Rate-Dependent Left Bundle Branch Block: An Electro-mechanical Study.

BACKGROUND: Rate-dependent left bundle branch block (rdLBBB) is a unique clinical entity diagnosed in 0.5%-1% of the patients undergoing exercise testing. OBJECTIVE: This study aimed to evaluate the electromechanical features in this acute and transient onset of left bundle branch block (LBBB) during exercise testing, and to assess its natural history. METHODS: Fifty patients with rdLBBB were retrospectively analyzed. If a stress echocardiography was feasible, they were prospectively included for a supine bicycle echocardiography to evaluate electromechanical dyssynchrony. RESULTS: RdLBBB fulfills the criteria of LBBB in 95% of the patients. Sixty percent of the patients with rdLBBB progressed to a permanent LBBB within 3 years, with a lower heart rate at onset of rdLBBB and left ventricular ejection fraction (LVEF) as predictors. rdLBBB was characterized by acute mechanical dyssynchrony with septal flash (SF). Patients with lower baseline LVEF had more pronounced SF and a higher strain stage (p = 0.001). The electromechanical dyssynchrony was associated with more left ventricular dysfunction during exercise (p = 0.045). CONCLUSION: RdLBBB is a relative rare clinical entity, which is characterized by sudden onset and typical LBBB, fulfilling criteria of LBBB. The degree of dyssynchrony during rdLBBB seems to be determined by the baseline LVEF, with possible acute LV dysfunction during exercise.