A bifascicular block (BFB) is an electrical conduction problem in the heart, signaling a delay or blockage in the transmission of electrical signals to the lower chambers (ventricles). This condition involves a defect in two of the three main pathways, or fascicles, that carry the electrical impulse through the ventricles. A blockage on these pathways causes the signal to be rerouted, which slows down the coordinated muscle contraction necessary for the heart to pump blood effectively. A bifascicular block is often discovered incidentally during a routine examination and does not always present with symptoms.
The Heart’s Electrical Wiring
The heart’s rhythm is controlled by an electrical impulse that begins in the upper chambers and travels downward through a specialized system of fibers. After moving through the atrioventricular (AV) node, the signal enters the Bundle of His, which quickly divides to send the impulse to the left and right ventricles. The Right Bundle Branch (RBB) carries the impulse to the right ventricle, while the Left Bundle Branch (LBB) targets the left ventricle.
The LBB further splits into two subdivisions: the Left Anterior Fascicle (LAF) and the Left Posterior Fascicle (LPF). This creates a three-pathway system for the electrical signal to reach the ventricles: the RBB, the LAF, and the LPF. A bifascicular block is defined as a failure in two of these three pathways, typically a block in the RBB combined with a block in either the LAF or the LPF.
When two paths are blocked, the electrical signal must rely on the one remaining open pathway to reach the entire ventricular muscle. This rerouting causes a measurable delay in the electrical signal. The delayed signal propagation leads to an uncoordinated contraction of the ventricles, which can be visualized on a heart rhythm test.
Underlying Causes and Risk Factors
Damage to the heart’s electrical wiring that results in a bifascicular block often stems from underlying structural heart disease or age-related changes. The most common cause is a degenerative disease of the conduction system, where normal aging leads to fibrosis, or scarring, of the electrical tissue. This fibrotic change can slowly disrupt the pathways over time, a process sometimes called Lenègre-Lev disease.
Ischemic heart disease, which occurs when blood flow to the heart muscle is reduced, is another frequent cause. Damage from a previous heart attack, particularly one affecting the wall that separates the ventricles, can destroy the electrical fibers. Chronic conditions like hypertension (high blood pressure) and aortic valve disease also increase the risk by causing strain and structural changes within the heart muscle.
Other causes include congenital heart defects or inflammatory conditions like myocarditis. Electrolyte imbalances, such as high potassium levels, can temporarily interfere with the electrical signal transmission. Treating these underlying conditions is crucial for managing the overall risk associated with the block.
How Bifascicular Block is Detected
A bifascicular block is most often discovered when a person has an electrocardiogram (EKG) for an unrelated reason, as the condition is frequently asymptomatic. The EKG is the specific test that identifies the block by measuring the heart’s electrical activity. It visualizes the delayed and rerouted electrical signal as characteristic patterns on the tracing.
Diagnosis requires a combination of two specific electrical patterns. The first pattern is a Right Bundle Branch Block (RBBB), which shows a widening of the QRS complex, the part of the tracing that represents ventricular activation. This RBBB pattern signifies the blockage of the impulse traveling down the right branch.
The second pattern involves a deviation in the electrical axis of the heart, which shows where the overall electrical flow is directed. If the RBBB is combined with a Left Anterior Fascicular Block (LAFB), the signal is forced through the unblocked Left Posterior Fascicle, resulting in a Left Axis Deviation (LAD). This RBBB-LAFB combination is the most common presentation of a bifascicular block.
A combination of RBBB with a Left Posterior Fascicular Block (LPFB) is less frequent and causes a Right Axis Deviation (RAD). This combination is less common because the LPF has a dual blood supply, making it more resistant to damage. The axis deviation occurs because the signal travels through the remaining healthy fascicle first, causing the bulk of the electrical activity to shift.
Treatment Approaches and Long-Term Outlook
For many people, an isolated bifascicular block that causes no symptoms requires no immediate intervention beyond regular monitoring. The focus for these asymptomatic individuals is treating any underlying conditions, such as managing high blood pressure or addressing coronary artery disease. Periodic follow-up EKGs allow doctors to watch for signs that the blockage is worsening over time.
The main concern is the risk of progression to a complete heart block, meaning all three electrical pathways become blocked. This progression can lead to a slow heart rate, causing symptoms like fainting (syncope) or extreme dizziness. The rate of progression is estimated at about one to four percent per year for those without symptoms.
A permanent pacemaker is the standard treatment when the bifascicular block becomes symptomatic. The pacemaker provides a reliable electrical backup, ensuring the heart maintains a steady rhythm even if the third pathway fails. A pacemaker is recommended for symptomatic patients who show evidence of severe conduction disturbance.
The prognosis for people with an asymptomatic bifascicular block is generally favorable, and the condition often does not affect life expectancy on its own. However, the presence of the block can indicate more extensive underlying heart disease. The long-term outlook is closely tied to the severity of associated cardiac conditions, which must be carefully managed.