Bundle Branch Block

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A comprehensive topic overview

Introduction

The heart has an intricate electrical system responsible for ensuring the most e
Any change in this electrical conduction system can a
This article will focus on the physiology, pathophysiology, and clinical consequences of damage to the bundle branches,
including left bundle branch block (LBBB) and right bundle branch block (RBBB).
You may also be interested in our guides to reading an ECG and understanding an ECG.

The cardiac conduction system

Electrical impulses travel through the heart via a speci
pacemaker before the impulse spreads throughout the atria and towards the atrioventricular node (AVN).
The depolarisation wave travels through the heart's septum via the Bundle of His and Purkinje
into the left and right bundle branches.
The right bundle branch depolarises the right ventricle, and the left bundle branch depolarises the left ventricle
simultaneously. The septum is depolarised by the left bundle branch, resulting in the septum being depolarised from left
to right.
For more information, see the Geeky Medics guide to the heart's conduction system.

ECG basics

The ECG is a graphical representation of the net direction of electrical depolarisation in the heart at any one time.
3
Di
is that V1 views the heart from the right and V6 from the left).
An upwards spike means the net depolarisation is heading towards that lead. A downward spike means the net
depolarisation is heading away from that 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 greater impact on the ECG trace.
The right and left ventricles should depolarise simultaneously, producing one uniform R wave.
Ventricular depolarisation using normal pathways is complete within 120ms. Depolarisation takes longer when these
pathways are disrupted or changed in any way, causing broad QRS complexes. A broad QRS complex always indicates
abnormal ventricular depolarisation.
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For more information on ECG interpretation, see the Geeky Medics guides to reading an ECG and understanding an ECG.
Table 1. Components of an ECG trace.
ECG component DeP wave Atrial depolarisation
PR interval Conduction through the AVN to the ventricles
QRS complex Ventricular depolarisation
Q wave The
R wave Any upwards de
S wave Any downward de
T wave Ventricular repolarisation
In normal cardiac conduction (Figure 1)\:
1. The sino-atrial node acts as the initial pacemaker
2. Depolarisation reaches the atrioventricular node
3. Impulses travel simultaneously down the bundle of His via the left and right bundle branches. The septum is depolarised
from the left.
4. Both the left and right ventricular walls are depolarised simultaneously.
Figure 1. Normal cardiac conduction
system
The main feature of bundle branch blocks is the broadening of QRS complexes. It is important to ensure other causes of
broad complexes are excluded. For more information, see the Geeky Medics guide to atrioventricular blocks.
As the problem is below the atria, the P waves and the PR intervals are normal.

Right bundle branch block (RBBB)

Diagnostic criteria
The diagnostic criteria for RBBB are\:
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Broad QRS complex\: >120 ms (3 small squares)
RSR’ pattern in V1-V3\: an initial small upward de
large upward de
Wide, slurred S wave in lateral leads\: I, aVL, V5-V6
WiLLiaM MaRRoW mnemonic
The WiLLiaM MaRRoW mnemonic can be used to quickly recognise left and right bundle branch blocks by looking
at V1 and V6\:
WiLLiaM refers to the ECG appearance of left bundle branch block (see next section)
MaRRoW refers to the ECG appearance of right bundle branch block
The middle letters of the names help you remember which bundle branch block each name is referring two (two Ls
in WiLLiaM = left bundle branch block, two Rs in MaRRoW = right bundle branch block).
Each name's
To recognise right bundle branch block, we use the name MaRRoW and look at the
wave), then another large upward de
W\: complexes in V6 resemble a W\: initial small downward de
wave), and then a wide downward de
Figure 2. ECG features of right bundle branch block
Pathophysiology
In right bundle branch block (Figure 3)\:
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 e
pathway.
Figure 3. Right bundle branch block
Clinical relevance
RBBB can be either physiological or the result of damage to the right bundle branch. 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)

Diagnostic criteria
The diagnostic criteria for LBBB are\:
Broad QRS complex\: >120 ms (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-V6WiLLiaM MaRRoW mnemonic
As discussed above, the WiLLiaM MaRRoW mnemonic can be used to quickly recognise left and right bundle branch
blocks by looking at V1 and V6.
To recognise left bundle branch block, we use the name WiLLiaM and look at the
W\: complexes in V1 resemble the letter W\: deep downward de
M\: complexes in V6 resemble the letter M\: broad, notched or 'M' shaped R wave in V6
Figure 4. ECG features of left bundle branch block
Pathophysiology
When viewed from the right-hand side (V1), net depolarisation travels away (towards the left), resulting in negative ECG
dend
downward
de
When viewed from the left-hand side (V6), where the net depolarisation is travelling towards the detector, de
positive on the ECG. Again, there will be two peaks (RR) due to the delay in left ventricular depolarisation.
1
In left bundle branch block (Figure 4)\:
1. The sino-atrial node acts as the initial pacemaker
2. Depolarisation reaches the atrioventricular node
3. Depolarisation down the bundle of His occurs only via the right bundle branch. The septum is abnormally depolarised
from right to left.
4. The right ventricular wall is depolarised as normal.
5. The left ventricular walls are eventually depolarised by the right bundle branch, this occurs by a slower, less e
pathway.
Figure 5. Left bundle branch block
Clinical relevance
LBBB is always pathological. Left bundle branch block may be due to conduction system degeneration or myocardial
pathologies such as ischaemic heart disease, cardiomyopathy and valvular heart disease.
LBBB may also occur after cardiac procedures, which damage the left bundle branch or His bundle. A STEMI presenting
as chest pain with LBBB is exceedingly rare.Branches of the left bundle branch
Due to the relatively greater mass of the left ventricle, disruptions in the depolarisation of the left ventricular muscle can
cause cardiac axis changes. The left bundle branch splits into anterior and posterior fascicles.
LBBB = Left anterior fasicular block (LAFB) + Left posterior fasicular block (LPFB)
Each branch of the left bundle branch may be damaged in isolation. Anterior fascicle block, which is much more common,
causes left axis deviation. Posterior fascicle block may cause right axis deviation. However, the posterior fascicle does
much less work than the anterior fascicle, so it can be blocked without any obvious ECG changes.
The right ventricular muscle does not have enough mass to signi
Figure 6. The left bundle branch splits
into anterior and posterior fascicles

Other types of block

Bifascicular block involves both right bundle branch block and the blockade of one of the fascicles of the left bundle
branch.
Trifascicular block is present when a 3rd-degree heart block exists alongside bifascicular block.

Key points

Consider bundle branch block in an ECG trace with broad complexes.
A broad complex tachycardia requires di
concurrent bundle branch block.
Left bundle branch block is always pathological and indicates signi
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.

References

Burns, R. B. (2021, December 10). L e f t B u n d l e B r a n c h B l o c k . Available from\: [LINK]
Buttner, E. B. (2021, March 5). R i g h t b u n d l e b r a n c h b l o c k . Available from\: [LINK]
Hampton, J. (2013). E C G M a d e E a s y .

Reviewer

Dr Matt Jackson
Consultant Cardiologist
Dr Ben Marrow
Cardiology RegistrarRelated notes
Acute Coronary Syndrome (ACS)
Acute Heart Failure
Atrial Fibrillation (AF)
Atrioventricular Block
Brugada Syndrome