Left bundle branch block (LBBB) is a distinct cardiac conduction abnormality visible on the surface electrocardiogram, reflecting a delay in the electrical activation of the left ventricle. Recognizing this pattern is essential, as it alters the heart’s sequence of contraction and provides vital clues about underlying cardiac health. The primary goal of identifying LBBB on an ECG is to understand its morphology and differentiate it from other conduction disturbances.
Fundamental Mechanics of a Normal ECG
To appreciate the changes in LBBB, one must first understand the standard progression of an ECG tracing. The process begins with atrial depolarization, seen as the P wave, followed by the impulse traveling through the atrioventricular node and into the ventricles. The ventricular depolarization is the most prominent event, represented by the QRS complex, which normally lasts less than 100 milliseconds. This rapid, synchronized activation is facilitated by the left and right bundle branches, ensuring the ventricles contract almost simultaneously.
Defining the LBBB Morphology
In LBBB, the electrical impulse cannot travel efficiently through the left bundle branch. Consequently, the left ventricle depolarizes later than the right ventricle, as the signal must spread cell-to-cell from the right side through the myocardium. This delay reshapes the QRS complex on the ECG, widening it to greater than 120 milliseconds and creating a characteristic pattern of positive and negative deflections that depend on the viewing angle of the electrode.
ECG Criteria for Diagnosis
The diagnosis of LBBB relies on specific criteria applied to the surface ECG. These criteria focus on the wide and notched appearance of the QRS complex in the lateral leads and the absence of certain septal signs. The following table summarizes the key diagnostic requirements used by clinicians worldwide.
Visual Description in Lead V1
Lead V1 offers a direct view of the interventricular septum and is crucial for identifying LBBB. Instead of the expected small r wave followed by a deep S wave, the ECG displays a wide, deep S wave. Often, this is preceded by a small initial positive deflection, and the entire complex appears as a broad, asymmetric QS or rS pattern. The terminal portion of the complex lacks the normal septal q wave, reinforcing the block.
Visual Description in Leads V5, V6, I, and aVL
Viewing the left ventricle from the lateral and high lateral walls provides a contrasting image to lead V1. In leads I, aVL, V5, and V6, the QRS complex appears as a wide, smooth monophasic R wave. This upward deflection dominates the tracing, often with a slurred upstroke or terminal notch indicating delayed activation. The transition from the negative deflections in V1 to these tall, broad positive waves is a hallmark visual feature of left bundle branch block.
