The lower, back wall of the left ventricle is reached a little late, from above, turning the axis to +110° or beyond: rS in I and aVL, qR in II, III and aVF, with a QRS under 120 ms. Rare alone, and a diagnosis of exclusion.

On the trace

Read the tracing above in this order. Tap any lead to see its own measurements; the Normal button in the box, and the Normal switch under the tracing, draw the ECG without the block in grey behind it.

How to recognise it

These are the criteria of the 2009 AHA/ACCF/HRS recommendations on intraventricular conduction.

Mechanism

The posterior fascicle runs to the inferior and posterior walls of the left ventricle. When it fails, the impulse enters the left ventricle through the anterior fascicle alone. The upper, front wall depolarises first, pointing up and to the left: the small r in I and aVL, the small q in II, III and aVF. The lower, back wall is reached a few tens of milliseconds later, from above, and its late force points down and to the right: the deep S in I and aVL, the tall R in the inferior leads.

As in every fascicular block, the axis turns towards the part of the ventricle activated last, here down and right. The rest of the conduction system still works, so the QRS stays under 120 ms.

›Go deeperShow less

Why it is a diagnosis of exclusion

Right axis deviation is common, and LPFB is its rarest cause. A large right ventricle, from pulmonary hypertension, lung disease or congenital heart disease, turns the axis right; so does acute strain on the right heart from a pulmonary embolism; so does a lateral myocardial infarction, by removing the leftward forces; and so does a thin, vertical heart in a young adult. LPFB is only called when none of these explains the axis, and a change from an older tracing is the strongest support.

Why it is rare

The posterior fascicle is short and broad, fanning out early, and it has a double blood supply, from both the left anterior descending artery and the posterior descending artery. Blocking it takes extensive disease, which is why isolated LPFB is rare and why, when it appears, it usually comes with other conduction damage.

Bifascicular block

LPFB more often comes with right bundle branch block than alone: bifascicular block, with only the left anterior fascicle still conducting, and more likely than RBBB with LAFB to progress to complete heart block. Choose With RBBB beside the title for its tracing.

Clinical impact

The left posterior fascicle stops conducting

↓

The lower wall of the left ventricle is reached late, from above

↓

Usually a sign of extensive heart diseaseLook for a cause

↓

With RBBB: one fascicle leftWatch

LPFB alone is rare. It is seen with coronary artery disease, particularly an infarct involving both the anterior descending and the posterior descending arteries, with cardiomyopathy, hypertension, and degenerative disease of the conduction system. Its main importance is what it adds to right bundle branch block: with both, the whole left ventricle depends on the anterior fascicle.

Management

1

Exclude the commoner causes

Right ventricular hypertrophy, lung disease, pulmonary embolism and a lateral infarct explain a right axis far more often. An echocardiogram and an older tracing settle most cases.

2

Look for heart disease

When LPFB is confirmed, look for the coronary and structural disease that usually lies behind it.

3

Watch the conduction system

With RBBB, or with syncope, monitor for intermittent complete heart block. A pacemaker is indicated when high-grade block is found, and alternating bundle branch block is itself an indication.

Differential

Right ventricular hypertrophy

Also a right axis, but with a tall R in V1, right atrial enlargement, and ST-T strain in V1 to V3.

Pulmonary embolism and lung disease

Acute right heart strain or chronic lung disease turn the axis right, often with a fast rate, S1Q3T3, or low voltages and a vertical P axis.

Lateral myocardial infarction

Removes the leftward forces, giving Q waves in I and aVL rather than the small initial r of LPFB.

Left anterior fascicular block

The mirror image: an axis of −45° or beyond, qR in I and aVL, rS in II, III and aVF.

References

1. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, Part III: Intraventricular Conduction Disturbances — Circulation, 2009
2. Hemiblocks revisited — Circulation, 2007
3. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay — Circulation, 2019
4. 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy — European Heart Journal, 2021