LBBB is a totally a non-homogenous concept: defining 4 phenotypes of response to CRT in a highly experienced center

Cardiac resynchronization (CRT) is a well-established technique used to treat significant HF patients who associate LBBB and severe LV dysfunction. CRT indications refer to a certain clinical profile but not to the underlying pathology. However, the primitive/secondary nature of the LBBB and its fine ECG characteristics predict CRT response.

We therefore sough to classify patients implanted with CRT in a highly experienced center according to LBBB fenotypes that have a long-term predictive value of their response.

We retrospectively analyzed a complex database – demographics, risk factors, disease characterization (LGE-MRI included), biological – of patients implanted with CRT within CEHB starting in 2010 until present day (n = 1230 cases). Patients were clinically and structurally evaluated at 1 week, 1 monts, 6 months and 1 years (selected cases by re-MRI). We defined 4 main profiles of CRT candidates (from inclusion), according to the disease nature and long-term evolution. We finally selected 20 patients from each subtype in order to perform statistical analysis for classical parameters that define clinical and structural long-term response.

The 4 profiles that were defined from the database are: pure electrical LBBB, LBBB with limited substrate, LBBB with advanced substrate and burnt-out myocardium with LBBB. CRT-response is optimal in case of pure electrical disease, with rapid correction of asynchrony and subsequent reverse-remodeling which raises questions regarding the need for further OMT. Patients with limited non-critical ischemic substrate also have a rapid favorable evolution while non-ischemic structural disease with low expression of the disease fenotype have a good initial response with uncertain long-term prognosis (depends on the underlying disease). The critically ischemic patient can also obtain a benefit ahead of myocardial revascularization if the heart rate allows for non-ischemic lateral myocardium at rest. Extended substrate of non-ischemic nature already indicates a bad prognosis, usually in such cases the LBBB being a consequence of the disease; these patients usually follow a clinical stability period post-CRT during which they should be prepared for cardiac function replacement of some sort. In case of burnt-out myocardium we usually perform atypical CRTs ("emergency"); some patients reach this state because of very late referral for CRT. Even in cases of initially electrical disease, left un-addressed in time they can become structural.

Complete and correct definition of LBBB is essential for a coherent response to CRT. Establishing that LBBB is the cause or an additional component to a complex pathology and also defining the presence, nature and extent of myocardial substrate in patients with CRT indication, allows for stratifying them into at least 4 categories of LBBB-fenotypes which have a predictive impact on their long-term evolution.