Conduction Blocks and Cardiac Pacing

and Alwyn Scott2

School of Computer Science, University of Manchester, Manchester, UK

Cardiology High Dependency Unit, Papworth Hospital NHS Foundation Trust, Cambridge, UK


Heart blockAV blockSA blockPacemakersPacingBundle branch blocks


Any part of the conduction system can be blocked preventing or delaying impulses reaching subsequent parts of the conduction system. These blocks should be classified as conduction delays or blocks, not as arrhythmias even though they can cause the appearance of rhythm irregularities.

Bundle Branch Blocks

A bundle branch block refers to the blocking of the electrical impulse down one of the bundle branches (Fig. 5.1), referred to as a left bundle branch block (LBBB) or a right bundle branch block (RBBB).


Fig. 5.1
The right and left bundle branches

Disease or damage of the conduction system can result in a bundle branch block. Table 5.1 shows some of the common causes for bundle branch blocks.

Table 5.1
Causes of bundle branch blocks

Ischemic heart disease

Myocardial infarction


Fibrosis of the conduction system


Pulmonary embolism

Atrial septal defect

Other congenital heart disease

Normal variant

The salient feature seen in both left and right bundle branch blocks are the presence of widened QRS complexes in all leads (a QRS duration of >0.10 s/2.5 small squares). With this in mind it is also necessary to consider and rule out other causes of QRS prolongation, including; ventricular paced rhythms, idioventricular rhythm, broad complex tachycardia and premature ventricular beats.

Right Bundle Branch Blocks

A RBBB (Fig. 5.2) is often seen in older patients and does not require treatment. The best leads to use for the identification of a RBBB are V1 and V6. The QRS duration needs to exceed 120 ms/0.12 s. Normal septal depolarization occurs, followed by depolarization of the left bundle branch and then the right; due to the block the impulse then travels through the slower myocardial tissue, leading to an increase in QRS duration. Repolarization is affected by the abnormal depolarization, so the T wave is deflected in the opposite direction to the terminal portion of the QRS complex. Table 5.2 shows the changes seen in leads V1 and V6 forming the rsR pattern in V1 and qRS pattern in V6.


Fig. 5.2
A right bundle branch block

Table 5.2
Changes seen in leads V1 and V6 in the presence of a RBBB



Small r wave

Small q wave

s wave

Tall R wave

Secondary R wave

Deep S wave

T wave deflected in opposite direction of the terminal portion of the QRS complex

T wave deflected in opposite direction of the terminal portion of the QRS complex

Incomplete Right Bundle Branch Block

When the morphological changes for a RBBB exist and the QRS duration is >100 ms but <120 ms, it is referred to as an incomplete right bundle branch block.

Left Bundle Branch Blocks

Left bundle branch blocks (Fig. 5.3) are not often seen in healthy individuals and are often associated with chronic coronary heart disease. In a LBBB the septal depolarization takes place from the right to the left. Left ventricular depolarisation is carried out through the slower myocardial tissue increasing the QRS duration. As with a RBBB leads V1 and V6 are the best leads to view for changes associated with a LBBB (Table 5.3).


Fig. 5.3
A left bundle branch block

Table 5.3
Changes seen in leads V1 and V6 in the presence of a LBBB



QS or rS pattern

No q waves

Small r wave in around 30 % of cases

Notched R wave

Positive notch present following Q wave

Negative notch following R wave

T wave deflected in opposite direction to the QRS complex

T wave deflected in opposite direction to the QRS complex

Patients with advanced disease of the conduction system and coronary heart disease often present with a LBBB and an axis deviation. These changes are associated with increased mortality.

If a patient presents with chest pain and new LBBB they should be treated as a medical emergency and undergo cardiac catheterisation or thrombolysis according to local policy. As a LBBB has associated ST elevation and Q waves, the ECG becomes a less reliable indicator of myocardial infarction.

Sgarbossa’s criteria is a point based system to help identify the possibility of a STEMI in patients with a LBBB (Table 5.4). A score of three or more suggests the presence of a STEMI. The use of serial ECGs and the examination of primary T wave changes are generally more sensitive than the criteria.

Table 5.4
Sgarbossa’s criteria for detecting acute myocardial infarction in the presence of LBBB



ST elevation ≥1 mm in a lead with positively deflected QRS complexes


ST elevation ≥5 mm in a lead with negatively deflected QRS complexes


ST depression ≥1 mm in lead V1/V2/V3


Incomplete Left Bundle Branch Block

An incomplete left bundle branch block has the same morphology as a LBBB with the exception of the QRS duration, which is less than 120 ms/0.12 s.

Rate Dependent/Transient Bundle Branch Blocks

It is also possible for a bundle branch block to appear intermittently. Infection, cardiac catheterization, MI and heart failure are known causative factors. There is also a form of rate dependant bundle branch block that is caused when a rapid heart rate does not allow the bundle branch time to recover and is still in a refractory state, preventing the next impulse passing through the bundle branch.

Features and Criteria for Identifying Left and Right Bundle Branch Blocks

There are several features that help practitioners tell the difference between a left and right bundle branch block as summarised in Table 5.5.

Table 5.5
Features of left and right bundle branch blocks



QRS negatively deflected in V1

QRS positively deflected in V1

Widened QRS complexes in all leads

Widened QRS complexes in all leads

QS pattern in V1

rsR pattern in V1

T-wave inversion in V6

qRS pattern in V6

ST-segment elevation (sometimes) seen in V1–V4

T-wave inversion in V6

William Morrow/William Marrow

Is a mnemonic often taught to help practitioners remember the key features of left and right bundle branch blocks. Essentially the first and last letter show the deflection of the QRS complex in leads V1 and V6 respectively with ‘W’ being negatively deflected and ‘M’ being positively defected. The ‘LL’ in WilLLam = LBBB, the ‘RR’ in MaRRow or MoRRow = RBBB. Table 5.6 shows an example of this.

Table 5.6
The mnemonic William Marrow/Morrow

QRS deflection in V 1
Left/right block
QRS deflection in V 6














If a patient presents with a RBBB and left or right axis deviation, one of the fascicles of the left bundle branch is also blocked. A block in the anterior or posterior fascicles of the left bundle branch is termed a hemiblock. The direction of axis deviation tells the practitioner if the block is in the left anterior or posterior fascicle, as demonstrated in Table 5.7. When a RBBB exists with a hemiblock, it is termed a bifascicular block (Fig. 5.4). A LBBB is a form of bifascicular block as both fascicles of the left bundle are blocked.

Table 5.7
Left anterior and posterior hemiblocks (with additional features)

Left anterior hemiblock

Left posterior hemiblock

RBBB + left axis

RBBB + right axis

 Left axis deviation ≥ −40°

 Right axis deviation ≥ 120°

 qR pattern in lead I

 rS pattern in lead I

 rS pattern in lead II

 qR pattern in lead II (without right ventricular hypertrophy criteria)


Fig. 5.4
Example of a bifascicular block (RBBB + right axis deviation = left posterior hemiblock)

Trifascicular Block

Trifascicular block refers to a blockage of all three fascicles and is a bilateral bundle branch block. There are two types of trifascicular block, complete or incomplete. A complete trifascicular block is caused by a RBBB that leads to 3rd degree AV nodal block or ventricular standstill. AV nodal blocks are discussed later in the chapter in the section about AV blocks. An incomplete trifascicular block (Fig. 5.5) however is detected by the presence of a bifascicular block with a 1st degree AV nodal block (also discussed later in the chapter).


Fig. 5.5
An incomplete trifascicular block (RBBB + right axis deviation + 1st degree AV block)

Atrioventricular Nodal Blocks

AV nodal blocks are blocks that inhibit the ability of the AV node to conduct an impulse to the subsequent conduction system. There are three main types of AV block, known as; 1st, 2nd and 3rd degree AV nodal block. These blocks can either be permanent or temporary. AV blocks are best seen by examining the ECGs rhythm strip.

1st Degree AV Block

This form of block does not normally require treatment, it is identified by a delay between atrial depolarization and ventricular depolarisation, seen on the ECG as an increase in the PR interval >200 ms/0.20s (5 small squares) (Fig. 5.6). Although the PR interval is increased, the increase is the same for each beat (Fig. 5.7). Each P wave is followed by a QRS complex and the rhythm is usually regular. Causes of 1st degree AV block are summarized in Table 5.8.


Fig. 5.6
A 12-lead ECG showing 1st degree AV block (prolonged PR interval best seen in rhythm srip, lead V1)

May 29, 2017 | Posted by in CARDIOLOGY | Comments Off on Conduction Blocks and Cardiac Pacing
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