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).