Bundle Branch Block

Bundle Branch Block

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The heart has a complex electrical system responsible for ensuring the most efficient sequence of cardiac contraction. Any change in this electrical system can affect cardiac output. This article will focus on the physiology, pathophysiology, and clinical consequences of damage to the bundle branches.

The cardiac conduction system

The electrical impulse travels through the heart via a specific conduction pathway. The sinoatrial node (SAN) serves as the initial pacemaker before the impulse spreads throughout the atria and towards the atrioventricular node (AVN).

The depolarisation wave then travels through the heart's septum through the Bundle of His and Purkinje fibres. These are organised into the left and right bundle branches. The right bundle branch depolarises the right ventricle while the left bundle branch depolarises the left ventricle simultaneously. The septum itself is depolarised by the left bundle branch, meaning it is depolarised from left to right.

ECG basics

The ECG is a graphical representation of the net direction of electrical depolarisation in the heart at any one time. Different leads look at the heart from different angles (it is important to note that V1 views the heart from the right and V6 from the left).

An upwards spike indicates that the net depolarisation is heading towards that lead. A downward spike indicates the net depolarisation is heading away from the lead. There is a greater muscle mass on the left side of the heart compared to the right, so depolarisation within the left ventricles has a stronger effect on the ECG trace. The right and left ventricles should typically depolarise simultaneously to produce one uniform R wave.

Ventricular depolarisation using normal pathways is complete within 120ms. When these pathways are disrupted or changed in any way, depolarisation takes longer, leading to broad QRS complexes. A broad QRS complex is always indicative of abnormal ventricular depolarisation.

Table 1. Components of an ECG trace.

ECG Components in Normal Cardiac Conduction

The following table details the different components of an electrocardiogram (ECG) and their definitions:

  • P wave: Atrial depolarisation
  • PR interval: Conduction through the AVN to the ventricles
  • QRS complex: Ventricular depolarisation
  • Q wave: The first downward deflection
  • R wave: Any upwards deflection
  • S wave: Any downward deflection after an R wave
  • T wave: Ventricular repolarisation

In normal cardiac conduction (Figure 1), the sino-atrial node acts as the initial pacemaker. Depolarisation reaches the atrioventricular node and impulses travel simultaneously down the bundle of His via the left and right bundle branches. The septum is depolarised from the left and both left and right ventricular walls are depolarised simultaneously.

The main feature of bundle branch blocks is the broadening of QRS complexes. It is thus important to exclude other causes of broad complexes. For more information, consult a guide to atrioventricular blocks.

As the problem is below the atria, the P waves and PR intervals remain normal.

Right bundle branch block (RBBB)

The diagnostic criteria for RBBB are:

  • QRS > 120 ms (3 small squares)
  • RSR’ pattern in V1-V3
  • Wide, slurred S wave in lateral leads -I, aVL, V5-V6

A quicker simpler way to recognise and interpret this in a clinical setting is to use the MarRoW mnemonic.

Right bundle branch block: MaRroW

Complexes in V1 resemble the letter M (the first letter of MaRroW) and complexes in V6 resemble a W (the sixth letter of MaRroW)

In right bundle branch block:

  1. The sino-atrial node acts as the initial pacemaker
  2. Depolarisation reaches the atrioventricular node
  3. Depolarisation through the bundle of His occurs only via the left bundle branch. The left branch still depolarises the septum as normal.
  4. The left ventricular wall depolarises as normal.
  5. The right ventricular walls are eventually depolarised by the left bundle branch, this occurs by a slower, less efficient pathway.

Right Bundle Branch Block (RBBB)

RBBB can either be physiological or the result of damage to the right bundle branch. Possible causes of damage include underlying lung pathology (COPD, pulmonary emboli, cor pulmonale), primary heart muscle disease (ARVC), congenital heart disease (e.g. ASD), ischaemic heart disease and primary degeneration of the right bundle.

Left Bundle Branch Block (LBBB)

When viewed from the right hand side (V1), net depolarisation travels away (towards the left), resulting in negative ECG deflections. The first downward deflection represents the right ventricle, and the slightly delayed second downward deflection corresponds to the depolarisation of the left ventricle.

When viewed from the left hand side (V6), where the net depolarisation is travelling towards the detector, deflections are positive on the ECG. Again, there will be two peaks (RR) due to the delay in left ventricular depolarisation.

In left bundle branch block:

  • The sino-atrial node acts as the initial pacemaker
  • Depolarisation reaches the atrioventricular node
  • Depolarisation down the bundle of His occurs only via the right bundle branch. The septum is abnormally depolarised from right to left.
  • The right ventricular wall is depolarised as normal.
  • The left ventricular walls are eventually depolarised by the right bundle branch, this occurs by a slower, less efficient pathway.

Left Bundle Branch Block

Left bundle branch block (LBBB) is a common ECG abnormality seen in clinical practice. Its diagnostic criteria are as follows:

  • QRS duration > 120ms (3 small squares)
  • Dominant S wave in V1
  • Broad, monophasic R wave in lateral leads - I, aVL, V5-V6
  • Absence of Q waves in lateral leads
  • Prolonged R wave > 60ms in leads V5-V6

A simpler way to recognise and interpret LBBB, particularly in a clinical setting, is the WiLliaM mnemonic. It states that complexes in V1 resemble the letter W and complexes in V6 resemble the letter M.

LBBB is always pathological, and can be due to conduction system degeneration or result from myocardial pathologies such as ischaemic heart disease, cardiomyopathy and valvular heart disease. It may also occur after cardiac procedures which damage the left bundle branch or His bundle. A STEMI presenting with chest pain and LBBB is very rare.

Branches of the Left Bundle Branch

The left bundle branch splits into anterior and posterior fascicles, and can be damaged in isolation. Anterior fascicle block, which is more common, causes left axis deviation while posterior fascicle block may cause right axis deviation. However, due to the greater mass of the left ventricle, disruptions of its depolarisation can cause cardiac axis changes. The right ventricle does not have a large enough mass to significantly deviate the cardiac axis.

LBBB is equal to left anterior fasicular block (LAFB) plus left posterior fasicular block (LPFB).

Bundle Branch Block

Bundle branch block is seen on an electrocardiogram (ECG) trace when there is a broadening of the QRS complex. Broadening of the QRS complex may be seen in a variety of cardiac pathologies and so it is important to differentiate between bundle branch block and other causes of this pattern.

Right bundle branch block is more common than left bundle branch block and can be a normal variant. This requires a QRS duration greater than 110 ms and a monophasic R wave in V1.

The WiLliaM and MaRroW technique is often used to assess the QRS complex morphology of V1 and V6 and differentiate whether the bundle branch block is RBBB or LBBB.

The Left Bundle Branch

Left bundle branch block is almost always pathological and is associated with significant damage to the conduction system. It is seen with a QRS duration greater than 120 ms and a monophasic R wave in V1. It is further divided into anterior and posterior fascicles.

Other Types of Block

Bifascicular block is a combination of right bundle branch block and blockade of one of the fascicles of the left bundle branch.

Trifascicular block occurs when a third-degree atrioventricular block is present alongside bifascicular block.

Key Points

  • Consider bundle branch block in an ECG trace with broad complexes.
  • A broad complex tachycardia requires differentiation between ventricular tachycardia or sinus tachycardia with concurrent bundle branch block.
  • Left bundle branch block is always pathological and indicates significant damage to the cardiac conduction system.
  • When assessing whether a broad QRS complex is LBBB or RBBB, the appearances of V1 and V6 are often enough to provide the answer using the WiLliaM and MaRroW technique.


  1. Burns, R. B. (2021, December 10). Left Bundle Branch Block. Available from: [LINK]
  2. Buttner, E. B. (2021, March 5). Right bundle branch block. Available from: [LINK]
  3. Hampton, J. (2013). ECG Made Easy.

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