Left Anterior Fascicular Block (LAFB)

Anatomy & Mechanism

The left bundle branch divides into two fascicles:

Anterior (superior) fascicle — thin, long, supplied mainly by the LAD; conducts to the anterolateral LV
Posterior (inferior) fascicle — thicker, shorter, dual blood supply (LAD + RCA); more resistant to block

Block of the anterior fascicle forces depolarization to proceed first through the posterior fascicle (inferiorly and rightward), then swings superiorly and leftward — producing the characteristic marked left axis deviation. Because the block is within the fascicle (not the bundle itself), it does not significantly widen the QRS.

ECG Criteria

Parameter	Finding
QRS duration	< 120 ms (normal)
Frontal axis	−45° to −90° (marked left axis deviation)
Lead I, aVL	qR pattern (small q, tall R)
Leads II, III, aVF	rS pattern (small r, deep S)
Precordial	Delayed R-wave transition (late transition)
ST-T waves	Normal

Axis threshold: Most authorities use more negative than −45° as the cutoff. Harrison's states it is "probably the most common cause of marked left axis deviation in adults."

Below is the ECG from Goldman-Cecil Medicine (Fig. 42-5A) showing classic LAFB — left axis deviation ~−60°, normal QRS duration, qR in I/aVL, rS in inferior leads:

Epidemiology & Causes

Found in ~1% of otherwise healthy adults
Prevalence of any bundle branch block rises from ~1% at age 50 to ~17% at age 80
Common causes:
- Degenerative/fibrotic disease of the conduction system (most common)
- Coronary artery disease / ischemic heart disease
- Hypertensive heart disease
- Cardiomyopathy
- Aortic valve disease (including post-TAVR)
- Chagas disease
- Infiltrative disease (amyloid, sarcoid)
- Myocarditis

Clinical Features

Typically asymptomatic — LAFB itself causes no hemodynamic compromise
No significant ventricular dyssynchrony (unlike LBBB)
Clinical importance lies in what it accompanies or progresses to

Combinations & Clinical Significance

LAFB + RBBB = Bifascicular Block

Two of three fascicles are blocked. The ECG shows:

Wide QRS (≥120 ms) with RBBB morphology (rsR′ in V1, wide S in I/V5-6)
Left axis deviation (−45° to −90°)
High risk for progression to complete heart block, especially in acute MI

Bifascicular Block + PR Prolongation = Trifascicular Block

Implies disease in all three fascicles. Very high risk for complete AV block — pacemaker often placed prophylactically, especially with acute MI.

Evaluation

Compare to prior ECGs — determine if new or chronic
Echocardiography — assess LV function, structural disease
Stress test (echo or nuclear) — rule out ischemic heart disease
Cardiac MRI — if infiltrative cardiomyopathy suspected
Rhythm monitoring — if associated with prolonged PR, alternating blocks, or symptoms of syncope/presyncope

Management & Treatment

Isolated LAFB, asymptomatic: No specific treatment; treat underlying cause; follow-up
LAFB + symptoms (syncope/presyncope): Evaluate for higher-degree block; consider EP study
Bifascicular block + syncope: Pacemaker implantation preferred over implantable cardiac monitor, regardless of block morphology or PR interval
Bifascicular block + new PR prolongation: Prophylactic pacemaker often placed due to high risk of progression to complete heart block
In acute MI with new bifascicular block: Pacemaker often placed prophylactically

Prognosis

Isolated LAFB in a structurally normal heart: prognosis is benign; risk not significantly elevated above baseline
With underlying heart disease: increased mortality risk
LAFB + RBBB (bifascicular): portends higher long-term risk of requiring a permanent pacemaker, but prophylactic pacing is not indicated without symptoms such as syncope

Summary Table — LAFB vs. Other Fascicular Blocks

Block	Axis	QRS Width	Key Morphology
LAFB	−45° to −90°	Normal (<120 ms)	qR in I/aVL; rS in II, III, aVF
LPFB	+90° to +180°	Normal (<120 ms)	rS in I/aVL; qR in III, aVF
RBBB	Normal	Wide (≥120 ms)	rsR′ in V1; wide S in I/V6
RBBB + LAFB	−45° to −90°	Wide (≥120 ms)	RBBB + left axis
LBBB	Variable/left	Wide (≥120 ms)	Broad notched R in I/V5-6; no septal q

Sources: Goldman-Cecil Medicine, 26th Ed. (Ch. 42); Harrison's Principles of Internal Medicine, 22nd Ed. (Ch. 247); Fuster & Hurst's The Heart, 15th Ed.

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