A bundle branch block refers to a condition that arises from a blocked electrical pathway in the heart, causing the left and right sides of your heart to beat out of sync.

When a person has a “right” bundle branch block, the right side of the heart beats slightly later than the left side. Similarly, when a person has a “left” bundle branch block, the left side of the heart beats a bit behind the right side.

A right bundle branch block is less serious than a left bundle branch block, which can be a sign of an underlying heart condition, such as coronary artery disease. For instance, about 33% of patients with heart failure have a left bundle branch block.

A bundle branch block is likeliest to affect adults aged 60 and older, as well as those with high blood pressure and heart disease. The condition is also more common in men than in women.

For most people, a bundle branch block doesn’t cause or contribute to health problems. However, when the condition causes symptoms that require treatment, a pacemaker can help sync the heartbeat on both sides of the heart.

A bundle branch block is a heart condition caused by a change in the flow of electrical signals within the heart. Most people with this condition don’t experience any symptoms, so they don’t realize they have it. That is not usually a problem, since it typically doesn’t negatively affect a person’s health or quality of life.

In a healthy heart, electrical signals travel through the heart and make the heart muscle contract (or beat) with a normal rhythm. The electrical impulses follow a path that starts at the sinoatrial node, an area of the heart that acts as a natural pacemaker. The electrical impulses first flow to the chambers at the top of the heart (the left and right atria) before reaching the chambers at the bottom of the heart (the left and right ventricles). To reach these lower chambers, the heart’s electrical impulses move through a bundle of fibers like an electrical wire within the heart that splits in two, forming a left bundle branch and a right bundle branch.

In an individual with a bundle branch block, the fibers within the left or right bundle branch become damaged, affecting the ability of the electrical impulses to travel along both branches simultaneously. This can cause the left and right ventricles to contract or beat out of sync. In certain cases, this can lead to the heart pumping out less blood with each heartbeat, resulting in patients developing symptoms like shortness of breath.

Scar tissue, which can form after a heart attack or as a result of inflammation or an infection in the heart, may damage the left or right bundle branch, leading to an abnormal heart rhythm.

Different conditions can cause a bundle branch block, including:

• High blood pressure (hypertension)
• Heart disease
• Inflammation of the heart muscle (myocarditis)
• Heart-muscle disease (cardiomyopathy)
• Hardening of the arteries within the heart (arteriosclerosis)
• A blood clot that enters the lungs (pulmonary embolism)
• A congenital (or present at birth) heart abnormality

It’s important to know that in about half of cases, the exact cause of a bundle branch block is unknown.

Most people with a bundle branch block don’t experience any symptoms. For this reason, most don’t know they have the condition. However, for those who notice symptoms, they may experience:

• Chest pain
• Dizziness
• Fainting

People might be at increased risk of a bundle branch block if they:

• Are male
• Are age 60 or older
• Have hypertension
• Have heart disease
• Previously had a heart attack
• Previously had heart surgery
• Have an enlarged heart
• Have a hole in a heart wall that separates the heart’s chambers
• Have coronary artery disease
• Have myocarditis
• Have cardiomyopathy
• Have experienced arteriosclerosis
• Have had a pulmonary embolism
• Have had rheumatic fever
• Were born with a congenital heart abnormality
• Have high levels of potassium in the blood
• Take certain medications for heart-rhythm disorders (arrhythmia)

Sometimes, a bundle branch block is diagnosed incidentally if an individual without symptoms sees a doctor for another reason. For example, the condition might be discovered during a well visit or when a patient goes for an electrocardiogram or echocardiogram to identify a different condition.

Other times, a person might see their doctor to discuss symptoms, including chest pain and dizziness. A bundle branch block can be diagnosed after a doctor learns about a patient’s medical history and after the results of diagnostic tests have been reviewed. It isn’t possible to diagnose a bundle branch block during a physical examination.

During the medical history part of the visit, it’s important to tell your doctor if you have high blood pressure or heart disease. You should also mention if you’ve had a heart attack, a pulmonary embolism, or if you were born with a heart abnormality.

To rule out other conditions, blood tests, a chest X-ray, or an echocardiogram (an ultrasound of the heart) may be used.

A diagnosis of a bundle branch block can only be confirmed with an electrocardiogram. This test records the heart’s electrical activity and is used to discover heart-rhythm disorders. Different electrical activity patterns will be present if a patient has a left bundle branch block, a right bundle branch block, or a normal heartbeat.

Most people with no symptoms of a bundle branch block do not need treatment after being diagnosed. They should be able to continue living their lives normally.

When symptoms of a bundle branch block cause chest pain or fainting, doctors may recommend a pacemaker to restore a controlled flow of electrical activity within the heart. A pacemaker will be surgically implanted under the skin and near the heart, enabling electrical impulses to fire properly within the heart so that the left and right sides beat in unison.

People with a left bundle branch block who don’t need treatment for the condition upon diagnosis should continue to see their doctors regularly. A left bundle branch block can be a predictor of high blood pressure or other heart problems. If symptoms or complications develop, a patient may need a pacemaker.

People with an asymptomatic bundle branch block typically do not experience negative health effects. However, if treatment is necessary, a pacemaker should help to restore normal electrical signals within the heart, improving the heart rhythm.

“At Yale, we have innovative ways to treat patients with bundle branch blocks,” says Mark Blitzer, MD, a Yale Medicine cardiologist and electrophysiologist. “At times, traditional pacemakers are all that is required. More frequently, though, we will implant special types of pacemakers that can not only prevent the heart from going dangerously slow but also resynchronize the heart’s contraction pattern. We can do this by pacing both sides of the heart at the same time or by using a new technique, where we implant the pacemaker lead directly into the heart’s conduction system. In these cases, we can often make patients feels more energetic and actually make the heartbeat stronger again. At Yale, we are involved in research pushing this exciting new technology forward.”