What is the pathophysiology of bundle branch blocks?

Pathophysiology of Bundle Branch Blocks

Bundle branch blocks develop primarily as a consequence of degenerative lesions in specialized conducting tissue or from various cardiac pathologies, including ischemic heart disease, hypertension, cardiomyopathies, myocarditis, channelopathies, cardiac tumors, sarcoidosis, Chagas' disease, and congenital heart disease. 1

Anatomical Basis

• The normal cardiac conduction system consists of the sinus node, atrial muscle, atrioventricular node, His bundle, bundle branches, fascicles, Purkinje fibers, and ventricular muscle 1
• The His bundle divides into right and left bundle branches, with the left bundle further dividing into anterior and posterior fascicles 1
• Bundle branch blocks occur when conduction is delayed or blocked within one or more branches of the His-Purkinje system 1

Pathophysiological Mechanisms

Right Bundle Branch Block (RBBB)

• Represents conduction interruption/delay through the right bundle branch 1
• Uncommon in healthy individuals but may represent an idiopathic, isolated, and clinically benign finding 1
• QRS duration exceeds 120 ms with characteristic ECG patterns in right precordial leads 1
• May be associated with arrhythmogenic right ventricular cardiomyopathy (ARVC) when accompanied by epsilon waves or delayed S-wave upstroke 1

Left Bundle Branch Block (LBBB)

• Represents conduction block in the main left bundle or both fascicles 1
• Very rare in otherwise healthy individuals 1
• Strong ECG marker of underlying structural cardiovascular disorder 1
• May occur as an early and isolated manifestation of ischemic heart disease or cardiomyopathy, often years before structural changes in the left ventricle can be detected 1
• Causes electrical and mechanical ventricular dyssynchrony, affecting regional myocardial function 2
• Intermittent, rate-dependent LBBB may occur as a precursor and carry the same clinical significance as stable LBBB 1
• Recent studies suggest LBBB often involves proximal conduction block within the left-sided His fibers rather than diffuse disease throughout the distal conduction system 3, 4

Hemiblocks

• Left anterior hemiblock is more common in men and increases with age 1
• Estimated prevalence in the general population (age <40 years) is 0.5-1.0% 1
• Isolated left anterior hemiblock is usually an incidental ECG finding but may be associated with cardiovascular disorders 1
• Isolated left posterior hemiblock is very rare, usually associated with RBBB 1
• Combinations of bundle branch block and hemiblock (bifascicular block) reflect more extensive involvement of the specialized conduction system and carry increased risk of clinically significant atrioventricular block 1

Mechanical Consequences

• LBBB causes abnormal pattern of cardiac activation affecting regional myocardial function 2, 5
• In LBBB, there is prolongation of the interval from QRS onset to left ventricular ejection 5
• Common mechanical abnormalities in LBBB include 5:
  • Delayed mitral valve closure (23% of patients)
  • Prolonged left ventricular isovolumetric contraction time (41% of patients)
  • Both delayed mitral valve closure and prolonged isovolumetric contraction time (18% of patients)
  • Delayed onset of ventricular contraction (26% of patients)

Clinical Implications

• LBBB affects diagnostic accuracy of ischemia detection on ECG and stress testing 2
• LBBB may be associated with progressive cardiac conduction disease (Lènegre disease), an autosomal dominant condition linked to SCN5A gene mutations 1
• Patients with heart failure with reduced ejection fraction and LBBB may respond positively to cardiac resynchronization therapy 6, 2
• Demonstration of complete bundle branch block should lead to a cardiological work-up including exercise testing, 24-hour ECG, and imaging to evaluate underlying pathological causes 1
• Bifascicular block (LBBB, or RBBB with left posterior hemiblock) warrants ECG screening in siblings of young patients 1

Recent Advances in Understanding

• His bundle pacing can normalize wide QRS patterns in some patients with LBBB, suggesting proximal rather than diffuse conduction system disease 3, 4
• Electrophysiology studies with left-sided recordings have shown that conduction block associated with LBBB pattern is most often proximal, usually within the left-sided His fibers 3
• Patients with intact Purkinje activation and intraventricular conduction delay are less likely to benefit from His bundle pacing 3