Systolic improvement and mechanical resynchronization does not require electrical synchrony in the dilated failing heart with left bundle-branch block.

Background: Biventricular (BiV) and left ventricular (LV) pacing similarly augment systolic function in left bundle-branch block (LBBB)-failing hearts despite different electrical activation. We tested whether electrical synchrony is required to achieve mechanical synchronization and functional benefit from pacing.

Methods And Results: Epicardial mapping, tagged MRI, and hemodynamics were obtained in dogs with LBBB-failing hearts during right atrial, LV, and BiV stimulation. BiV and LV both significantly improved chamber hemodynamics (eg, 25% increase in dP/dt(max) and aortic pulse pressure) compared with atrial pacing-LBBB, and this improvement correlated with mechanical resynchronization. Electrical dispersion, however, decreased 13% with BiV but increased 23% with LV pacing (P<0.01).

Conclusion: Improved mechanical synchrony and function do not require electrical synchrony. Mechanical coordination plays the dominant role in global systolic improvement with either pacing approach.