Bundle branch block (BBB) is a condition in which the electrical impulses traveling through the heart are slowed or blocked completely from traveling along their normal route through the ventricles. The condition tends to produce no symptoms, although a complete heart block can cause a person to feel either faint (syncope) or as if they are going to faint (presyncope). In those circumstances, the heartbeat may be so slow that an artificial pacemaker is advisable, even if the heart is otherwise healthy.

BBB is relatively rare among young people, and among older people, it is usually caused by an underlying heart condition, such as damage to the heart muscle from a heart attack, or it may occur in conjunction with another condition (e.g., heart failure). Therefore, tests will be ordered to find out the cause of the bundle branch block. A treatment plan will be designed based on what those tests find. In many cases, people will not need treatment for BBB.

Bundle branch block (BBB) is an abnormality in a very specific area of the heart that can result in a slower heart rhythm (bradycardia). Normally, electrical impulses travel from the sinoatrial node in the upper-right chamber of the heart (the right atrium) to the atrioventricular (AV) node and then to the bundle of His. The electrical impulses then travel down both the left and right bundles to stimulate both the left and right ventricles. As a result, the heartbeat is coordinated between the upper and lower chambers, as well as the right and left sides of the heart.

BBB arises when there is a defect in either of the bundle branches as they branch off from the bundle of His. Unless defects have blocked transmission completely, the electrical impulses will take a less direct route (or a longer period of time) in order to reach the ventricles.

Depending on where the defect(s) may be located, a person may be diagnosed with one of several specific types of bundle branch block. They include:

• Left bundle branch block (LBBB). Defects are present in both the front (anterior) and back (posterior) divisions (fascicles) of the left bundle branch. If only the front division is involved, then people will be diagnosed with an anterior fascicular block (also known as an anterior divisional block or left anterior hemiblock). If only the back division is involved, then people will be diagnosed with a posterior fascicular block (also known as a posterior divisional block or left posterior hemiblock). LBBB affects the pumping action of the left ventricle by affecting the timing of contraction of different walls of the left ventricle, which is the heart's main pumping chamber 
  
  
• Right bundle branch block (RBBB), in which the defect occurs in the right bundle branch. RBBB affects the timing of contraction of the right ventricle, which is responsible for pumping oxygen-poor blood to the lungs.
  
  
• Bifascicular block, in which defects occur in two of the three divisions of the bundle branches. More specifically, people with this condition may be diagnosed with “RBBB with left anterior hemiblock,” “RBBB with left posterior hemiblock” or LBBB.
  
  
• Trifascicular block, in which there is a form of bifascicular block (as described above) as well as a first-degree atrioventricular (AV) block (a condition in which the electrical impulses are slowed or blocked as they pass through the AV junction). This suggests a more advanced degree of conduction system disorder.

Because all forms of BBB involve delays or electrical blockages within the ventricles, they are all types of intraventricular conduction deficits. Bundle branch blocks may also be categorized as either partial (incomplete) or complete, depending on which patterns of electrical activity are detected on an electrocardiogram (EKG). BBB may be transient depending on the heart rate and the degree of exercise.

It is useful to distinguish between BBB and coronary blockage, as people may think they mean the same thing. A coronary blockage is an obstruction within the arteries that supply the heart muscle, often due to the buildup of plaque. This affects the delivery of oxygen-rich blood to the heart, and can increase the likelihood of triggering a heart attack. In contrast, a BBB simply relates to the flow of electricity through the heart. An obstruction along the heart’s conduction system could result in an abnormal heartbeat (arrhythmia).

Sometimes, bundle branch block (BBB) develops for no known reason. Other times, it may be due to a variety of causes, such as: 

The prevalence of BBB increases with age. In younger patients, BBB frequently exists with no symptoms or underlying heart disease. Among older patients, BBB is more likely to exist concurrently with underlying heart disease, and the long-term outlook for these patients is worse than for similar heart patients without BBB. Among younger patients with BBB, physicians often look for other cardiac conditions, such as coronary heart disease and cardiomyopathy, that may be in the early developing stages.

Diagnosis of a bundle branch block (BBB) begins with a complete medical history and physical examination. A physician will also order a painless test called an electrocardiogram (EKG), which measures the heart’s electrical activity. Once a clear diagnosis of BBB has been made, the physician will try to determine what is causing it.

Tests that may be ordered to determine the cause of bundle branch block include:

• Echocardiogram. This test uses sound waves to visualize the structures and functions of the heart. A moving image of the patient’s beating heart is played on a video screen, where a physician can study the heart muscle’s thickness, size and function. All four of the heart’s chambers can be assessed and measured. The image also shows the motion pattern and structure of the four valves, revealing any potential leakage (regurgitation) or narrowing (stenosis). During this test, a Doppler ultrasound may be done to evaluate cardiac blood flow and to assess the presence and/or severity of any valvular heart disease. This test may also include a stress echocardiogram, which combines a standard echocardiogram with a stress test to evaluate heart function during exercise.
  
  
  
  
  
• Nuclear stress test. A test that includes all the procedures performed during a standard exercise stress test, as well as producing clear pictures of heart muscle on a video monitor. The physician is not only able to compare the heart’s electrical activity at rest and under exertion, but can also determine which parts of the heart are healthy and functioning, and which are not. The test involves the administration of a radionuclide substance called a tracer. Both nuclear and echocardiography stress testing can help in determining whether these abnormalities are related to the presence and severity of coronary artery disease.
  
  
• Electrophysiology study. An invasive test that can be useful in assessing the severity of electrical disturbances that can indicate the presence of a bundle branch block. In some cases preventative implantation of a pacemaker is recommended if the risk of progression to a complete heart block is very high.

In addition to treating any underlying heart disease that may be present, treatment for bundle branch block (BBB) is generally not necessary unless the heart rate is too slow and fainting is present. In those circumstances, physicians may recommend surgery to implant an artificial pacemaker. Whether or not treatment is advised, people with BBB are encouraged to maintain regular contact with their physician to monitor for any unexpected cardiac changes.

A temporary pacemaker may also be necessary for certain cardiac procedures, such as cardiac catheterization. In this procedure, a long, thin catheter is guided into the heart to perform tests or other procedures. Patients with BBB are at risk for complete heart block in these circumstances, and temporary pacing is often recommended.

The best strategy for preventing BBB is to take preventive steps against the development of heart disease.

In patients who suffer from congestive heart failure due to a weakened heart muscle, the presence of a bundle branch block can further worsen the heart function. It is possible to improve the heart function by implantation of a cardiac resynchronising pacemaker. This works by correcting the abnormality in the timing of the contraction of the walls of the left ventricle. This preventative measure can relieve the symptoms of congestive heart failure.

Preparing questions in advance can help patients have more meaningful discussions with their doctor. Patients may wish to ask their physicians the following questions related to bundle branch block (BBB):

1. Can I still have BBB even if I feel no symptoms?
   
   
2. Are there any tests available that can confirm a diagnosis of BBB?
   
   
3. Is my BBB considered to be partial or complete? What is the difference?
   
   
4. Which specific type of BBB do I have? What is my BBB different from other types?
   
   
5. Does my BBB occur in conjunction with any other condition? Which condition?
   
   
6. Are there any medications I can take to improve my condition?
   
   
7. Will my condition require me to wear an implantable pacemaker?
   
   
8. How urgently do I need to receive treatment for my BBB? What will happen to me if I delay treatment?
   
   
9. Is it safe to become pregnant if I have a BBB? Could a BBB threaten my pregnancy in any way?