Left Anterior Fascicular Block (LAFB) on ECG

Characteristic ECG Findings of LAFB

• LAFB causes a shift of the QRS vector in a posterior and superior direction, resulting in larger R waves in leads I and aVL and smaller R waves but deeper S waves in leads V5 and V6 1
• QRS duration is usually normal or only slightly prolonged (less than 120 ms) 1, 2

Clinical Implications

• LAFB may be an isolated, benign finding or may indicate underlying cardiovascular disease 3
• LAFB can occur in association with various cardiac conditions including hypertensive heart disease 4, 5
• LAFB can occur in association with congenital heart defects, such as atrioventricular canal defects 6

Impact on Other ECG Diagnoses

• The American College of Cardiology recommends that R-wave amplitude in leads I and aVL are not reliable criteria for left ventricular hypertrophy (LVH) in the presence of LAFB 4, 1
• Criteria that include the depth of the S wave in left precordial leads improve detection of LVH when LAFB is present 4, 1

Differential Diagnosis

• Left axis deviation may result from left ventricular hypertrophy itself 5
• Left axis deviation may result from age-related changes, with a tendency toward leftward axis with increasing age 4, 5

Clinical Evaluation

• The European Society of Cardiology recommends a thorough cardiological work-up, including exercise testing, 24-hour ECG monitoring, and cardiac imaging to evaluate for underlying pathology when complete bundle branch block or hemiblock is found in an athlete 3
• ECG screening of siblings is recommended when bifascicular block patterns are identified in young athletes 3

Prognostic Significance

• Isolated LAFB is generally considered to have a benign prognosis 3
• When associated with other conduction abnormalities, there is an increased risk of developing clinically significant AV block 3

Treatment and Management of Left Anterior Fascicular Block (LAFB)

Clinical Approach and Management

• The European Society of Cardiology recommends monitoring for progression to more advanced conduction disorders in patients with LAFB associated with other conduction abnormalities 7, 8
• The American College of Cardiology suggests that permanent pacing is not indicated for fascicular block with first-degree AV block without symptoms, but may be considered for persistent second-degree AV block in the His-Purkinje system with alternating bundle-branch block or third-degree AV block after ST-elevation MI 9
• The American College of Cardiology also recommends permanent pacing for persistent and symptomatic second- or third-degree AV block 9

Special Considerations

• LAFB in the context of acute myocardial infarction may indicate more severe coronary artery disease and has been associated with worse outcomes, according to the American College of Cardiology 9
• The European Heart Journal suggests that more vigilant monitoring is warranted for LAFB in the context of acute myocardial infarction due to potential for progression to higher-grade AV block 7