Left bundle branch block (LBBB) is a distinctive conduction abnormality visible on the surface electrocardiogram, representing a delay or failure of electrical activation within the left bundle branch of the His-Purkinje system. Recognizing this pattern is essential because it alters the typical sequence of ventricular depolarization, changing the appearance of the QRS complex and often signaling underlying cardiac pathology. The hallmark of LBBB is a widened QRS duration, generally exceeding 120 milliseconds, accompanied by specific morphological changes in the limb and precordial leads that reflect the abnormal spread of electrical activity across the ventricles.
Fundamental ECG Characteristics of LBBB
The primary diagnostic criteria for LBBB revolve around the widening of the QRS complex and its characteristic configuration in specific leads. These criteria are applied systematically to distinguish LBBB from other causes of wide QRS complexes and to ensure accurate identification. The following parameters define the classic electrocardiographic signature of a left bundle branch block.
Width and Morphology Criteria
The first and most critical feature is a QRS duration of 120 milliseconds or greater, reflecting the delayed activation of the left ventricle. The terminal portion of the QRS complex in the left precordial leads (V5, V6, and often V4) must display a monophasic or biphasic pattern with a prominent, broad, notched R wave, commonly described as a "rabbit ear" configuration. This morphology results from the electrical current moving away from the recording electrode in these leads as activation proceeds from right to left.
Lead-Specific Findings
In the limb leads, the QRS complex typically shows a predominantly negative or biphaser pattern in leads I and aVL, with the terminal force directed inferiorly. Conversely, the right precordial leads, V1 and V2, usually exhibit a deep, wide S wave, reflecting the initial septal depolarization moving toward the inactive left bundle branch. The presence of an rSR' pattern in V1 is a common, though not universal, finding that underscores the altered depolarization sequence.
Physiological Mechanism and Visual Appearance
To truly understand what LBBB looks like, it is helpful to visualize the underlying electrical event. In a normal heart, the left bundle branch activates the left ventricle almost simultaneously with the right bundle branch activating the right ventricle, leading to a balanced and efficient contraction. When a left bundle branch block occurs, the left ventricle is depolarized solely via cell-to-cell conduction from the right ventricle, a much slower process that drags out the QRS complex.
