(See also Overview of Abnormal Heart Rhythms Overview of Abnormal Heart Rhythms Abnormal heart rhythms (arrhythmias) are sequences of heartbeats that are irregular, too fast, too slow, or conducted via an abnormal electrical pathway through the heart. Heart disorders are... read more 
and Overview of Heart Block Overview of Heart Block Heart block is a delay in the conduction of electrical current as it passes through the conduction system of the heart, including the atrioventricular node, bundle of His, or both bundle branches... read more .)
The electrical current that controls the contraction of heart muscle starts in the sinoatrial node, flows through the heart's upper chambers (atria) and then goes to the heart's lower chambers (ventricles) through a sort of electrical junction box called the atrioventricular node (AV node). The AV node is located in the lower part of the wall between the atria near the ventricles and provides the only electrical connection between the atria and ventricles. The bundle of His is a group of fibers that conducts electrical impulses from the atrioventricular node to the lower chambers of the heart (the ventricles). The bundle of His divides into two bundle branches. The left bundle branch conducts impulses to the left ventricle, and the right bundle branch conducts impulses to the right ventricle. Conduction may be blocked in the left or right bundle branch.
The left bundle branch further divides into two branches, called the anterior and posterior fascicles. If conduction is blocked through one of these fascicles, the blockage is called a hemiblock or a fascicular block.
Tracing the Heart’s Electrical Pathway
The sinoatrial (sinus) node (1) initiates an electrical impulse that flows through the right and left atria (2), making them contract. When the electrical impulse reaches the atrioventricular node (3), it is delayed slightly. The impulse then travels down the bundle of His (4), which divides into the right bundle branch for the right ventricle (5) and the left bundle branch for the left ventricle (5). The impulse then spreads through the ventricles, making them contract. |
Bundle branch block usually causes no symptoms. Right bundle branch block is not serious in itself and may occur in apparently healthy people. However, it may also indicate significant heart damage due to, for example, a previous heart attack Acute Coronary Syndromes (Heart Attack; Myocardial Infarction; Unstable Angina) Acute coronary syndromes result from a sudden blockage in a coronary artery. This blockage causes unstable angina or heart attack (myocardial infarction), depending on the location and amount... read more 
.
Left bundle branch block tends to be more serious. In older people, it often indicates coronary artery disease Overview of Coronary Artery Disease (CAD) Coronary artery disease is a condition in which the blood supply to the heart muscle is partially or completely blocked. The heart muscle needs a constant supply of oxygen-rich blood. The coronary... read more 
due to atherosclerosis Atherosclerosis Atherosclerosis is a condition in which patchy deposits of fatty material (atheromas or atherosclerotic plaques) develop in the walls of medium-sized and large arteries, leading to reduced or... read more 
.
Bundle branch block can be detected by electrocardiography Electrocardiography Electrocardiography (ECG) is a quick, simple, painless procedure in which the heart’s electrical impulses are amplified and recorded. This record, the electrocardiogram (also known as an ECG)... read more 
(ECG). Each type of block produces a characteristic pattern.
Usually, no treatment is needed.
More Information
The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
American Heart Association: Arrhythmia: Information to help people understand their risks of arrhythmias as well as information on diagnosis and treatment
