Bundle Branch Blocks

Definition | Aetiology | Pathophysiology | Risk Factors | Signs and Symptoms | Investigations | Diagnosis | Management

Definition

A bundle branch block (BBB) occurs when there is an obstruction or blockage in the electrical impulses traveling through the ventricles, which are the lower chambers of the heart. This blockage causes the electrical signal to move more slowly than it should, leading to delayed and altered depolarization of the ventricles. There are two types of BBB:

• Left Bundle Branch Block (LBBB)
• Right Bundle Branch Block (RBBB)

Aetiology

The heart's electrical impulses initiate from the sinoatrial (SA) node and spread through the atria to the ventricles via the bundle branches. Damage to either the right or left bundle branch can disrupt this signal, potentially leading to a bundle branch block. Below is an image (Image 1) illustrating how the heart's electrical signal propagates:

Image 1: This image shows the heart's conduction system, starting from the sinoatrial (SA) node, which is the natural pacemaker of the heart. The electrical impulses travel from the SA node through the atria to the atrioventricular (AV) node, then down the bundle of His, and finally through the right and left bundle branches that stimulate the ventricles. When one of these bundle branches is blocked, the electrical signal is delayed, leading to a bundle branch block.

Common causes of bundle branch block include:

• Left Bundle Branch Block (LBBB):
  • Cardiomyopathy
  • Hypertension
  • Myocarditis
  • Myocardial Infarction (MI)
• Right Bundle Branch Block (RBBB):
  • Myocarditis
  • Myocardial Infarction (MI)
  • Congenital Heart Defects
  • Pulmonary Hypertension
  • Pulmonary Embolism (PE)

Pathophysiology

Bundle branch block occurs when one of the bundle branches (right or left) is damaged and unable to properly conduct electrical impulses. This damage may result from underlying heart disease, a myocardial infarction, or heart surgery. As a consequence:

• The mechanism of ventricular depolarization is altered.
• Electrical impulses are delayed and may follow an abnormal pathway through the ventricles, which can lead to an elongated QRS complex on the ECG.
• This slowed or altered conduction can impact the efficiency of the heart's pumping action.

Risk Factors

Several factors can increase the risk of developing a bundle branch block:

• Age: The risk increases with age as the heart undergoes degenerative changes.
• Underlying Health Conditions: Conditions such as hypertension, coronary artery disease, and cardiomyopathy are significant risk factors.

Signs and Symptoms

Bundle branch block is often asymptomatic and may only be detected during an ECG. However, some individuals may experience symptoms, including:

• Breathlessness: Due to decreased cardiac efficiency.
• Bradycardia: Slowed heart rate, especially in severe cases.
• Abnormal Heartbeat: Palpitations or irregular heart rhythms.
• Syncope or Presyncope: Fainting or feeling faint, particularly if the block affects both bundle branches.

Investigations

Diagnostic investigations for bundle branch block include:

• ECG: The primary tool for diagnosing bundle branch block by identifying characteristic changes in the QRS complex.
• Echocardiogram: To assess underlying structural heart disease that may be contributing to the block.
• Blood Tests: To evaluate for contributing factors such as electrolyte imbalances or myocardial infarction.

Diagnosis

Bundle branch block is diagnosed primarily through ECG findings:

Right Bundle Branch Block (RBBB)

An RBBB is indicated on an ECG by the following features:

• QRS complex duration >120 ms.
• Terminal R wave in lead V1, creating an "M" shape.
• Slurred S wave in lead I.

The mnemonic MaRRoW can be used to remember these features: the QRS complex in V1 resembles an "M" (first letter of MaRRoW), and in V6, it resembles a "W" (last letter of MaRRoW). Below is an ECG image illustrating these changes:

Image 2: This ECG shows the characteristic features of Right Bundle Branch Block (RBBB). Notice the "M" shape in lead V1 and the "W" shape in lead V6, which correspond to the delayed depolarization of the right ventricle.

Left Bundle Branch Block (LBBB)

An LBBB is indicated on an ECG by the following features:

• QRS complex duration >120 ms.
• Dominant S wave in V1.
• Broad, monophasic R wave in leads I, aVL, V5-V6.
• Absence of Q waves in lateral leads.
• Prolonged R wave peak time in V5-V6.

The mnemonic WiLliaM can be used to remember these features: the QRS complex in V1 resembles a "W" (first letter of WiLliaM), and in V6, it resembles an "M" (last letter of WiLliaM). Below is an ECG image illustrating these changes:

Image 3: This ECG shows the characteristic features of Left Bundle Branch Block (LBBB). Notice the "W" shape in lead V1 and the "M" shape in lead V6, which indicate the delayed depolarization of the left ventricle.

Management

The management of bundle branch block depends on the presence and severity of symptoms:

• Asymptomatic Patients: No specific treatment is required, but regular monitoring may be advised.
• Symptomatic Patients: A pacemaker may be recommended if the bundle branch block causes significant symptoms or is associated with heart block, to maintain proper heart rhythm and prevent complications.
• Address Underlying Conditions: Treatment of underlying conditions such as hypertension, coronary artery disease, or myocardial infarction is crucial in managing bundle branch blocks.

References

1. National Institute for Health and Care Excellence (NICE) (2024) Guidelines for the Management of Bundle Branch Blocks. Available at: https://www.nice.org.uk/guidance/ng187 (Accessed: 26 August 2024).
2. British Heart Foundation (2024) Understanding Bundle Branch Blocks. Available at: https://www.bhf.org.uk (Accessed: 26 August 2024).
3. British National Formulary (BNF) (2024) Cardiovascular System: Bundle Branch Blocks. Available at: https://bnf.nice.org.uk/ (Accessed: 26 August 2024).

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