Bundle Branch Block Morphology on ECG

Bundle Branch Block Morphology on ECG

Understanding bundle branch block (BBB) patterns on ECG is a core skill for clinicians, residents, and cardiology trainees. BBB alters ventricular depolarization, producing characteristic QRS morphologies that are best appreciated by systematically comparing leads V1 and V6. Once you train your eye, these patterns become instantly recognizable and clinically meaningful.

Normal Ventricular Conduction on ECG

In normal conduction, electrical impulses travel simultaneously down the right and left bundle branches, leading to near-synchronous activation of both ventricles.

In lead V1, the QRS complex is usually predominantly negative, reflecting left ventricular dominance, while in lead V6 there is a tall, narrow R wave representing efficient left ventricular depolarization. The QRS duration remains narrow, typically less than 120 ms, and ST-T segments follow the QRS direction smoothly.

These normal patterns form the baseline against which bundle branch blocks are identified.

Left Bundle Branch Block (LBBB): ECG Morphology Explained

In left bundle branch block, conduction through the left bundle is delayed or blocked. As a result, the right ventricle depolarizes first, and the left ventricle is activated later through slow cell-to-cell conduction.

Lead V1 in LBBB

V1 typically shows a predominantly negative QRS complex with a QS or rS pattern. The classic description is a “W-shaped” complex. This reflects delayed left ventricular activation moving away from V1. The QRS is wide, usually ≥120 ms, and the initial septal q wave is absent because septal activation is reversed.

Lead V6 in LBBB

V6 shows a broad, upright, monophasic R wave, often described as an “M-shaped” appearance. The R wave is tall and slurred, with no preceding Q wave. This is a hallmark feature of LBBB and represents delayed left ventricular depolarization spreading toward lateral leads.

ST-T changes in LBBB

Secondary ST-T changes are common. ST segments and T waves are typically discordant to the major QRS deflection. This is a physiological consequence of abnormal depolarization and should not be mistaken for ischemia unless disproportionate or meeting specific criteria.

Clinical significance of LBBB

LBBB is often associated with structural heart disease such as cardiomyopathy, hypertension, aortic valve disease, or ischemic heart disease. New-onset LBBB in the setting of chest pain raises concern for acute myocardial infarction and requires urgent evaluation.

Right Bundle Branch Block (RBBB): ECG Morphology Explained

In right bundle branch block, right ventricular activation is delayed, while left ventricular depolarization occurs normally.

Lead V1 in RBBB

V1 characteristically shows an rSR′ or rsR′ pattern, producing an “M-shaped” QRS complex. The terminal R′ wave reflects delayed right ventricular depolarization. The QRS duration is prolonged, but the initial part of the QRS is usually normal.

Lead V6 in RBBB

V6 demonstrates a broad, terminal S wave, sometimes described as a “W-shaped” appearance. This occurs because delayed right ventricular activation moves away from the lateral leads.

ST-T changes in RBBB

As with LBBB, secondary repolarization changes are common. T-wave inversion and ST depression may be seen in the right precordial leads and are usually benign when consistent with classic RBBB morphology.

Clinical significance of RBBB

RBBB may be seen in structurally normal hearts and can be incidental, especially in younger individuals. However, it can also be associated with pulmonary embolism, chronic lung disease, right ventricular strain, ischemic heart disease, or congenital heart conditions.

A Simple Memory Aid: WiLLiaM–MaRRoW

To quickly differentiate BBB patterns, many clinicians use visual mnemonics:

LBBB

V1: W appearance

V6: M appearance

RBBB

V1: M appearance

V6: W appearance

This simple mental image helps reinforce lead-based pattern recognition during real-world ECG interpretation.

Why Rate and Disease Matter

It is important to remember that ECG patterns shown in diagrams are representative. Heart rate, ventricular hypertrophy, ischemia, pacing, electrolyte disturbances, and underlying structural disease can all modify QRS morphology. Therefore, BBB diagnosis should always be made in clinical context rather than relying on a single feature in isolation.

Practical Takeaway

When faced with a wide QRS complex on ECG, always look at V1 and V6 together. Identify whether the dominant pattern is an M or a W in each lead, assess QRS duration, and correlate with clinical findings. Mastery of these basic morphological principles turns bundle branch block interpretation from memorization into pattern recognition, a skill that becomes faster and more intuitive with practice.