Bundle Branch Block

How the heart's wiring works

Each heartbeat starts in the sinus node in the upper right chamber. The signal passes through the AV node and then down the bundle of His, which splits into the right bundle branch (to the right ventricle) and the left bundle branch (to the larger left ventricle). Normally both ventricles are activated almost simultaneously, producing a narrow QRS complex on the ECG.

When one branch is blocked, the signal travels slowly through heart muscle instead, so that ventricle activates late. The ECG shows a wide QRS (120 milliseconds or more) with a characteristic shape.

Right vs left bundle branch block

Incomplete bundle branch block (QRS 110 to 119 ms) is a milder delay and is often normal, especially incomplete RBBB in young athletes.

Bifascicular block combines RBBB with a block of one of the two divisions of the left bundle. With a lengthened PR interval or fainting, it can signal risk of complete heart block.

Causes

Right bundle branch block

• no identifiable cause in many healthy people
• chronic lung disease and pulmonary hypertension
• pulmonary embolism, which can cause new RBBB
• congenital heart disease, such as atrial septal defect
• heart surgery or catheter procedures
• Brugada syndrome, an inherited rhythm disorder with an RBBB-like pattern

Left bundle branch block

• long-standing high blood pressure
• coronary artery disease and previous heart attack
• cardiomyopathy and heart failure
• aortic valve disease and after valve replacement (TAVI)
• age-related fibrosis of the conduction system
• myocarditis and infiltrative diseases such as sarcoidosis

Symptoms

Bundle branch block itself rarely causes symptoms. When symptoms occur, they usually come from the underlying heart disease or from more advanced conduction problems:

• fainting or near-fainting (syncope)
• dizziness or a very slow heartbeat
• breathlessness and fatigue from heart failure
• chest pain from coronary disease

Diagnosis

• ECG: confirms the type of block.
• Echocardiogram: checks heart structure and pumping function, recommended for LBBB and for RBBB with symptoms or other abnormalities.
• Holter or longer-term rhythm monitoring if there is fainting or dizziness.
• Stress testing or coronary imaging for possible coronary disease. LBBB makes standard exercise ECGs hard to interpret, so imaging-based stress tests are used.
• Electrophysiology study in selected people with unexplained fainting.

A new LBBB with chest pain may indicate a heart attack, and emergency teams treat it with high suspicion.

Treatment

• No treatment is needed for isolated bundle branch block without symptoms.
• Treat the underlying condition: blood pressure control, coronary disease treatment, heart failure medicines, valve repair.
• Pacemaker: if bundle branch block is accompanied by fainting from intermittent complete heart block, or alternating RBBB and LBBB.
• Cardiac resynchronization therapy (CRT): a special pacemaker with a lead to each ventricle. For people with heart failure, reduced pumping (ejection fraction 35% or less) and LBBB with a wide QRS, it improves symptoms and survival.
• Conduction system pacing: newer techniques pace the heart's own conduction pathway and can correct some bundle branch blocks.

Outlook

Isolated RBBB in a healthy person usually has an excellent outlook. LBBB is associated with higher cardiovascular risk because of the conditions that cause it, but if the heart is otherwise normal on testing, the outlook is often good with periodic follow-up. Over years, LBBB can lead to a gradual weakening of the left ventricle in some people, so monitoring is sensible.

Frequently asked questions

Is bundle branch block serious?

Often not. Right bundle branch block is common and usually harmless in people without symptoms. Left bundle branch block is more often linked to heart disease such as high blood pressure, coronary disease or cardiomyopathy, so it needs evaluation. The seriousness depends on the cause.

What causes right bundle branch block?

RBBB can occur in healthy people with no cause found. It can also be caused by lung disease, pulmonary hypertension, a blood clot in the lung, congenital heart defects such as an atrial septal defect, or heart surgery.

What is the difference between RBBB and LBBB?

RBBB delays signals to the right ventricle and is often benign. LBBB delays signals to the left ventricle, which pumps blood to the body, and is more often linked to underlying heart disease and can affect pumping efficiency.

Does bundle branch block need a pacemaker?

Most people do not need a pacemaker. One is recommended if bundle branch block occurs with fainting caused by intermittent complete heart block, or alternating blocks. People with heart failure and LBBB may benefit from a resynchronization pacemaker.

Can you exercise with bundle branch block?

Most people can exercise normally, especially with RBBB and a structurally normal heart. If you have LBBB, symptoms such as fainting or chest pain, or underlying heart disease, ask your cardiologist which activities are safe.