From the Guidelines
Left anterior fascicular block (LAFB) on ECG typically requires no specific treatment as it is generally a benign conduction abnormality, and management should focus on identifying any underlying cardiac conditions through a thorough history, physical examination, and appropriate cardiac workup. The ECG diagnosis of LAFB is based on specific criteria including a frontal plane axis between 45° and 90°, qR pattern in lead aVL, R-peak time in lead aVL of 45 ms or more, and QRS duration less than 120 ms, as outlined in the American Heart Association electrocardiography and arrhythmias committee guidelines 1.
When LAFB is found incidentally in an asymptomatic patient without evidence of structural heart disease, routine follow-up with periodic ECGs is sufficient. However, if LAFB is new or associated with symptoms such as syncope, palpitations, or chest pain, further evaluation is warranted, including:
- Echocardiography to assess for structural heart disease
- Ambulatory ECG monitoring to detect arrhythmias
- Possibly stress testing to evaluate for coronary artery disease
LAFB may be a marker of underlying cardiac pathology such as coronary artery disease, cardiomyopathy, or degenerative conduction system disease, particularly when it progresses to bifascicular block (LAFB plus right bundle branch block) or trifascicular block, as noted in guidelines for cardiac pacing and cardiac resynchronization therapy 1. The significance of LAFB lies in its potential to progress to more advanced conduction abnormalities, though this progression is relatively uncommon in isolated LAFB. No medications are specifically indicated for LAFB itself, but treatment of any underlying cardiac condition is essential, and cardiac pacing may have a place in patients with neuromuscular disease and any degree of fascicular block, with or without symptoms.
From the Research
ECG Workup for Left Anterior Fascicular Block
- Left anterior fascicular block (LAFB) is identifiable from an abnormal electrocardiogram (ECG) 2
- ECG criteria for diagnosis of LAFB include marked left axis deviation (LAD) and delayed intraventricular conduction 3
- LAFB can be associated with an increased risk of heart failure, and non-specific intraventricular conduction delay due to lesions of the conduction bundles and slow cell to cell conduction can also contribute to heart failure 2
Management of Left Anterior Fascicular Block
- The presence of LAFB can lower the accuracy of clinically diagnosing coronary artery disease (CAD) 4
- LAFB patients have a higher risk of death and cardiac death, with the major cause of death being cardiac death, including myocardial infarction, myocarditis, and cardiac rupture 4
- The prognosis of LAFB patients can be affected by the presence of CAD, myocardial infarction, and myocarditis 4
- Different ECG patterns of left main coronary artery occlusion can signify varying degrees of ischemic severity, and LAFB can be associated with left ventricular depolarization delay secondary to extensive ischemia 5
Diagnostic Considerations
- Phase analysis can be used to assess the use of LAFB, and left ventricular phase and amplitude maps can be evaluated to determine the contraction patterns of the anterior and inferior segments 6
- The simulation results of LAFB ECG patterns can provide a promising starting point to further investigate the underlying mechanism of heart failure with LAFB 2
- The duration of QRS and wave amplitude can be used to differentiate between LAFB ECG patterns and slowed conduction velocity in the ventricular myocardium 2