ECG Topic - Fascicular Blocks (LAFB LPFB, and Bifascicular)

Last Updated November 2026

Introduction

The left bundle splits into two fascicles: the left anterior fascicle and the left posterior fascicle. Block one of them and you get a fascicular block. The QRS stays narrow, but the axis swings hard to one side. In the ED you need to answer three questions: Is the axis abnormal? Is it a fascicular block? Is there also an RBBB?

The Anatomy in One Picture

• Left anterior fascicle: a thin, tight cord. It supplies the anterior and lateral walls of the LV. Easy to block, so LAFB is common.
• Left posterior fascicle: loose fibers that fan out over the inferior and posterior walls. You need a big lesion to block it. Hard to block, so LPFB is rare.

Left Anterior Fascicular Block (LAFB)

What Happens

The impulse enters the LV through the posterior fascicle, so the LV fires from the bottom up.

1. First: a small force points down and right → small r in II, III, aVF and a small q in I, aVL.
2. Then: a big, late force points up and left → tall R in I, aVL and a deep S in II, III, aVF.

LAFB Criteria

1. Left axis deviation, specfically −30° to −90°
2. qR (or a tall R) in lead I
3. rS in lead III (and usually II and aVF)

The 2-Second Left Axis Check

• Lead I up
• aVF down
• Lead II down

That combination only fits an axis between −30° and −90°. With a narrow QRS, that's LAFB.

Examples of LAFB

Bonus: R-Wave Peak Time (LITFL)

Measure from the start of the QRS to the top of the R in aVL. In LAFB it's more than 45 ms, because that late force arrives late. Useful when the axis is borderline.

LAFB vs Inferior MI

Both give you a left axis. Look at the start of the QRS in II, III and aVF:

• LAFB: starts with a small r, then a deep S (rS).
• Inferior MI: starts with a Q (Qr or QS).

Some people say you can't call LAFB when there are inferior Q waves. An axis between −30° and −90° is abnormal no matter what, and a Q wave can hide the small r. If you miss a fascicular block in an MI, the patient can go into complete heart block. If you overcall it, no one is harmed. Don't be too strict.

Don't Be Fooled

• In LAFB, the voltage in lead aVL may meet voltage criteria for LVH (R wave height > 11 mm).
• Tall R in aVL ≠ LVH. LAFB points the main force straight at aVL, so the R can get tall (over 11 mm). With no strain pattern, be careful calling LVH.
• LAFB is common and often stable. A new LAFB in a patient with chest pain is different: think ischemia.

Left Posterior Fascicular Block (LPFB)

What Happens

The impulse enters the LV through the anterior fascicle, so the LV fires from the top down. It's the mirror image of LAFB.

1. First: a small force points up and left → small r in I, aVL and a small q in II, III, aVF.
2. Then: a big, late force points down and right → deep S in I, aVL and a tall R in II, III, aVF.

Criteria (Garcia)

1. Axis +90° to +180° (the right quadrant, not the extreme right/northwest)
2. S in lead I and q in lead III
3. No RAE and no RVH

Examples of LPFB

It's a Diagnosis of Exclusion

No finding proves LPFB. You get there by ruling out everything else that causes a right axis. You could read thousands of ECGs and be lucky to find one.

Side by Side

Bifascicular Block

Three fascicles carry the impulse: the right bundle, the left anterior fascicle and the left posterior fascicle. A bifascicular block is RBBB + LAFB or RBBB + LPFB. Now one fascicle carries every beat.

How to Read It (Garcia's Steps)

1. QRS ≥ 120 ms?
2. Slurred S in I and V6? RSR′ in V1? → RBBB.
3. Now do the axis:
• I up, aVF down, II down → RBBB + LAFB
• I down, aVF up, q in III (and no RVH/RAE) → RBBB + LPFB

Which One Should Worry You?

Examples

Bifasicular Block - RBBB+LAFB

Bifasicular Block - RBBB+LPFB

Bifascicular block (RBBB+LAFB) w/ 2nd degree degree av block

Bifascicular block (RBBB+LAFB) w/ 3rd degree degree av block

High-Yield Pearls

• Narrow QRS + big axis shift = think fascicular block.
• I up, aVF down, II down = LAFB. Takes 2 seconds.
• Inferior Q waves + left axis? Think inferior MI, but don't be too strict about calling LAFB too.
• Right axis? Ask "Is this LPFB?" Then rule out RVH, RAE, PE and lateral MI.
• Northwest axis is never LPFB.
• Wide QRS? Find the RBBB first, then check the axis for a bifascicular block.
• RBBB + LPFB is the unstable one. Any bifascicular block in an acute MI → be ready to pace.

Quick Algorithm

1. Look at Lead I and aVF.
2. I up, aVF down → check Lead II.
• II down → LAFB (if the inferior leads start with a small r).
• II up → normal (0° to −30°).
3. I down, aVF up → right axis → "Is this LPFB?" Rule out RVH, RAE, PE, lateral MI.
4. Both down → northwest axis. Not a fascicular block.
5. QRS wide? Look for RBBB → bifascicular block?

Practice - Name That Block

These are drawn single beats in a 12-lead layout. V2–V5 are left blank because you don't need them for these cases. For each one: quadrant → QRS shapes → V1/V6 → name it.

Exercise 1

Exercise 2

Exercise 3

Exercise 4

Exercise 5

Exercise 6

Exercise 7

Sources

1. Garcia TB. 12-Lead ECG: The Art of Interpretation. 2nd ed. Burlington, MA: Jones & Bartlett Learning; 2015. Chapter 13, Bundle Branch Blocks and Hemiblocks.
2. Burns E, Buttner R. Left Anterior Fascicular Block (LAFB). Life in the Fast Lane ECG Library. https://litfl.com/left-anterior-fascicular-block-lafb-ecg-library/
3. Burns E, Buttner R. Left Posterior Fascicular Block (LPFB). Life in the Fast Lane ECG Library. https://litfl.com/left-posterior-fascicular-block-lpfb-ecg-library/

This post is for education and not medical advice.