What Is a Bundle Branch Block?
Electrical Conduction in the Heart
All of the cells of the heart have a negative charge and have a property called automaticity–the capability to spontaneously generate a rhythmic impulse of depolarization–an electrical change that can propagate to the next cell and begin depolarization there. In this way, an impulse can travel along a path.
When a cell depolarizes, it becomes refractory (silent), immune to a subsequent depolarization until it re-polarizes in preparation for the next depolarization. This allows a dominant (faster rhythmic) cell to lead the way in the depolarizations. In the heart, the dominant pace making cells are in the sinoatrial (SA) node.
From the SA node, the impulse causes the contraction of the atrium and travels to the junctional area between the atria and the ventricles. There, the atrioventricular (AV) node receives the impulse and sends it farther down a bundle of fibers–the Bundle of His–which separates into right and left main bundle branches. These themselves splinter into finer fibers that spread throughout the ventricles. Thus, a wave of depolarization travels from the SA node to the AV node, Bundle of His, right and left bundle branches, finally terminating in the Purkinje fibers.
The fact that the impulse has to travel, which takes time, means the wave of depolarizations traveling inferior and from right to left cause a sequential contraction of atria first and ventricles second; this generates a directional pumping action–out–for blood within the heart, one-way valves preventing back-flow.
A block at any point in the depolarization path can cause the directional pumping action of the heart to fail, the severity of which is based on where and how extensively the dissociation occurs. Such a failure results in an arrhythmia. Some failures are mild but others can significantly alter the cardiac output.
Blocks in the bundle branches are within the ventricles themselves, causing delays in the propagation of the wave of impulses. This is called “intraventricular conduction delay” (IVCD). The blocks can be total or partial (“fascicular”), accounting for a varied range of conduction and rhythm abnormalities associated with varying degrees of cardiac output impairment.
An ECG is a recording of the depolarizations and repolarizations as they travel through the heart. Electrodes placed on the skin around the heart allow different views of this wave from different directions. From this, a bundle branch block can be diagnosed, as well as can other electrical abnormalities in the heart.
Left Bundle Branch Block (LBBB)
LBBB occurs often indicates one of 4 conditions:
- Coronary artery disease
- Aortic valve disease
Right Bundle Branch Block (RBBB)
RBBB is typically due to:
- A congenital structural defect, such as an atrial septal defect
- Acquired heart disease, such as valvular disease or ischemia/myocardial infarction