Left Anterior Fascicular Block Overview

Overview

Lecture covers left anterior fascicular block (LAFB) from an EKG coding reference guide.
Focus on recognition, mechanism, EKG features, causes, and clinical significance.
Part of a multi-lecture series; prior parts cover normals, rhythms, AV conduction, and voltage/hypertrophy.

Conduction path: sinus node → AV node → His bundle → right bundle + left bundle.
Left bundle splits into anterior and posterior fascicles.
LAFB = anatomical or functional block of left anterior fascicle.
With LAFB, left ventricular depolarization relies on the left posterior fascicle.
Anterior fascicle: thin, organized fibers supplying the anterolateral left ventricle.
Anterior fascicle is more susceptible due to smaller arterial supply and thin fibers.

Main criterion: Left axis deviation between −45° and −90°.
Lead patterns:
- Lead I and aVL: QR complexes (initial Q then prominent R).
- Inferior leads (II, III, aVF): RS complexes (initial R then deep S).
R-wave peak time in aVL may be >45 ms (from QRS onset to R peak).
QRS duration generally within normal limits; may be slightly prolonged.
Axis example: machine-calculated axis often around −60° in LAFB.
Probable LAFB: axis between −30° and −45° sometimes labeled probable LAFB.
Criteria do not apply in congenital heart disease with infancy left axis deviation.

Block of the anterior fascicle causes initial depolarization vectors to be altered:
- Early vector toward inferior leads (small upward deflection).
- Dominant later vector (from posterior fascicle) directed superior-left, away from inferior leads producing S waves.
- In lateral leads (I, aVL) the initial vector is away (Q), later vector toward produces R.
This shift produces characteristic QR in lateral leads and RS in inferior leads plus leftward axis.

Can occur in healthy individuals; more significant with heart disease.
Associated conditions:
- Hypertension and left ventricular hypertrophy
- Coronary artery disease and infarction affecting anterior fascicle
- Dilated or hypertrophic cardiomyopathy
- Degenerative conduction disease
- Myocarditis, amyloidosis, hyperkalemia
LAFB is more common than left posterior fascicular block.
Isolated LAFB often benign but can progress to bifascicular block (LAFB + RBBB) or complete heart block.

Primary diagnostic focus: left axis deviation (−45° to −90°).
Look for QR in I and aVL; RS in II, III, aVF.
R-wave peak time >45 ms in aVL supports diagnosis.
Beware: LAFB can mimic an anterior septal myocardial infarction or mask an inferior MI.
In congenital heart disease, neonatal left axis deviation invalidates criteria.

Left Anterior Fascicular Block (LAFB): Block of left anterior fascicle causing leftward shift of QRS axis.
Left Axis Deviation: Frontal plane QRS axis shifted left; diagnostic range for LAFB is −45° to −90°.
QR Complex: Q wave followed by R wave morphology in a lead.
RS Complex: R wave followed by deep S wave morphology in a lead.
R-wave Peak Time: Interval from QRS onset to peak of R wave; >45 ms in aVL may be noted.

Review prior lectures on axis determination and conduction system anatomy.
Practice interpreting EKGs for axis and lead-specific QRS patterns (I, aVL, II, III, aVF).
Consider differential causes when LAFB is present; correlate clinically for heart disease.
Remember pitfalls: possible mimicry of anterior septal MI and masking of inferior MI.
Use calipers to measure R-wave peak time and confirm >45 ms when suspected.