HealthSheets™


Understanding Left Bundle Branch Block

Left bundle branch block is a problem in the heart’s electrical system. One of the 2 main wires in the heart is not functioning correctly. This results in the heart's main pumping chambers to squeeze out of sync. It doesn’t usually occur in young or healthy people. It’s more common in older adults and people with serious heart disease. It can worsen the heart’s ability to pump in people with heart failure.

What is left bundle branch block?

The heart uses electrical signals to keep pumping normally. With left bundle branch block, some signals are slowed when they pass through the heart. There is a problem with the left branch of the system that sends the electrical signal to the left ventricle. The signal can’t travel down this path the way it should. The signal still gets to the left ventricle, but it is slow compared with the right. Because of this, the left ventricle contracts a little later than it should. This can cause the heart to pump less efficiently. This condition may be intermittent but most of the time it is permanent.

What causes left bundle branch block?

Left bundle branch block occurs in some people with no know risk factors or heart problems. But, most of the time it is associated with some type of heart abnormality. They include:

  • Any disease of the heart’s electrical system that may be seen with advancing age

  • Severe disease of the arteries that send blood to the heart muscle (coronary artery disease)

  • Long-term high blood pressure (hypertension) with thickened heart muscle

  • Heart valve disease, such as aortic stenosis

  • Enlarged, weakened heart muscle (cardiomyopathy)

  • Infection of the heart muscle (myocarditis) from any cause

  • Heart attack (myocardial infarction or MI)

Symptoms of left bundle branch block

Left bundle branch block doesn’t often cause symptoms on its own. But in some cases you may have symptoms such as:

  • Dizziness

  • Fainting

  • Trouble breathing with physical activity

If you have heart failure, left bundle branch block can make symptoms worse.

Diagnosing left bundle branch block

Your healthcare provider will ask about your health history and symptoms. He or she will give you a physical exam. You may have tests such as:

  • Electrocardiography (ECG). Sticky pads are put on your chest. Wires are attached and connected to a machine. The machine checks your heart rhythm. The results of the ECG may show abnormal changes that mean left bundle branch block. You may have left bundle branch block diagnosed while having electrocardiography (ECG) for other reasons. Left bundle branch block may be the first sign of an undiagnosed heart problem.

  • Echocardiography (Echo). Sound waves are used to check the blood flow and movement of the heart.

  • Exercise stress testing. An ECG is done while you exercise. This test checks your heart under stress.

  • Cardiac CT or MRI. These tests help visualize the muscle layers and structures of the heart.

  • Blood tests. These are done to check fat (lipid) and sugar (glucose) levels in the blood.

You will likely be referred to a heart doctor (cardiologist).

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