What is left anterior hemiblock (left anterior fascicular block)?

What is Left Anterior Hemiblock (Left Anterior Fascicular Block)?

Left anterior hemiblock (LAHB), also called left anterior fascicular block (LAFB), is a conduction abnormality where electrical impulses are blocked in the anterior-superior fascicle of the left bundle branch, causing a characteristic shift in the heart's electrical axis and specific ECG changes. 1, 2

Anatomical and Physiological Basis

• The left bundle branch divides into fascicles that conduct electrical impulses through the left ventricle, with LAHB representing blockage in the anterior-superior fascicle 3, 4
• When this fascicle is blocked, the QRS vector shifts in a posterior and superior direction, fundamentally altering the depolarization sequence of the left ventricle 1, 2
• Despite anatomical evidence supporting three fascicles, the bifascicular concept (anterior and posterior divisions) remains the practical teaching standard 3

Diagnostic ECG Criteria

Four mandatory criteria must be present to diagnose LAHB: 2

• Left axis deviation between -45° and -90° (the hallmark finding) 2
• qR pattern in lead aVL (small q wave followed by tall R wave) 2
• R-peak time in lead aVL ≥45 ms 2
• QRS duration <120 ms (distinguishes it from complete bundle branch block) 1, 2

Additional ECG Features

• Larger R waves appear in leads I and aVL 1, 2
• Smaller R waves but deeper S waves develop in leads V5 and V6 1, 2
• Small q waves in leads I and aVL with tall R waves 2

Important Diagnostic Pitfalls

• These criteria do not apply to patients with congenital heart disease where left axis deviation exists from infancy 2
• Left ventricular hypertrophy itself can cause left axis deviation and must be excluded 1, 2
• Age-related leftward axis shift in elderly patients can mimic LAHB 1, 2
• R-wave amplitude in leads I and aVL are unreliable for diagnosing left ventricular hypertrophy when LAHB is present; use criteria incorporating S-wave depth in V5-V6 instead 1, 2

Clinical Significance and Associations

Prevalence and Demographics

• LAHB is more common in men and increases with advancing age 5
• Estimated prevalence in the general population under age 40 is 0.5-1.0%, similar to athletic populations 5

Associated Conditions

• Coronary artery disease is a major cause, with significant left anterior descending artery lesions commonly present 3
• Can occur with hypertensive heart disease 1
• Associated with congenital heart defects, particularly atrioventricular canal defects where the AV node is displaced inferiorly with relative hypoplasia of the left anterior fascicle 5, 1
• May indicate degenerative disease of the conduction system, often more widespread than the ECG terminology suggests 4

Prognostic Implications

The clinical significance of LAHB depends critically on context: 1, 6

• Isolated LAHB is generally benign in asymptomatic individuals without structural heart disease 5, 1, 2
• When associated with other conduction abnormalities, there is increased risk of developing clinically significant AV block 1, 2
• In patients with suspected coronary artery disease, LAHB is associated with increased risk of cardiac death even after adjusting for other clinical factors 6
• In acute myocardial infarction, LAHB indicates more severe coronary disease and worse outcomes, warranting vigilant monitoring for progression to higher-grade AV block 5, 1, 2, 3
• Patients with acute MI and intraventricular conduction defects (except isolated LAHB) have unfavorable short- and long-term prognosis with increased sudden death risk 5

Clinical Evaluation Requirements

When LAHB is discovered, particularly in athletes or young individuals, the European Society of Cardiology recommends: 5, 1

• Thorough cardiological work-up including exercise testing 5, 1
• 24-hour ECG monitoring 5, 1
• Cardiac imaging to evaluate for underlying structural pathology 5, 1
• ECG screening of siblings when bifascicular block patterns are identified in young athletes 5, 1

Special Monitoring Considerations

• In atrioventricular canal defects, late-onset complete heart block can occur as late as 15 years after surgery, requiring regular symptom monitoring and ECG screening 5
• Transient LAHB can occur during angina pectoris (at rest or exercise-induced), likely due to ischemia in the anterolateral wall from proximal left anterior descending artery lesions 3