Left bundle branch block cardiomyopathy (LBBB-CMP): from the not-so-benign finding of idiopathic LBBB to LBBB-CMP diagnosis and treatment.

Introduction Idiopathic left bundle branch block (iLBBB) is an uncommon finding. Its benignity has been increasingly questioned, though its natural history remains poorly clarified. Similarly, LBBB-cardiomyopathy (LBBB-CM) has been also increasingly recognized as a distinct entity, where electromechanical dyssynchrony seems to play a central role in left ventricular dysfunction (LVD) development. Still, it remains a scarcely studied topic. There is an urgent need for investigation and evidence reinforcement in these areas.

Objectives: two main objectives: (1) to explore the natural history of "asymptomatic" iLBBB carriers; (2) to characterize the outcomes and therapeutic approach used in a "real-world" cohort of possible LBBB-CMP patients (pts).

Methods: tertiary care centre retrospective study of pts with iLBBB and possible LBBB-CMP, screened from a large hospital electrocardiographic database from 2011 to 2017 (LBBB = 347). To assign the 1st objective, only pts with left ventricular ejection fraction (LVEF) ≥ 50% and available follow-up (FU) data were included (n = 152). Regarding the 2nd objective, possible LBBB-CMP pts were selected and defined as iLBBB pts with LVD (LVEF < 50%) and no secondary causes for LVD (n = 53). Data were based on pts' careful review of medical records.

Results: focusing our 1st objective, 152 iLBBB carriers were identified. Median FU time were 8 years, and 61% were female. During FU, approximately 25% developed LVD, 20% needed ≥ 1 cardiovascular (CV) hospitalization, and 15% needed a cardiac device implantation. The majority (2/3) of pts with LVD on FU (n = 35) had no secondary causes for LVD, being classified as possible LBBB-CMP pts. Time-to-LVD analysis showed no differences between pts with a known cause for LVD vs LBBB-CMP pts (Log-rank = 0.713). Concerning the 2nd objective, 53 possible LBBB-CMP pts were identified. Median FU time were 10 years, and 51% were female. During the FU, 77% presented heart failure (HF) symptoms, and 42% needed ≥ 1 CV hospitalization, mainly due to HF. Half presented severe LVD at some point in time, and 55% needed a cardiac device, most of them a cardiac resynchronization therapy (CRT) device. Comparing CRT with non-CRT pts, no differences were found in terms of medical therapy, but better outcomes were observed in CRT group: LVEF improvement was higher (median LVEF improvement of 11% in non-CRT vs 27% in CRT; p < 0.001), and fully recovery from LVD was more frequent (50% of CRT vs 14% non-CRT; p = 0.028).

Conclusion: our data strengthen current evidence on natural history of iLBBB, showing significant CV morbidity associated with the presence of iLBBB, and reinforces the need for a serial and proper FU of these carriers. Our data on "real-world" possible LBBB-CMP pts shows high rates of CV events, namely HF-related events, and supports the growing evidence pointing out CRT as this subgroup of pts' cornerstone of treatment. In conclusion, our work sheds additional light on these largely unknown topics and underlines the urgent need for larger and prospective studies addressing the identification of LVD development predictors in iLBBB carriers, as well as the establishment of diagnostic criteria and therapeutic approach for LBBB-CMP.