His Bundle Pacing but not Left Bundle Pacing Corrects Septal Flash in Left Bundle Branch Block Patients

Marina Strocchi1, Aurel Neic2, Julien Bouyssier3, Karli Gillette2, Mark Elliot1, Justin Gould1, Jonathan Behar1, Baldeep Sidhu1, Martin Bishop1, Edward Vigmond3, Gernot Plank2, Christopher Rinaldi1, Steven Niederer1
1King's College London, 2Medical University of Graz, 3LYRIC University of Bordeaux


Background: His bundle pacing (HBP) and left bundle pacing (LBP) are novel delivery methods for cardiac resynchronisation therapy (CRT) for left bundle branch block (LBBB) patients. Septal flash, an abnormal pre-ejection motion of the septum towards the left ventricle (LV) arising from dyssynchronous activation, has been shown in the past to be a robust and independent predictor for CRT response. We investigated whether HBP and LBP can correct abnormal septal motion with the use of computational modelling.

Methods: We modelled ventricular activation, ECGs and mechanical contraction on a four-chamber heart mesh, inclusive of transmural ventricular myofiber orientation and His-Purkinje network. We applied this framework to four heart failure patients geometries. Our mechanics model accounts for ventricular preload, ventricular afterload and the effect of the pericardium on the heart by means of spatially varying Robin boundary conditions. We simulated synchronous activation, LBBB, LBBB+HBP and LBBB+LBP. We computed the peak of normal septal displacement before LV ejection, with negative displacements towards the LV and positive towards the right ventricle (RV).

Results LBBB results in increased leftwards septal motion compared to synchronous activation (synchronous: -0.4±0.5mm vs LBBB: -3.8±1.1mm). HBP results in reduced LV latest activation times (LAT) compared to baseline (LBBB: 140±30ms vs HBP: 107±6ms). This results in correction of septal flash, with pre-ejection septal motion similar to synchronous activation (HBP: -0.4±0.5mm). LBP also reduces LV LAT (107±5ms), but leads to slow RV activation (RV LAT LBBB: 106±13ms vs LBP: 131±8ms). RV pressure rise during isovolumic contraction is slower than LV pressure rise, causing the septum moves to the right (0.9±0.6mm).

Conclusion: HBP resynchronises ventricular activation without affecting RV activation, resulting in restored normal septal motion. LBP shortens LV activation but leads to slow RV activation, leading to abnormal rightwards septal motion.