What is the significance of Left Bundle Branch Block (LBBB)?

From the Guidelines

Left Bundle Branch Block (LBBB) is a significant electrical conduction abnormality in the heart that can indicate underlying heart disease and may require further cardiac evaluation and treatment, including cardiac resynchronization therapy (CRT) in patients with heart failure. The presence of LBBB can complicate the diagnosis of heart attacks and alter normal ECG patterns. According to the 2012 ACCF/AHA/HRS focused update of the 2008 guidelines for device-based therapy of cardiac rhythm abnormalities 1, LBBB is a key factor in determining the eligibility for CRT in patients with systolic heart failure. The guidelines recommend CRT for patients with LVEF less than or equal to 35%, sinus rhythm, LBBB with a QRS duration greater than or equal to 150 ms, and NYHA class II, III, or ambulatory IV symptoms on GDMT.

The significance of LBBB is further emphasized by the 2019 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay, which highlights the importance of echocardiography in patients with suspected structural heart disease, including those with LBBB 1. The guideline also notes that patients with LBBB have a higher likelihood of left ventricular systolic dysfunction compared to those with normal ECGs.

More recently, the 2021 ACC/AHA key data elements and definitions for heart failure provide further insight into the significance of LBBB, defining it as a cardiac conduction abnormality seen on the ECG, where activation of the left ventricle of the heart is delayed, causing the left ventricle to contract later than the right ventricle 1. The document emphasizes the importance of accurate diagnosis and treatment of LBBB, particularly in patients with heart failure.

Key points to consider in the management of LBBB include:

• The presence of LBBB can indicate underlying heart disease, such as coronary artery disease, cardiomyopathy, or heart failure
• LBBB can complicate the diagnosis of heart attacks and alter normal ECG patterns
• Patients with LBBB may require further cardiac evaluation, including echocardiography, stress testing, or coronary angiography
• CRT may be beneficial in patients with LBBB and heart failure, particularly those with a QRS duration greater than or equal to 150 ms and NYHA class II, III, or ambulatory IV symptoms on GDMT.

Overall, LBBB is a significant electrical conduction abnormality that requires careful evaluation and management to improve patient outcomes. The most recent and highest quality study, the 2021 ACC/AHA key data elements and definitions for heart failure 1, provides the most up-to-date guidance on the diagnosis and treatment of LBBB, emphasizing the importance of accurate diagnosis and treatment in patients with heart failure.

From the Research

Significance of Left Bundle Branch Block (LBBB)

• LBBB is a significant condition that occurs frequently in heart failure and may trigger a vicious cycle of progressive left ventricular dysfunction 2.
• A correct diagnosis of LBBB is pivotal to guide cardiac resynchronisation therapy 2.
• LBBB causes an abnormal pattern of cardiac activation and affects regional myocardial function, leading to electrical and mechanical ventricular dyssynchrony 3.
• The altered pattern of cardiac activation in LBBB influences ischemia detection on the surface electrocardiogram, and affects stress testing and imaging modalities dependent on wall motion and thickening 3.

Diagnostic and Therapeutic Implications

• Restoration of synchrony by biventricular pacing can improve symptoms and longevity in carefully selected patients with LBBB 3.
• Left bundle branch pacing (LBBP) is a potentially optimal alternative to cardiac resynchronization therapy (CRT) with biventricular pacing (BiVP) for patients with heart failure and reduced left ventricular ejection fraction (LVEF) 4.
• LBBP-CRT has been shown to have greater LVEF improvement than BiVP-CRT in heart failure patients with nonischemic cardiomyopathy and LBBB 4.

Importance of Accurate Diagnosis

• Conventional electrocardiographic (ECG) criteria for LBBB may result in false-positive diagnoses, particularly in cases of left ventricular hypertrophy (LVH) or dilatation 5.
• Strict LBBB ECG criteria, which include LV conduction delay, QRSd ≥140 ms men or ≥130 ms women, and mid-QRS notching in at least two leads, can improve specificity in cases of LVH/dilatation and incomplete LBBB 5.
• Accurate diagnosis of LBBB is critical for selecting patients who will benefit from cardiac resynchronization therapy (CRT) 5.

Clinical Outcomes

• Patients with idiopathic LBBB (iLBBB) are at risk of developing left ventricular dysfunction (LVD) and cardiovascular events, highlighting the need for serial and proper follow-up of these patients 6.
• Cardiac resynchronization therapy (CRT) has been shown to improve outcomes in patients with LBBB-cardiomyopathy (LBBB-CMP), including improved left ventricular ejection fraction (LVEF) and reduced risk of heart failure-related events 6.