Bundle branch blocks represent a specific pattern of delayed electrical conduction within the ventricles, fundamentally altering the sequence of cardiac activation. This condition occurs when the bundle branches, the specialized pathways responsible for rapidly distributing the electrical impulse to the left and right ventricles, become partially or completely obstructed. The resulting delay or failure of depolarization creates distinct changes in the surface electrocardiogram (ECG), serving as a critical diagnostic marker for underlying cardiac pathology.
Understanding the Cardiac Conduction System
The heart's rhythmic contraction is initiated by the sinoatrial node, the natural pacemaker located in the right atrium. This electrical impulse travels through the atrioventricular node and into the His bundle, which divides into the right and left bundle branches just below the membranous septum. The right bundle branch innervates the right ventricle almost exclusively, while the left bundle branch supplies the left ventricle via its anterior and posterior fascicles. A block at any point within this system disrupts the efficient, near-synchronous contraction of the ventricles, leading to the characteristic wide QRS complex observed on an ECG.
Classification by Anatomical Location
Clinically, bundle branch blocks are primarily categorized based on the specific limb of the conduction system that is impaired. This anatomical classification is essential for interpreting the ECG and understanding the pathophysiology. The two main categories are right bundle branch block (RBBB) and left bundle branch block (LBBB), with further subdivisions possible within the left bundle branch depending on the fascicle involved.
Right Bundle Branch Block (RBBB)
Right bundle branch block is characterized by a delay or failure of activation in the right ventricle. On an ECG, this manifests as a wide QRS complex exceeding 120 milliseconds, with a distinctive 'rsR' pattern (often described as a rabbit's ear) in the right precordial leads (V1-V2). The terminal portion of the complex appears wide and slurred in the left precordial leads (V5-V6). RBBB is frequently observed in otherwise healthy individuals but is also commonly associated with conditions such as pulmonary hypertension, atrial septal defect, and chronic lung disease.
Left Bundle Branch Block (LBBB)
Left bundle branch block involves a delay in the activation of the left ventricle, typically due to disease within the left bundle branch or its fascicles. The ECG findings are more complex, featuring a wide QRS complex with a monophasic R wave in the lateral leads (I, aVL, V5-V6) and deep S waves in the right precordial leads (V1-V2). The absence of a septal q wave and the presence of a broad, notched R wave in lateral leads are hallmark features. LBBB is almost always pathological, strongly associated with hypertension, aortic valve disease, ischemic heart disease, and cardiomyopathies.
Fascicular Blocks: The Subdivisions of LBBB
Because the left bundle branch divides into anterior and posterior fascicles, blocks can occur at a more specific level, known as hemiblocks or fascicular blocks. These lesions often present with a more subtle ECG pattern than a complete LBBB and can be an early indicator of underlying diffuse left ventricular disease. Identifying the specific fascicle involved is crucial for risk stratification and understanding the progression of cardiac disease.
Left Anterior Fascicular Block (LAFB)
Left anterior fascicular block, or hemiblock, results from interruption of the left anterior fascicle. The ECG diagnosis is based on a consistent left axis deviation, typically between -45° and -90°, alongside a small, rS pattern in lead aVF and a qR pattern in lead I. LAFB is the most common type of hemiblock and, while often considered a benign finding in the absence of other conduction abnormalities, can signify underlying structural heart disease when new-onset in older patients.
