Left bundle branch block (LBBB) and right bundle branch block (RBBB) represent the most common specific patterns observed on a standard 12-lead ECG when analyzing the ventricular conduction system. Both conditions signify a delay in the electrical activation of either the left or right ventricle, respectively, yet they carry distinct implications for underlying cardiac pathology and clinical management. While the diagnosis of a bundle branch block often appears straightforward, the nuances differentiating LBBB vs RBBB on ECG are critical for accurate risk stratification and guiding appropriate intervention. Understanding these differences requires a detailed analysis of the surface ECG morphology, the underlying pathophysiology, and the associated clinical correlations.
Fundamental Mechanisms of Bundle Branch Block
The heart's electrical impulse originates in the sinoatrial node and travels through the atrioventricular node, the His bundle, and subsequently divides into the right and left bundle branches. These fascicles ensure near-synchronous depolarization of the ventricles. A block within one of these branches disrupts this synchronization, forcing the ventricles to depolarize via slower cell-to-cell conduction rather than the specialized fast-conducting system. This fundamental delay is what generates the wide QRS complex exceeding 120 milliseconds seen on the ECG. The direction of the initial vector and the subsequent activation sequence determine the specific waveform patterns that differentiate LBBB from RBBB, making the surface ECG a non-invasive window into the conduction system's integrity.
ECG Characteristics of Right Bundle Branch Block
Identifying RBBB relies on recognizing a classic triad of findings across the precordial leads. The hallmark is a wide, slurred S wave in leads I and V6, reflecting delayed activation of the left ventricle. Concurrently, a tall, wide R wave, often denoted as R', appears in V1 and V2, representing the late activation of the right ventricle. The final defining feature is a wide, slurred S wave in leads V1 and V2. This specific morphology indicates that the septum is activated from right to left (the normal direction) initially, but the left ventricle requires a secondary impulse spreading cell-by-cell, which takes significantly longer. The QRS duration in RBBB consistently exceeds 120 ms, providing the primary quantitative threshold for diagnosis.
Typical Morphology in Right Bundle Branch Block
Lead I: Broad, slurred S wave.
Lead V1: Broad, slurred R wave or rsR' pattern.
Lead V6: Slurred S wave.
QRS Duration: Greater than 120 ms.
ECG Characteristics of Left Bundle Branch Block
LBBB presents with a more complex morphology due to the reversal of the normal ventricular activation sequence. In a typical LBBB, the left ventricle is activated late and inefficiently via cell-to-cell conduction, while the right ventricle is activated normally. This results in a QRS complex that is wide, notched, and predominantly negative in the right-sided leads (V1) and wide, monophasic, and positive in the lateral leads (I, aVL, V5, V6). A crucial diagnostic feature is the absence of a q wave in the lateral leads, as the septum is activated right-to-left, obliterating this initial septal depolarization. Furthermore, the ST segments and T waves typically display a directional discordance, meaning they are deflected opposite to the main QRS complex, which is a secondary repolarization phenomenon caused by the altered sequence of activation.
Typical Morphology in Left Bundle Branch Block
Lead I, aVL, V5, V6: Broad, notched R wave without q wave.
Lead V1: Broad, deep S wave with minimal or no R wave.
II, III, aVF: Variable, often with rS pattern.
ST-T Waves: Concordant opposite to the QRS main deflection.
