Left Anterior Fascicular Block: Typical Axis Deviation
The typical axis deviation for a left anterior fascicular block (LAFB) is between -45° and -90° in the frontal plane. 1
Diagnostic Criteria for LAFB
According to the American Heart Association/American College of Cardiology Foundation/Heart Rhythm Society guidelines, the diagnostic criteria for LAFB include:
- Frontal plane axis between -45° and -90°
- qR pattern in lead aVL
- R-peak time in lead aVL of 45 ms or more
- QRS duration less than 120 ms 1
It's important to note that all four criteria should be present to make a definitive diagnosis of LAFB.
Clinical Significance and Interpretation
LAFB represents a block in the anterior fascicle of the left bundle branch, which alters the normal ventricular activation sequence. This results in:
- Delayed activation of the anterosuperior region of the left ventricle
- Initial forces directed inferiorly and to the right
- Terminal forces directed superiorly and to the left
Correlation Between Axis Deviation and LAFB
Research has shown that the degree of left axis deviation correlates with the likelihood of LAFB:
- At -30°: approximately 41% show delayed intrinsicoid deflection in aVL
- At -45°: approximately 69% show delayed intrinsicoid deflection in aVL
- At -60°: approximately 82% show delayed intrinsicoid deflection in aVL
- At -75° or greater: 100% show delayed intrinsicoid deflection in aVL 2
This suggests that the more negative the axis, the more likely a true LAFB is present.
Important Considerations
Left axis deviation alone is not sufficient for diagnosis: While left axis deviation is a key component of LAFB, it should not be considered synonymous with LAFB. The additional criteria of qR pattern in aVL, prolonged R-peak time, and normal QRS duration must also be present 2.
Age-specific considerations: The diagnostic criteria for LAFB do not apply to patients with congenital heart disease in whom left-axis deviation is present in infancy 1.
Coexisting conditions: LAFB may coexist with other conduction abnormalities, particularly right bundle branch block (RBBB). When LAFB occurs with RBBB, the diagnosis can be more challenging but remains possible by examining the initial portion of the QRS complex 3.
Clinical implications: While isolated LAFB is generally considered benign, it may be a marker for underlying cardiac disease, including coronary artery disease, cardiomyopathy, or degenerative conduction system disease 4, 5.
By understanding the typical axis deviation and other diagnostic criteria for LAFB, clinicians can accurately identify this conduction abnormality and determine whether further cardiac evaluation is warranted.