A Left Posterior Fascicular Block (LPFB) describes a minor disruption within the heart’s intricate electrical wiring system. This condition is a specific type of conduction delay that occurs in the lower chambers of the heart, the ventricles. It is a finding frequently noted on an electrocardiogram (ECG) during a routine check-up or when investigating other heart-related concerns. Understanding this specific finding requires a look at the heart’s normal electrical highway and how the block alters the signal’s path.
Understanding the Heart’s Conduction System
The heartbeat begins with an electrical signal generated in the sinoatrial (SA) node, often called the heart’s natural pacemaker. This signal travels across the upper chambers, the atria, causing them to contract. The impulse then pauses briefly at the atrioventricular (AV) node, which acts as a gateway before passing the signal to the ventricles.
From the AV node, the signal moves into the Bundle of His, which divides into the main left and right bundle branches. These branches carry the impulse down the septum, the wall separating the two ventricles. The right bundle branch activates the right ventricle, while the left bundle branch activates the larger left ventricle.
The left bundle branch splits into two specialized pathways, known as fascicles: the left anterior fascicle and the left posterior fascicle. The anterior fascicle is thinner and supplies the front and upper part of the left ventricle. The posterior fascicle is broader, thicker, and has a robust dual blood supply, directing the impulse to the back and lower walls. These fascicles ensure the coordinated contraction of the entire left ventricle, which is necessary for efficient blood pumping.
Defining Left Posterior Fascicular Block
A Left Posterior Fascicular Block occurs when the electrical impulse cannot travel down the posterior fascicle. This blockage means the signal cannot directly activate the inferior and posterior sections of the left ventricle. Instead, the electrical current is forced to reroute through an alternate, slower route.
This detour involves the signal traveling first down the unobstructed left anterior fascicle. The impulse activates the upper and anterior parts of the ventricle normally, but then must spread indirectly, cell-by-cell, through the muscle tissue to reach the blocked posterior region. This use of muscle tissue, rather than the fast conduction system, creates a slight but measurable delay in the activation of the left ventricle.
The electrical activation of the left ventricle is completed, but the overall sequence is altered and delayed, which the ECG detects. Because the posterior fascicle is thick and has two sources of blood flow, an isolated block in this location is considered less common than a block in the more fragile anterior fascicle.
How LPFB is Identified on an ECG
LPFB is identified by observing a characteristic pattern on an electrocardiogram (ECG). The block does not significantly widen the QRS complex, the part of the tracing representing ventricular activation, which remains under 120 milliseconds. The primary evidence of LPFB lies in the change in the heart’s electrical axis, which describes the general direction of the electrical current.
LPFB causes the electrical signal to be strongly redirected, resulting in a Right Axis Deviation (RAD). This means the overall electrical vector is shifted to the right, generally falling between +90 and +180 degrees on the frontal plane of the ECG. This shift occurs because the rerouted electrical signal is late in activating the lower and back walls of the heart.
Cardiologists look for distinct patterns in the limb leads, which view the heart from the front. LPFB is specifically identified by an rS pattern in leads I and aVL, where the initial positive deflection (r-wave) is small and the negative deflection (S-wave) is deep. Simultaneously, the inferior leads, such as III and aVF, display a qR pattern, showing a small initial negative deflection (q-wave) followed by a tall positive deflection (R-wave).
A diagnosis of LPFB is often one of exclusion, meaning other common causes of RAD must first be ruled out. Conditions like right ventricular hypertrophy, myocardial infarction, or a naturally vertical heart position can also cause a rightward axis shift. The combination of specific wave patterns, appropriate axis deviation, and the absence of other underlying causes allows for a confident diagnosis of isolated LPFB.
Causes, Risks, and Management
LPFB is frequently associated with an underlying cardiac condition that has damaged the heart muscle or the conduction system. Common causes include chronic high blood pressure (hypertension), which stiffens the heart muscle, and ischemic heart disease, where reduced blood flow leads to tissue injury. Other factors include cardiomyopathy and aortic valve disease, which increases the heart’s workload.
When LPFB is found in isolation, without other conduction issues, it is generally considered a relatively benign condition. The block itself rarely causes noticeable symptoms or significantly impacts life expectancy. The most significant concern arises when LPFB is found alongside a Right Bundle Branch Block (RBBB), creating a bifascicular block.
This combination of blockages increases the risk of the electrical system failing completely, potentially leading to a complete heart block. Management of LPFB primarily focuses on identifying and treating the underlying cause, such as controlling blood pressure or managing coronary artery disease. If the LPFB is asymptomatic and found without evidence of structural heart disease, no specific treatment is necessary, but regular monitoring is often recommended.