![]() ![]() The American Heart Association (AHA), American College of Cardiology (ACC), and Heart Rhythm Society (HRS) collaborated on the currently adopted definition in 2009 and was restated in 2018 ( Surawicz et al., 2009 Kusumoto et al., 2019). Most definitions of LBBB are based on surface ECG patterns. How to define left bundle branch block? Surface ECG Conduction system pacing (CSP) and hybrid pacing are emerging strategies for CRT but may not be suitable for all types of LBBB patterns. CRT response has been shown to depend on the mechanism of the bundle branch block ( Caputo et al., 2018 Jastrzębski et al., 2018). Recent observations of suggest that the LBBB pattern recognized on surface ECG may distinct underlying pathophysiology: complete conduction block through the left bundle of the His-Purkinje system, wide LBBB-like QRS due to left ventricular hypertrophy or diffuse fibrosis leading to interventricular conduction delay with preserved His-Purkinje activation (IVCD with IPA), or a combination of both proximal block and concomitant distal disease ( Upadhyay et al., 2019a Tung and Upadhyay, 2020). However, despite this, there remains a significant percentage of patients who do not experience reduction in morbidity or mortality from CRT ( Mullens et al., 2009). Multiple guidelines have adopted this definition and emphasized presence of LBBB as a criterion when selecting patients for CRT ( Tracy et al., 2012 Glikson et al., 2021 Allaw et al., 2022). Large clinical trials have shown that the presence of LBBB on surface ECG is one of the best predictors of CRT response ( Linde et al., 2008 Moss et al., 2009). Cardiac resynchronization therapy (CRT) via biventricular pacing (BiVP) has been established to reduce mortality in patients with electrical dyssynchrony and left ventricular systolic dysfunction ( Moss et al., 2009 Zareba et al., 2011 Gold et al., 2012 Goldenberg et al., 2014). The clinical significance of LBBB has changed over the years from being first perceived as a non-harmful electrocardiogram (ECG) finding to more recently being associated with poorer prognosis, particularly in patients with severe, symptomatic heart failure ( Master et al., 1940 Tabrizi et al., 2007). As various approaches to CRT continue to be studied, physiologic phenotyping of the LBBB pattern remains an important consideration.įor over a century, left bundle branch block (LBBB) has been recognized as a clinical entity. Newer hybrid approaches which combine CSP and traditional BiVP-CRT and may hold promise for patients with IP or mixed-level block. Conduction system pacing (CSP) has emerged as an alternative approach to deliver CRT and correct QRS in patients with conduction block. Despite this, however, there remains a significant number of non-responders to BVP. Currently, BiVP-CRT is indicated for all QRS duration ≥150 ms and may be considered for BBB patterns from 130 to 149 ms with robust clinical data to support its use. More recent data has challenged this assertion, revealing that LBBB pattern may include distinct underlying pathophysiology, including patients with complete conduction block, either at the left-sided His fibers or the proximal left bundle, intact Purkinje activation with wide LBBB-like QRS, and patients demonstrating both proximal block and distal delay. ![]() The presence of conduction block was assumed to correlate with commonly applied criteria for LBBB. Seminal trials demonstrating mortality benefit from CRT were conducted in patients with wide left bundle branch block (LBBB) pattern on electrocardiogram (ECG) and evidence of clinical heart failure. 2Section of Cardiology, Center for Arrhythmia Care, University of Chicago Medicine, Chicago, IL, United StatesĬardiac resynchronization therapy (CRT) via biventricular pacing (BiVP) is an established treatment for patients with left ventricular systolic heart failure and intraventricular conduction delay resulting in wide QRS.1Department of Internal Medicine, University of Chicago Medicine, Chicago, IL, United States. ![]()
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