What are the implications of a left bundle branch block (LBBB)?

From the Guidelines

Left bundle branch block (LBBB) is a significant cardiac conduction abnormality that can lead to impaired electromechanical coupling, reduced ventricular systolic function, and increased risk of heart failure and mortality, particularly in patients with QRS duration ≥150 milliseconds. The implications of LBBB are far-reaching, and its presence can mask signs of myocardial infarction on electrocardiogram (ECG) and may indicate underlying cardiac disease [ 1 ]. LBBB itself doesn't require specific treatment, but it's essential to identify and address any underlying cause such as coronary artery disease, hypertension, cardiomyopathy, or valvular heart disease.

Key Considerations

• LBBB can cause regional mechanical delay within the left ventricle, resulting in reduced ventricular systolic function, altered myocardial metabolism, functional mitral regurgitation, and adverse remodeling with ventricular dilatation [ 1 ].
• The presence of LBBB with QRS duration ≥150 milliseconds is a significant predictor of worsening heart failure, sudden cardiac death, and total death [ 1 ].
• Cardiac resynchronization therapy (CRT) may be beneficial in patients with heart failure and LBBB with QRS duration ≥150 milliseconds to improve cardiac function and reduce mortality [ 1 ].
• Patients with LBBB should undergo cardiac evaluation, including echocardiography to assess for structural heart disease and possible stress testing to evaluate for coronary artery disease.

Management and Treatment

• CRT is recommended as a Class I indication for patients with LBBB and QRS duration ≥150 milliseconds, as it has been shown to improve ventricular systolic function, reduce metabolic costs, ameliorate functional mitral regurgitation, and induce favorable remodeling with reduction of cardiac chamber dimensions [ 1 ].
• The differential classification of CRT benefit based on QRS duration is supported by multiple analyses, which have consistently demonstrated that a significant clinical benefit from CRT is greatest among patients with QRS duration ≥150 milliseconds [ 1 ].

From the Research

Implications of Left Bundle Branch Block (LBBB)

The implications of LBBB can be significant, particularly in patients with heart failure. Some key points to consider include:

• LBBB may be due to conduction system degeneration or a reflection of myocardial pathology, and can also develop following aortic valve disease or cardiac procedures 2
• Patients with heart failure with reduced ejection fraction and LBBB may respond positively to cardiac resynchronization therapy (CRT) 2
• LBBB can lead to severe left ventricular dyssynchrony, resulting in acute heart failure, particularly in cases where the block is acceleration-dependent 3
• CRT has been shown to improve hemodynamics in patients with heart failure and LBBB by resynchronizing left ventricular dyssynchrony 3

Cardiac Resynchronization Therapy (CRT) Options

Several options are available for delivering CRT, including:

• Lead placement via the coronary sinus, which is the mainstay approach of CRT 2
• Left bundle branch pacing (LBBP), which has been suggested as an alternative means to deliver CRT 4, 5
• His bundle pacing (HBP), which has been compared to LBBP and biventricular pacing (BVP) in terms of on-treatment outcomes 5
• Biventricular pacing (BVP), which has been shown to be less effective than LBBP and HBP in improving left ventricular ejection fraction (LVEF) and New York Heart Association (NYHA) class 5

Outcomes of LBBP

The outcomes of LBBP have been studied in several patient populations, including:

• Patients with heart failure with reduced ejection fraction and LBBB, who have been shown to respond positively to LBBP 4, 5
• Patients with mildly reduced and preserved left ventricular ejection fraction, who have been shown to experience improvements in cardiac function and remodelling with LBBP 6
• Patients with nonischemic cardiomyopathy, who have been shown to experience significant improvements in LVEF and NYHA class with LBBP 5, 6

Patient Selection for CRT

Patient selection for CRT is critical, and may involve:

• Evaluation for left ventricular dyssynchrony during exercise, particularly in patients with acceleration-dependent LBBB 3
• Assessment of LVEF and NYHA class, as well as other clinical and echocardiographic parameters 4, 5, 6
• Consideration of alternative CRT options, such as LBBP and HBP, in patients who may not be suitable for traditional BVP 5