The top left panel shows normal ventricular activation (blue arrow). Note left to right septal activation, then superiorly to septal border of the crista supraventricularis (CSV) and meets the RV free wall wavefront exiting from right bundle branch
The left lower panels show a possible pathway of the of the parallel wavefront (yellow arrow) which, after it reaches the septum moves superiorly to CSV towards the RV free wall. This early rightward activation may result in counterclockwise rotation of the QRS complex.
The right upper panel shows that in RBBB, the parallel wavefront after reaching the CSV and finding an unexcited RV free wall it continues rightward and decreases the duration of S wave. At higher voltage the parallel wavefront may pre-excite larger portion of RV free wall than at lower voltage with slower conduction. The degree of fusion would determine the duration of S wave while the block remains unresolved.
Right lower panel shows an alternative mechanism where the parallel pathway connects to the His Purkinje system distal to site of block. At higher voltage the parallel wavefront would arrive at this site earlier and no RBBB is seen, and at lower voltage the slower conduction in parallel wavefront allows conduction to occur via His bundle with manifest RBBB.
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Limitations
There are many difficulties in trying to explain phenomenon observed following active fixation of pacing lead with electrophysiologic principles obtained using atraumatic diagnostic catheters. The mechanism will remain a hypothesis until further studies increase our understanding of the complex area surrounding the His bundle.
No high-density mapping data was obtained to determine the exact pathway of the pre-excitation wavefront and we relied mainly on analysis of surface ECG.
Clinical Significance
Non-selective His bundle pacing may, in essence, be activating an additional pathway with behavior suggestive of a ‘parallel His bundle’ (25). In patients with His bundle disease, NS-HBP may deserve consideration as preferred mode of His bundle pacing.
Our study also suggests that the paradigm of lowering pacing voltage to prolong battery life may not apply to His bundle pacing as correction of conduction block often occurs at higher voltage pacing.
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