Basic Mechanism for Functional BBB-dependent ORT
When aberration is present during ORTs mediated by ipsilateral APs,
impulse travels down the contralateral bundle branch and lengthens VA
time[1-3]. Meanwhile, retrograde penetration into
the blocked bundle branch subsequent to transeptal conduction
perpetuates aberration during tachycardia, which can diminish
spontaneously or be restored by a ventricular extrastimulus peeling back
the refractoriness of the bundle[4].
As a macro-reentrant tachycardia, ORT can be theoretically non-inducible
in patients with APs if the conduction properties of AP or AV node do
not fulfill the prerequisite of reentry e.g., small circuit, long
refractory period, or rapid conduction. However, in the setting of
aberrancy, an extra transseptal conduction time of approximately
40ms[2] is added to VA conduction which helps the
wavefront avoid the refractoriness of the circuit components.
Conversely, the restoration of BBB can shorten intraventricular time and
terminate tachycardia.
In our report, all APs were from lateral regions of AV rings, which
provided more significant increase in TCL when BBB occurs, compared to
septal pathways. To induce tachycardia in this setting, anterograde
conduction should meet multiple requirements including (1) AP was
refractory; (2) AV node was excitable; and (3) the ipsilateral bundle
branch was unexcitable, which can explain the short clinical episodes
and late onset possibly associated with age-related infra-His conduction
disturbance (case 1 and 2). Therefore, timing and site of pacing are
important, and the refractory period of AV node, AP and His-Purkinje
system should be carefully measured.