Introduction

Science has made it well known that the heart is more than just a pump which ensures our organs receive an adequate blood supply. It’s also a finely tuned electrical system which ensures when and how the heart beats. It does this by guiding signals across the chambers to bring about contraction of the heart through its highly coordinated network of electrical pathways.

Disorders of impulse transmission may occur within the cardiac conduction system when one of these pathways becomes blocked, delayed, or damaged. Heart block is one of these conditions where the electrical pathways of the heart do not work properly, causing disruption in the contraction of the ventricles. Heart block can be conceptualised in terms of three cardiac regions, with three degrees of conduction failure in each region. The three regions of the heart block include the sino-atrial connections, the AV junction (AV Node and His Bundle), and the bundle branches along with their fascicles. 

The cardiac conduction system consists of five specialised tissues which transport electrical signals of the heart:

1. Sinoatrial node (SA node)- The main point where the electrical signal originates 
2. Atrioventricular node (AV node) 
3. Bundle of His
4. Right and left bundle branches
5. Purkinje fibers

As impulses arise in the SA node, they traverse through the atria, leading to contraction of the atria. The impulses will travel from the atria and reach the AV node, where there is some delay, which enables the contraction of the atria, which will push blood into the ventricles.

The impulse then travels from the AV node through the bundle of His, down the left and right bundle branches, which transmit the signals respectively to the left and right ventricles through the Purkinje fibres, eventually triggering the ventricles to contract. The left bundle branch further divides into the left posterior fascicle and left anterior fascicle, where the left posterior fascicle runs deep in the lower part of the heart and is surrounded by muscle tissue.

Understanding left posterior fascicular block 

A left posterior fascicular block or hemiblock occurs when impulses from the left bundle branch cannot pass through to the inferior and posterior part of the ventricle due to ischemic or mechanical changes in the left posterior fascicle. This causes a delay in impulses reaching the ventricles, since they will be conducted by the much slower cell-to-cell depolarisation instead.⁷

The left posterior fascicle is short, thick and commonly has a dual blood supply; therefore, a posterior fascicular block is less likely to occur as an isolated block.¹ It is more commonly associated with a right bundle branch block, which can be more serious, due to its tendency to progress into Atrioventricular blocks(AV blocks).²

Even though left posterior fascicular block itself might seem minor, it can be linked to more serious problems like poor blood flow (ischaemia), scarring of heart tissue (fibrosis), and underlying diseases. Therefore, understanding what causes left posterior fascicular block can help patients and caregivers take better steps toward early detection and proper treatment. 

Ischemia: A common cause of left posterior fascicular block

Although the heart’s chambers are filled with blood, the walls of the heart, consisting of muscles (also known as myocardium), have their own blood supply, which is exclusively provided by the coronary arteries. Ischaemia is when the blood supply to certain regions of the heart is compromised,leading to damage and eventually death of the affected (infarcted) area. This damage is not only restricted to the myocardium, but can also include the conduction pathways that run in the wall, resulting in irregular heart rhythms called arrhythmias.

This is commonly seen during heart attacks (myocardial infarction) involving the inferior wall of the ventricle, where a coronary artery is completely occluded by a blood clot or fatty deposit called plaque.¹ A  left posterior fascicular block may also indicate multi-vessel coronary artery disease, due to its dual blood supply from both the right and left coronary arteries.¹ 

Fibrosis: scarring that disrupts the heart’s signals

Fibrosis or scarring of the heart tissue occurs when damaged heart tissue heals by scar formation, much like how our skin develops a scar after a wound heals. When fibrosis develops in this area, it can block or slow the electrical signals, resulting in left posterior fascicular block. Unlike ischemia and myocardial infarction, which can happen suddenly, fibrosis usually develops gradually over time. This means that people with chronic long-standing diseases such as Hypertension, Cardiomyopathy or even natural effects of ageing can develop fibrosis and heart failure, which can interfere with the transmission of electrical signals throughout the heart.

Therefore, it is important to address the underlying condition, such as controlling blood pressure or treating heart failure, in order to slow its progression and reduce complications, mainly due to the fact that fibrosis can’t be reversed.

Inherited conduction defects

Familial Progressive Cardiac Conduction Defect (FPCCD), also known as Lenègre disease, is a condition that runs in families and causes fibrosis of the heart’s electrical system.^1,2 Over the years, the electrical pathways may start to slow down or fail completely, leading to different types of blocks, including Left Posterior Fascicular Block.¹

Lenègre disease is usually seen more commonly in Left anterior Fascicular block than in left posterior fascicular block and affects younger individuals who are otherwise healthy.⁴ People with this condition might even have family members who needed pacemakers at a young age or experienced sudden fainting or irregular heartbeats.

Aortic valve defects: A hidden cause of electrical blocks

Left posterior fascicular block is one of the lesser-known complications of aortic stenosis, which is a condition where the aortic valve, in the blood vessel aorta, that allows blood to flow from the left heart chamber to the rest of the body, becomes narrow and stiff.¹ The blood pumped from the left ventricle to the body decreases because the narrowed valve forces the heart muscle to work harder to pump blood. Over time, this extra effort can lead to thickening (hypertrophy) and damage of the heart muscle, especially around the area where the posterior fascicle is located.

In addition, patients with aortic stenosis,  especially older adults,  are often at risk of other heart conditions like calcification, where calcium deposits build up and stiffen both the valve and the surrounding tissues. These deposits can also affect the nearby electrical pathways and contribute to combined blocks affecting the other fascicles as well.⁵

Other possible causes 

While ischemia and fibrosis are the most common causes of a Left Posterior Fascicular Block (LPFB), there are several other less common but important factors that can affect the heart’s electrical system.

Trauma or surgery on the heart - Procedures such as valve replacement, bypass surgery, or even certain catheter-based treatments can accidentally damage delicate parts of the heart’s conduction system. When the left posterior fascicle is injured during surgery, it may stop working properly, leading to a block.

Infiltrative diseases- Conditions like cardiac Sarcoidosis or Amyloidosis occur when there is an accumulation of abnormal substances in the heart tissue.¹ These are also known as infiltrative cardiomyopathies since deposits can interfere with electrical signalling and cause blocks, including LPFB.

Congenital heart defects- Structural heart problems present from birth may affect the position and integrity of the heart’s electrical system. Studies have shown that LPFB might serve as an ECG biomarker for underlying heart disease and as a parameter to assess the risk of arrhythmias and sudden cardiac death in young people.⁸ Even if these defects are repaired with surgery, scarring from the procedure or altered anatomy can still affect how electrical signals move. Therefore, young people with LPFB should undergo further cardiac evaluations, including cardiac magnetic resonance imaging (CMR), to detect potential structural abnormalities.

Cardiomyopathy-This is a condition where the heart muscle becomes abnormally thickened (hypertrophic cardiomyopathy), stretched (dilated cardiomyopathy), or stiff (restrictive cardiomyopathy). When the heart wall, most often the left ventricle, thickens or stretches, it can put pressure on or damage the delicate electrical fibres of the left posterior fascicle that runs in the left ventricle. This can disrupt the electrical signals and can lead to LPFB over time.

Important diagnostic considerations of left posterior fascicular block

An LPFB usually does not cause symptoms, and can be an incidental finding which may appear in your routine health check-ups. An ECG is used to detect signs of right axis deviation, which occurs when the block in the left posterior fascicle causes the electrical wave to be diverted through to the left anterior fascicle.^{4, 7} Before diagnosing left posterior fascicular block, the doctor may exclude other causes of right axis deviation, including right ventricular hypertrophy, right-sided heart failure, pulmonary hypertension, large lateral wall MI, by means of imaging, blood workups and clinical examination.³

Summary

While fibrosis itself doesn’t always cause symptoms, it can increase the risk of irregular heart rhythms or heart failure if left unchecked. Even though LPFB is often found by accident and may seem harmless on its own, it should always prompt a closer look at the heart. It could be the first sign of a deeper issue, and identifying the underlying cause early can help prevent more serious complications down the road.