Introduction

Left posterior fascicular block (LPFB) is a rare intraventricular conduction disturbance characterised by blocked electrical impulses in the left posterior fascicle of the heart's conduction system. While LPFB is uncommon in the general population, its clinical significance becomes pronounced when associated with underlying cardiac conditions, particularly myocardial infarction (MI).^1,3 The appearance of LPFB on an electrocardiogram (ECG) is most reliably connected with inferior wall MI and is generally considered a marker of severe coronary artery disease, often reflecting two- or three-vessel involvement.^1,3 Understanding the association between LPFB and myocardial infarction is crucial for accurate diagnosis, risk stratification, and management of affected patients.^1,3

Anatomy and physiology of the LPF

There are two main fascicles: Anterior and Posterior. The left posterior fascicle (LPF) conducts electrical impulses to the posterior and inferior walls of the left ventricle.^2,3,7 Unlike the left anterior fascicle, the LPF is broader, shorter, and has a dual blood supply from both the left anterior descending (LAD) and right coronary arteries (RCA), making it more resistant to ischemic injury.⁷ Its protected anatomical course through the left ventricular inflow tract also reduces its vulnerability compared to the anterior fascicle.

Pathophysiology of LPFB

In LPFB, the left posterior fascicle is blocked, forcing the electrical impulse to travel via the left anterior fascicle to the lateral wall before spreading to the posterior-inferior regions.^1,2,3 This rerouting alters the sequence of ventricular activation, typically producing right axis deviation on the ECG, with an rS pattern in leads I and aVL and a qR pattern in leads III and aVF.⁹

Etiology of LPFB

Myocardial infarction as a primary cause

Myocardial infarction is a leading cause of LPFB.^5,6,8 During an MI, especially one involving the inferior or posterior walls, the blood supply to the LPF can be compromised, resulting in a conduction block. The dual blood supply of the LPF means that only extensive infarcts or those involving both the LAD and RCA territories are likely to produce an isolated LPFB.⁷

Other causes and risk factors

Other etiologies include:

• Cardiomyopathies (dilated, hypertrophic)
• Chronic hypertension
• Aortic stenosis
• Degenerative fibrosis with ageing
• Infiltrative diseases (amyloidosis, sarcoidosis)
• Myocarditis
• Surgical injury or trauma
• Genetic predispositions and autoimmune diseases
• Lifestyle factors contributing to cardiovascular disease (smoking, poor diet, inactivity)^[5][6][8]

Association of LPFB with myocardial infarction

Mechanisms linking MI and LPFB

The LPF’s relative resistance to ischemia means LPFB is uncommon in the general population and even among those with heart disease.^7,9 When LPFB does occur, it often indicates a large or strategically located infarct affecting both blood supplies. The conduction block may be transient or permanent, depending on the extent of myocardial damage.

Clinical implications

The presence of LPFB in the context of MI signifies more extensive myocardial injury and is frequently associated with other conduction abnormalities, such as right bundle branch block (RBBB). When LPFB and RBBB occur together (bifascicular block), the risk of progressing to complete heart block and sudden cardiac death increases.^1,5

 Prognostic significance

Isolated LPFB is rare and often benign, but its occurrence during or after MI is a marker of poor prognosis, reflecting significant conduction system involvement and a higher risk for arrhythmias and mortality.^1,5,8

Diagnosis

ECG Criteria for LPFB

The diagnosis of LPFB relies on specific ECG findings:

• Right axis deviation (frontal plane axis between +90° and +180°)
• rS pattern in leads I and aVL
• qR pattern in leads III and aVF
• QRS duration less than 120 ms⁹

These criteria must be interpreted in the context of clinical presentation. LPFB can be missed or masked by the presence of other conduction defects or infarct patterns.

Role of additional diagnostic tools

• Echocardiography: Evaluate ventricular function and detect wall motion abnormalities
• Cardiac biomarkers: Confirm MI
• Stress testing: May reveal exercise-induced conduction blocks
• Advanced imaging: Cardiac MRI can delineate fibrosis or infiltrative disease^1,5

Clinical presentation

LPFB is often asymptomatic and detected incidentally during ECGs performed for other reasons.^5,8 When symptoms are present, they are usually related to the underlying cause, such as chest pain, dyspnea, syncope, or palpitations during MI or heart failure episodes.

Management and treatment

Addressing underlying causes

• Acute MI: Reperfusion therapy (PCI or thrombolysis), antiplatelet agents, beta-blockers, and ACE inhibitors
• Heart failure: Guideline-directed medical therapy
• Hypertension: Blood pressure control
• Infiltrative or inflammatory disease: Disease-specific management

Specific considerations in MI

Patients with MI and new-onset LPFB, especially when combined with RBBB, should be monitored in a high-dependency setting due to the risk of complete heart block. Temporary or permanent pacemaker implantation may be required if advanced atrioventricular block develops.^9,5

Conclusion

In summary, the association of left posterior fascicular block with myocardial infarction is clinically significant, as the presence of LPFB often signals extensive underlying coronary artery disease and may indicate a more severe prognosis.^1,4,6 Although LPFB is rare, its detection in the context of acute MI should prompt clinicians to consider the possibility of more extensive myocardial injury or complications such as septal rupture.^1,6 Early recognition and appropriate evaluation of LPFB in patients with MI can aid in timely intervention and may improve clinical outcomes.^1,4,6

FAQs

 What is the left posterior fascicular block?

Left Posterior Fascicular Block (LPFB) is a cardiac condition where there's a delay or blockage in the electrical conduction through the left posterior fascicle in the heart. This can result from various factors, such as coronary artery disease, hypertension, or structural heart abnormalities 

How is LPFB associated with myocardial infarction?

LPFB can be caused by myocardial infarction (heart attack), which damages the heart muscle and interferes with the electrical pathways. When LPFB occurs with MI, it signifies more extensive myocardial injury and increases the risk of arrhythmias, heart failure, and mortality

Can LPFB be prevented?

While LPFB can't be entirely prevented, managing underlying heart conditions and adopting a heart-healthy lifestyle can reduce the risk. This includes:

• Regular Exercise: Maintaining a healthy weight and improving cardiovascular health
• Balanced Diet: Managing weight and blood pressure
• Avoiding Smoking and Excessive Alcohol: Reducing risk factors for heart disease

 What is the prognosis for LPFB?

The prognosis varies depending on the underlying cause and severity of the condition. With appropriate management and follow-up, most patients can lead normal lives without significant complications. However, patients with LPFB and underlying cardiac disease may have a worse prognosis.

Key points

• LPFB is a rare conduction abnormality often associated with significant underlying heart disease, particularly MI
• The left posterior fascicle's dual blood supply makes it more resistant to ischemic injury, but extensive infarcts can still cause LPFB
• ECG criteria for LPFB include right axis deviation, rS pattern in leads I and aVL, and qR pattern in leads III and aVF
• LPFB in MI patients signifies more extensive myocardial injury and increases the risk of arrhythmias, heart failure, and mortality