foci are protected by entrance block, then they would have to have identical cycle lengths, phase-shifted by exactly 180o to produce a constant cycle length during BVT, which also is improbable. If only one focus is protected by entrance block, then it could induce a second focus to fire at a fixed coupling interval, but the coupling interval would have to be exactly half of the first focus s cycle length to produce a constant cycle length during BVT.