Causes of Left Bundle Branch Block
Left bundle branch block results from interruption of conduction through the left bundle branch, most commonly due to underlying structural heart disease including ischemic heart disease, cardiomyopathies, hypertensive heart disease, degenerative conduction system disease, and less commonly from infiltrative processes, myocarditis, or iatrogenic causes following cardiac procedures. 1, 2
Primary Etiologic Categories
Structural Heart Disease (Most Common)
Ischemic heart disease is a critical etiology, particularly anterior myocardial infarction with persistent intraventricular conduction disturbances, which carries an unfavorable prognosis and increased risk of sudden cardiac death 3, 1
Hypertensive heart disease represents a major structural cause of LBBB, especially in older adults with long-standing hypertension 1
Cardiomyopathies of various types (dilated, hypertrophic, restrictive) produce LBBB through direct involvement of the left ventricular conduction system, with severe cardiomyopathy causing interruption of the left bundle and carrying a poor prognosis 3, 1
Degenerative Conduction System Disease
Primary degenerative lesions of the specialized conducting tissue occur as an isolated phenomenon, particularly in older individuals, representing age-related fibrosis of the conduction system 1
Lenegre disease (progressive cardiac conduction disease) is an autosomal dominant condition linked to SCN5A gene mutations affecting cardiac sodium channels, presenting with various conduction defects including LBBB in young individuals 4
Infiltrative and Inflammatory Processes
Sarcoidosis is an infiltrative cause that requires prophylactic pacing consideration even if AV block is transient, due to disease progression risk 1, 4
Amyloidosis may cause LBBB and requires prophylactic pacing consideration given the progressive nature of conduction system involvement 4
Myocarditis is an inflammatory cause of bundle branch blocks 1, 4
Iatrogenic and Procedural Causes
Aortic valve disease or cardiac procedures can lead to development of LBBB, representing a distinct iatrogenic category 2
Post-cardiac surgery conduction abnormalities may result in LBBB 5
Congenital Heart Disease
- Congenital heart malformations including atrioventricular canal defects and tricuspid atresia are associated with left anterior fascicular block and other conduction abnormalities 3
Critical Clinical Context
When LBBB Signals Serious Disease
LBBB is usually the expression of underlying cardiopathy and represents an independent risk factor for cardiovascular mortality, therefore further cardiac evaluation is indicated in each case 6, 7
LBBB may be the first manifestation of a more diffuse myocardial disease, making it essential to investigate for underlying structural pathology 7
In the context of acute myocardial infarction, LBBB identifies a subset of patients at high risk, particularly with anterior infarction and persistent intraventricular conduction disturbances 3
Pathophysiologic Consequences
LBBB induces abnormalities in left ventricular performance due to abnormal asynchronous contraction patterns, causing regional differences in workload that may lead to asymmetric hypertrophy and left ventricular dilatation 7
Asynchronous electrical activation causes increased wall mass in late-activated regions, which may aggravate preexisting left ventricular pumping performance or even induce dysfunction 7
"Latent cardiomyopathy" describes patients with LBBB and normal left ventricular dimensions/ejection fraction at rest but who present with abnormal increase in pulmonary artery pressure during exercise, lactate production during high-rate pacing, and signs of ischemia without coronary artery narrowing 7
Important Clinical Pitfalls
Congenital isolated complete LBBB is very rare in neonates, making acquired causes much more likely in adult presentations 3
LBBB-associated cardiomyopathy represents a potentially reversible form of cardiomyopathy in the absence of any other known etiology, with the majority of patients having reverse remodeling after cardiac resynchronization therapy 8
Alternating bundle branch block (LBBB and RBBB on successive ECGs) indicates severe conduction system disease with rapid progression to complete heart block and warrants permanent pacemaker implantation 1, 4