What are bundle branch blocks (BBB)?

From the Guidelines

Bundle branch blocks (BBB) are electrical conduction abnormalities in the heart that affect how electrical signals travel through the ventricles, and their presence can be a predictor for abnormal conduction properties, particularly in patients with syncope. There are two main types: left bundle branch block (LBBB) and right bundle branch block (RBBB). LBBB occurs when the left bundle branch is damaged or blocked, delaying activation of the left ventricle, while RBBB happens when the right bundle branch is affected, delaying right ventricular activation 1.

Diagnosis and Treatment

Diagnosis is typically made through an electrocardiogram (ECG), which shows a widened QRS complex (>120 ms) with characteristic patterns for LBBB and RBBB 1. Treatment for bundle branch blocks focuses on addressing any underlying heart conditions, and in many cases, no specific treatment is needed for the block itself if the patient is asymptomatic. However, in some cases of LBBB, cardiac resynchronization therapy (CRT) may be recommended to improve heart function and reduce symptoms of heart failure 1.

Management and Monitoring

Regular cardiac monitoring is important for patients with bundle branch blocks to track any progression or development of related heart conditions. Lifestyle modifications such as maintaining a heart-healthy diet, regular exercise, and avoiding smoking are also beneficial. The presence of BBB in patients with syncope warrants further evaluation, including electrophysiological study (EPS) and consideration of permanent pacing, especially if the HV interval is ≥70 ms or frank infranodal block is present 1. Understanding bundle branch blocks is crucial because they can impact heart function and may indicate underlying cardiac issues, and their management should prioritize reducing morbidity, mortality, and improving quality of life.

Key Considerations

• The diagnostic work-up for patients with BBB and syncope should follow established guidelines, including the use of EPS and implantable loop recorders (ILR) to determine the underlying cause of syncope 1.
• The therapeutic approach for patients with BBB and unexplained syncope depends on the ejection fraction (EF), with consideration of implantable cardioverter-defibrillator (ICD) or cardiac resynchronization therapy with defibrillator (CRT-D) for those with EF < 35% 1.
• The most recent guidelines and studies, such as the 2018 ACC/AHA/HRS guideline 1 and the 2013 ESC guidelines 1, should be consulted for the latest recommendations on the evaluation and management of patients with bundle branch blocks.

From the Research

Definition and Types of Bundle Branch Blocks

• Bundle branch blocks (BBB) are disorders of intraventricular conduction that can be stable or intermittent, and may appear and disappear with changes in heart rate 2.
• There are two types of bundle branch block: phase 3 block, which is believed to express a pathological increase in the duration of the recovery period of the bundle branch, and phase 4 block, which is best explained on the basis of enhanced phase 4 depolarization of the bundle branch system, with inability of excitation if the cardiac cycle is particularly prolonged 2.
• Left bundle branch block (LBBB) is frequently associated with structural heart disease, and predicts higher rates of morbidity and mortality 3.

Clinical Implications and Prognosis

• A proper characterization of frequency-dependent bundle branch blocks can provide useful prognostic information in some clinical situations, and may be due to an extensive damage of infrahisian system, which poses a high risk of developing advanced atrioventricular block requiring pacemaker implantation 4.
• The presence of LBBB independently identifies individuals with ∼3- to 7-fold greater odds of developing new segmental abnormal LV wall motion in patients with ECG-LVH undergoing antihypertensive treatment 5.
• LBBB-associated cardiomyopathy represents a potentially reversible form of cardiomyopathy, with the majority of patients having reverse remodelling after cardiac resynchronisation therapy (CRT) by left bundle branch pacing 3.

Mechanisms and Related Phenomena

• Functional bundle branch block can be caused by long-short aberrancy (usually physiologic), or acceleration/deceleration dependent aberrancy (usually pathologic) 6.
• Supernormal conduction may explain unusual conduction phenomena in patients with abnormal His-Purkinje function or poorly conducting accessory pathways 6.
• The gap phenomenon "paradox" is that with increasingly premature extrastimuli, progressive proximal conduction delay allows time for distal recovery of excitability 6.