A fascicular block represents a disruption in the electrical timing that coordinates the heart’s pumping action. It refers to a delay or complete blockage of the electrical impulse traveling through one of the specialized pathways within the left side of the heart. This condition involves the wiring system that controls every heartbeat, ensuring the heart chambers contract in a timed sequence. This type of block is often found incidentally during diagnostic testing but can signal a more serious underlying cardiac issue.
Understanding the Heart’s Electrical Wiring
The heart’s electrical activity begins with the sinoatrial (SA) node, the natural pacemaker located in the upper right chamber. This initial impulse travels through the atria to the atrioventricular (AV) node, which acts as a gatekeeper, introducing a brief delay before the signal continues. The electrical current then passes into the ventricles through the Bundle of His, which quickly splits into the Left and Right Bundle Branches.
The Right Bundle Branch carries the signal to the right ventricle, while the Left Bundle Branch divides almost immediately into two main subdivisions, known as fascicles. These two specialized pathways, the anterior fascicle and the posterior fascicle, distribute the electrical signal rapidly to the muscle tissue of the left ventricle. This entire system, known as the His-Purkinje network, ensures near-simultaneous contraction of the lower heart chambers.
The anterior fascicle directs the impulse toward the front and upper portion of the left ventricle. Conversely, the posterior fascicle conducts the impulse toward the back and lower portion of the left ventricle. This dual delivery system ensures the left ventricle, which pumps oxygenated blood to the entire body, contracts efficiently. A fault in this wiring can slow down or reroute the signal, which is what occurs in a fascicular block.
Defining Fascicular Block
A fascicular block, sometimes referred to as a hemiblock, occurs when the electrical impulse fails to travel normally down one of the two fascicles of the Left Bundle Branch. The blockage forces the electrical signal to detour around the affected pathway and travel through the remaining, intact fascicle to reach the heart muscle. This rerouting causes a measurable delay in the depolarization of the part of the ventricle supplied by the blocked fascicle.
Although the signal still reaches the entire left ventricle, the delayed arrival means the contraction of that section is out of sync with the rest of the heart muscle. This creates an alteration in the heart’s electrical pattern, most often detected incidentally during an electrocardiogram (ECG or EKG) evaluation. The characteristic sign of a fascicular block on an EKG is a shift in the heart’s electrical axis, rather than a significant widening of the overall electrical complex. The QRS complex, which represents ventricular depolarization, remains only slightly prolonged, typically not exceeding 0.11 seconds.
Identifying the Different Types
The type of fascicular block is categorized by the location of the conduction delay within the Left Bundle Branch. The Left Anterior Fascicular Block (LAFB) is the most frequently encountered type because the anterior fascicle is generally thinner and more vulnerable to damage from underlying conditions. In LAFB, the electrical signal must travel through the posterior fascicle first, causing the final electrical vector to deviate significantly toward the left and upward. This results in a left axis deviation on the EKG.
The Left Posterior Fascicular Block (LPFB) is considerably less common than its anterior counterpart. The posterior fascicle’s robust structure makes it more resistant to ischemic damage. When LPFB occurs, the electrical impulse is forced to use the anterior fascicle, resulting in a shift in the heart’s electrical vector toward the right and downward. This is seen on the EKG as a right axis deviation.
When a fascicular block is present alongside a block in the Right Bundle Branch (RBBB), the condition is termed a Bifascicular Block. This combination means two of the three main pathways to the ventricles—the Right Bundle Branch and one of the Left Bundle Branch fascicles—are blocked. This form is considered more concerning because it involves two major parts of the electrical highway, carrying a higher risk of progressing to a complete disruption of the heart’s electrical system.
Causes and Clinical Significance
Fascicular blocks, particularly the more common LAFB, can sometimes occur in individuals with structurally normal hearts, but they are most frequently associated with underlying cardiac disease. Common causes include long-standing hypertension, which can lead to left ventricular hypertrophy, or thickening of the heart muscle. Coronary Artery Disease (CAD) and a prior myocardial infarction (heart attack) are also frequent causes, as damage to the heart tissue can disrupt the electrical pathways.
Other conditions such as aortic valve disease, heart failure, and degenerative diseases that affect the heart’s structure can also contribute to a fascicular block. An isolated LAFB is often considered a benign finding that may not require specific treatment. However, it can be an indicator of a higher risk for future cardiac issues like atrial fibrillation or heart failure, so patients are typically monitored regularly to watch for any progression.
In contrast, LPFB is rarely found in otherwise healthy individuals and is more often a marker of severe, extensive underlying heart disease. A Bifascicular Block, which involves both RBBB and a fascicular block, carries a higher risk of progressing to a Complete Heart Block, where no electrical impulses travel from the atria to the ventricles. In these higher-risk cases, continuous follow-up and management of the underlying heart condition are essential, and a pacemaker may be considered for patients who develop symptoms or show signs of further conduction system failure.