Left Posterior Fascicular Block (LPFB) is a specific type of partial electrical conduction delay occurring within the lower left chamber of the heart. This delay, also known as a fascicular block or hemiblock, indicates a problem in one of the final branches of the heart’s electrical wiring system. The condition slows the electrical impulse as it tries to reach the posterior wall of the left ventricle.
LPFB is often asymptomatic, meaning the person experiences no noticeable physical symptoms. It is typically discovered incidentally during a routine electrocardiogram (ECG) performed for an unrelated reason. While an isolated LPFB is usually not immediately harmful, it signals the need for further investigation into the heart’s overall health.
The Heart’s Electrical Wiring
The coordinated beating of the heart is governed by a precise electrical system that initiates in the upper right chamber. The signal begins at the sinoatrial (SA) node, the heart’s natural pacemaker, and travels to the atrioventricular (AV) node, which momentarily delays the impulse. From the AV node, the signal moves into the ventricles via the Bundle of His, which divides into the Left and Right Bundle Branches.
The Left Bundle Branch splits into two main pathways: the Left Anterior Fascicle and the Left Posterior Fascicle. These fascicles are the final segments that distribute the electrical current directly to the left ventricular muscle, causing contraction. The Left Posterior Fascicle transmits the impulse to the lower, inferior, and posterior walls of the left ventricle.
A block in this posterior fascicle forces the electrical signal to bypass the intended route, traveling first down the anterior fascicle. This alternate pathway causes a slight delay and alters the sequence of activation, changing how the left ventricle contracts. Because the posterior fascicle is the most robust of the fascicles, a block here often suggests a significant underlying structural or disease process.
Why the Block Occurs
A Left Posterior Fascicular Block is considered a significant finding because the posterior fascicle is biologically resilient compared to its anterior counterpart. It possesses a dual blood supply from two different coronary arteries, making it less susceptible to damage from localized lack of blood flow (ischemia). Therefore, damage must be relatively extensive or severe to cause a block in this pathway.
LPFB is frequently associated with underlying structural heart disease that places chronic stress on the left ventricle. Hypertensive heart disease is a common contributor, as long-standing high blood pressure causes the heart muscle to thicken and disrupt the conduction system. Aortic valve disease, such as a narrowed or leaking valve, also increases the workload on the left ventricle, leading to similar structural changes.
Coronary artery disease, especially one that has caused a heart attack (myocardial infarction), is another frequent cause, as damaged heart tissue cannot conduct electricity effectively. The block can also result from idiopathic fibrosis, which is a scarring of the conduction tissue that occurs naturally with advanced aging.
Identifying Left Posterior Fascicular Block
Left Posterior Fascicular Block is overwhelmingly asymptomatic in isolation and does not typically cause immediate symptoms like dizziness or fainting. If symptoms occur, they are usually a consequence of the underlying heart condition, such as coronary artery disease, rather than the block itself. Identification of LPFB relies almost exclusively on the Electrocardiogram (ECG).
The ECG measures the heart’s electrical activity, translating the impulse movement into a waveform for analysis. When the posterior fascicle is blocked, the electrical activation of the left ventricle is redirected, creating a characteristic pattern. This pattern is known as Right Axis Deviation, where the overall electrical vector of the heart shifts to an angle greater than +90 degrees.
Specific changes in the QRS complex, which represents ventricular contraction, are required for diagnosis. These include a small Q-wave followed by a tall R-wave (a “qR” pattern) in the inferior leads (II, III, and aVF). Simultaneously, the lateral leads (I and aVL) show a small R-wave followed by a deep S-wave (an “rS” pattern), reflecting the abnormal electrical direction. The QRS duration remains relatively narrow (typically less than 0.12 seconds), distinguishing it from a complete bundle branch block.
Long-Term Outlook and Monitoring
When Left Posterior Fascicular Block is found in isolation, without other conduction defects or evidence of structural heart disease, the long-term outlook is generally benign. The condition itself does not require specific treatment, and life expectancy is not typically affected. Primary management revolves around treating the underlying condition that caused the block to form.
This involves controlling hypertension, managing high cholesterol, or treating existing coronary artery disease to prevent further heart damage. A thorough evaluation, often including an echocardiogram, is performed to exclude serious structural issues like cardiomyopathy. The main concern with LPFB is its potential to progress into a more serious conduction disorder, such as a Bifascicular Block (LPFB combined with a Right Bundle Branch Block) or a Trifascicular Block.
These combined blocks significantly increase the risk of developing a complete heart block, where the electrical signal cannot pass between the upper and lower chambers. Individuals with LPFB require regular monitoring, typically with periodic ECGs, to watch for progression. Pacemaker insertion is rarely needed for an isolated LPFB, but it may become necessary if the block advances or is accompanied by severe symptoms like syncope.