From the Guidelines
Left bundle branch block (LBBB) is primarily caused by conditions that damage or disrupt the heart's electrical conduction system, specifically the left bundle branch, with coronary artery disease, hypertension, and cardiomyopathies being the most common causes. The presence of LBBB on ECG markedly increases the likelihood that left ventricular systolic dysfunction will be diagnosed by echocardiogram 1. Other causes of LBBB include valvular heart disease, cardiac surgery complications, congenital heart defects, myocarditis, cardiac tumors, and degenerative conduction system disease in elderly patients. Some medications like antiarrhythmics (particularly sodium channel blockers) and high potassium levels can also induce LBBB.
Common Causes of LBBB
- Coronary artery disease, particularly after a myocardial infarction that affects the left ventricle or septum
- Hypertension, as long-standing high blood pressure leads to left ventricular hypertrophy that can damage the conduction system
- Cardiomyopathies, including dilated, hypertrophic, and infiltrative types, often result in LBBB due to structural heart changes
- Valvular heart disease, especially aortic stenosis and regurgitation, can cause LBBB through increased pressure load on the left ventricle
Diagnostic Evaluation
In patients with LBBB, a thorough cardiac evaluation is necessary to exclude underlying structural heart disease, which may include echocardiogram, stress test, ambulatory monitoring, and EPS 1. Cardiac MRI may be considered in selected patients with LBBB and normal left ventricular function by echocardiography where sarcoidosis, connective tissue disease, myocarditis, or other dilated cardiomyopathies are suspected on clinical grounds 1.
Clinical Significance
LBBB often indicates underlying cardiac disease and warrants thorough cardiac evaluation, as it can affect ventricular synchrony and potentially lead to heart failure over time. In younger patients without structural heart disease, LBBB may be rate-dependent, appearing only during periods of tachycardia. However, because rate-dependent LBBB, particularly if at slow rates, often occurs in the presence of structural heart disease, a more complete evaluation is necessary to exclude the latter 1.
From the Research
Causes of Left Bundle Branch Block (LBBB)
- LBBB is usually the expression of an underlying cardiopathy and represents an independent risk factor of cardiovascular mortality 2.
- It is frequently associated with structural heart disease, and predicts higher rates of morbidity and mortality 3.
- LBBB causes immediate electrical and mechanical dyssynchrony of the left ventricle (LV) and gradual structural damages in the Purkinje cells and myocardium 4.
- LBBB may be the first manifestation of a more diffuse myocardial disease 5.
- It may induce abnormalities in left ventricular performance due to abnormal asynchronous contraction patterns 5.
Underlying Conditions
- Cardiomyopathy (ejection fraction <35%) 3
- Structural heart disease 3, 4
- Myocardial disease 5
- Latent cardiomyopathy 5
Consequences
- Dyssynchronous left ventricular activation and inefficient contraction of the ventricles 2
- Reduced left ventricular ejection fraction (EF) 4
- Heart failure (HF) with preserved EF ≥50% (HFpEF), HF with moderately reduced EF 35%-50% (HFmrEF), and HF with reduced EF ≤35% (HFrEF) 4
- Asymmetric hypertrophy and left ventricular dilatation 5