The definition of left bundle branch block influences the response to cardiac resynchronization therapy.

Background: CRT has been proven to achieve most benefit in patients with left bundle branch block morphology (LBBB). However, ECG criteria to define LBBB significantly differ from each other. Objective of the study was to evaluate the impact of different ECG criteria for LBBB definition on survival, hospitalization for heart failure and reverse remodelling in patients who received cardiac resynchronization therapy (CRT).

Methods And Results: Three-hundred-sixteen consecutive patients were included in the analysis. Six different classifications were assessed in baseline ECGs of patients who received a CRT device: a QRS duration of ≥150 ms and LBBB according to AHA/ACC/HRS, ESC 2006, ESC 2009, ESC 2013 and the classification proposed by Strauss and colleagues. In univariate analysis, the ESC 2009 and 2013 and the Strauss classifications were significantly associated with a reduction in cumulative probability for heart failure (HF) and mortality (HR 0.60, 95%CI 0.42-0.86, HR 0.61, 95% CI 0.43-0.87 and HR 0.57, 95% CI 0.40-0.80, respectively). In multivariate analysis, the association with the combined endpoint was confirmed only for ESC 2009 and 2013 classifications and for Strauss. Moreover, the cumulative probability of all-cause death and HF hospitalizations was higher in patients who were negative for all the 5 LBBB classifications.

Conclusions: This study shows that the strength of the association of LBBB to outcome in CRT depends on the ECG classifications used to define LBBB, the simplest criteria (ESC 2009 and 2013) providing the best association with clinical endpoints in CRT.