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.