From the Guidelines
The workup for left anterior fascicular block (LAFB) primarily involves a thorough electrocardiogram (ECG) evaluation, which is the cornerstone of diagnosis, looking for left axis deviation, small Q waves in leads I and aVL, small R waves in leads II, III, and aVF, and a QRS duration less than 120 milliseconds, as defined in the 2019 ACC/AHA/HRS guideline 1. The diagnosis of LAFB is based on specific ECG criteria, including a frontal plane axis between 45 and 90 degrees, a qR pattern in lead aVL, an R-peak time in lead aVL of ≥45 ms, and an rS pattern in leads II, III, and aVF, as outlined in the 2019 guideline 1. After confirming LAFB on ECG, a comprehensive cardiac evaluation should be performed to identify any underlying causes, including:
- A detailed medical history focusing on cardiac risk factors
- Physical examination
- Laboratory tests including cardiac enzymes, electrolytes, and renal function
- Additional diagnostic tests such as echocardiography to assess cardiac structure and function, stress testing to evaluate for coronary artery disease, and occasionally cardiac MRI or CT angiography depending on clinical suspicion, as suggested in the context of arrhythmias and conduction defects 1. It's essential to note that LAFB itself is often asymptomatic and doesn't typically require specific treatment, but management focuses on addressing the underlying cause, which may include coronary artery disease, cardiomyopathy, valvular heart disease, or conduction system fibrosis, emphasizing the importance of a thorough evaluation to guide management decisions 1.
From the Research
Diagnosis of Left Anterior Fascicular Block
The diagnosis of left anterior fascicular block (LAFB) can be evaluated using several electrocardiographic criteria, including:
- Marked left axis deviation (LAD) and a delay in the time of inscription of the intrinsicoid deflection (ID) in lead aVL asynchronous to V6 2
- Left ventricular phase and amplitude maps evaluated in the 70-degree left anterior oblique projection 3
- Simulation of cardiac exciting sequence map and 12-lead ECG caused by the block in the left anterior fascicle and by the slowed conduction velocity in the ventricular myocardium 4
Electrocardiographic Criteria
The electrocardiographic criteria for isolated left anterior fascicular block include:
- Decrease in voltage in the chest leads and increase in voltage in the limb leads 5
- Changes in repolarization, including a decrease in the T wave of leads I and AVL and an increase in leads II, III, AVF, V5, and V6 5
- Small Q waves in V2 may simulate an anteroseptal myocardial infarction 5
Clinical Significance
The clinical significance of left anterior fascicular block during acute myocardial infarction includes: