What causes lbbb and rbbb

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Last updated: April 4, 2026

Quick Answer: Bundle branch blocks (LBBB and RBBB) are caused by a delay or blockage in the electrical pathways that control the heart's pumping action. This disruption prevents the heart's lower chambers (ventricles) from contracting in a coordinated manner, often due to underlying heart conditions, age-related changes, or sometimes without a clear cause.

Key Facts

LBBB and RBBB refer to blockages in the left or right bundle branches, respectively, which are part of the heart's electrical system.
These blocks can lead to a widened QRS complex on an electrocardiogram (ECG).
While some individuals with bundle branch blocks have no symptoms, others may experience shortness of breath or dizziness.
Underlying heart conditions like coronary artery disease, heart failure, and hypertension are common causes.
Bundle branch blocks are more common in older adults and can sometimes occur without any identifiable cause.

What are Bundle Branch Blocks (LBBB and RBBB)?

Bundle branch blocks (BBB), specifically Left Bundle Branch Block (LBBB) and Right Bundle Branch Block (RBBB), are conditions that affect the heart's electrical conduction system. The heart's rhythmic beating is controlled by electrical impulses that travel through specialized pathways. These pathways ensure that the heart's chambers contract in a coordinated sequence, allowing for efficient blood pumping. The key pathways responsible for transmitting these electrical signals to the ventricles (the lower chambers of the heart) are called the left and right bundle branches.

When there is a delay or complete blockage in one of these bundle branches, the electrical impulse cannot travel normally down that pathway. This results in the ventricles not contracting simultaneously. In LBBB, the left bundle branch is affected, meaning the left ventricle receives the electrical signal later than the right ventricle. Conversely, in RBBB, the right bundle branch is affected, leading to a delayed contraction of the right ventricle.

What Causes Left Bundle Branch Block (LBBB)?

LBBB is often associated with more significant underlying heart disease compared to RBBB. The primary causes of LBBB include:

Coronary Artery Disease (CAD): Blockages in the heart's arteries can damage the heart muscle and its electrical system, leading to LBBB.
Hypertension (High Blood Pressure): Long-standing high blood pressure can cause the heart muscle to thicken (hypertrophy) and strain, eventually affecting the conduction system.
Heart Failure: When the heart cannot pump blood effectively, it can lead to changes in the heart's structure and electrical pathways, predisposing it to LBBB.
Aortic Stenosis: Narrowing of the aortic valve can put extra strain on the left ventricle, contributing to LBBB.
Cardiomyopathy: Diseases of the heart muscle itself can disrupt the electrical signals.
Myocarditis: Inflammation of the heart muscle can also affect the bundle branches.
Age-Related Changes: The electrical system of the heart can degenerate with age, increasing the risk of LBBB.
Idiopathic: In some cases, especially in older individuals, LBBB can occur without any identifiable underlying heart condition. This is sometimes referred to as "age-related" or "degenerative" LBBB.

What Causes Right Bundle Branch Block (RBBB)?

RBBB is generally considered less serious than LBBB and is often found incidentally. Common causes include:

Congenital Heart Defects: Some individuals are born with structural abnormalities in the heart that affect the right bundle branch. Atrial septal defects (ASD) are a common example.
Pulmonary Embolism: A blood clot in the lungs can put increased pressure on the right side of the heart, potentially leading to RBBB.
Chronic Lung Disease: Conditions like Chronic Obstructive Pulmonary Disease (COPD) can strain the right ventricle, causing RBBB.
Right Ventricular Hypertrophy: Enlargement of the right ventricle due to various causes can affect the right bundle branch.
Coronary Artery Disease (CAD): While more commonly associated with LBBB, CAD can also affect the right bundle branch.
Myocarditis or Pericarditis: Inflammation of the heart muscle or the sac surrounding the heart can impact the electrical pathways.
Heart Surgery: Damage to the conduction system during cardiac surgery can sometimes result in RBBB.
Idiopathic: Similar to LBBB, RBBB can also occur without any apparent cause, particularly in younger individuals. It might be a normal variant in some people.

Diagnosis and Implications

Bundle branch blocks are typically diagnosed through an electrocardiogram (ECG or EKG). The ECG records the electrical activity of the heart, and the characteristic pattern of a BBB is seen in the widening of the QRS complex. Depending on the type of BBB and the presence of other symptoms or risk factors, further investigations such as echocardiograms, stress tests, or cardiac catheterization may be recommended to assess for underlying heart disease.

Many individuals with bundle branch blocks, especially RBBB, live normal lives without any symptoms. However, LBBB, particularly when newly diagnosed, can be a sign of significant heart disease and may warrant closer medical attention and management of underlying conditions. Symptoms, if present, can include fatigue, dizziness, fainting (syncope), or shortness of breath, particularly during exertion.