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Left Anterior Fascicular Block (LAFB) - ECG

Left Anterior Hemiblock (LAHB), Left Anterior Fascicular Block

Left Bundle Branch
Left Anterior Fascicular Block
Electrical Vector and Left Anterior Fascicular Block
ECG and Left Anterior Fascicular Block
Differential Diagnosis
Left Anterior Fascicular Block and Old Anteroseptal Infarct

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Left Bundle Branch

The Left Bundle Branch (LBB) divides into:

Left Anterior Fascicle

It directs forward and laterally
It activates the anterior and lateral parts of the left ventricle

Left Posterior Fascicle

It directs backward and downward
It activates the posterior and inferior parts of the left ventricle

The Left Anterior Fascicle is thinner than the Left Posterior Fascicle

Left Anterior Fascicular Block is therefore more common (98%)
Left Posterior Fascicular Block is very rare (2%)

Left Anterior Fascicular Block

electrical vector deviation, and left anterior hemiblock

In left anterior Fascicular Block:

The lower part of the left ventricle is activated first

Through the posterior fascicle
A small initial vector (V_I) is created

Then the upper lateral part is activated

Through the myocardium, because the left anterior fascicle is blocked
A main cardiac vector (V_H) is created and it turns upward to the left

Electrical Vector and Left Anterior Fascicular Block

ECG quick diagnosis of left anterior hemiblock (LAH), Positive I lead, Negative II III leads, narrow QRS complex

Left Anterior Fascicular Block (Quick Diagnosis)

Left anterior Fascicular Block is diagnosed using limb leads (I, II, III)
The main vector (V_H) points in the frontal plane upward to the left

Lead I is positive
Leads II and III are negative

ECG and Left Anterior Fascicular Block

ECG left anterior hemiblock (LAH), R peak time in lead aVL of 45ms or more

Narrow QRS complex < 0.12s

The QRS is slightly widened by about 20ms but does not exceed 0.12s

Left axis deviation > -30º (most commonly -45º to -90º)

Lead I is positive
Leads II and III are negative

Prolonged R wave peak time in aVL > 45ms

The time from the beginning of the QRS complex to the peak of the R wave is more than 45ms
Due to prolonged activation of the left ventricle

Specifically, the part that should be activated through the blocked anterior fascicle

ECG criteria, left anterior hemiblock (LAH), left axis deviation, qR pattern in aVL, R peak time, narrow QRS complex

Differential Diagnosis

Left axis deviation, left anterior hemiblock

When diagnosing Left Anterior Fascicular Block (LAFB), it is crucial to exclude all causes of left axis deviation:

Left Anterior Fascicular Block and Old Anteroseptal Infarct

Precordial Leads (V1-V2) are placed parasternally in the 4th intercostal space

Physiological P wave (V1-V2) is equifasic (positive-negative)

Leads (V1-V2) are used for differential diagnosis:

Left Anterior Fascicular Block vs. Old Anteroseptal Infarct

Correct position of the precordial electrodes, diagnosis anteroseptal myocardial infarction, left anterior hemiblock (LAF)

ECG erroneous diagnosis of anteroseptal myocardial infarction, Left anterior hemiblock (LAF)

Differential Diagnosis Using V1-V2

Properly placed V1-V2 electrodes in the 4th intercostal space

P wave is equifasic
Sometimes there is a QS configuration in V1-V2, which can indicate:
Left Anterior Fascicular Block, or Old Anteroseptal Infarct

If V1-V2 are placed lower (6th intercostal space)

P wave is monofasic positive
rS configuration (V1-V2) appears

This indicates Left Anterior Fascicular Block
Old infarct would still have QS configuration (without r wave)

If V1-V2 are placed higher (2nd intercostal space)

P wave is monofasic negative

Left axis deviation, frontal plane, left anterior hemiblock

ECG left anteriro fascicular block (LAFB), narrow QRS, left axis deviation, R peak time

Left Anterior Fascicular Block

Narrow QRS complexes (< 0.12s)
Left Axis Deviation (> -30º)

Positive QRS in lead I
Negative QRS in leads II and III

ECG left anterior hemiblock (LAH), small Q wave (I, aVL), small r wave (II, III, aVF), narrow QRS (80-110ms), prolonged R peak time

Left Anterior Fascicular Block

Narrow QRS complexes (< 0.12s)
Left Axis Deviation (> -30º)

Positive QRS in lead I
Negative QRS in leads II and III

ECG Trifascicular block, Left anterior hemiblock, First degree AV block, Right bundle branch block (RBBB)

Trifascicular Block (Left Anterior Fascicular Block + AV Block of 1st Degree + RBBB)

Trifascicular Block is a combination of 3 blocks:

Left Anterior Fascicular Block

Positive QRS in lead I
Negative QRS in leads II and III

AV Block of 1st Degree

Prolonged PQ interval > 0.2s

Right Bundle Branch Block (RBBB)

Wide QRS > 0.12s
rSR' in lead V1
Large S wave in lead V6

Sources

ECG from Basics to Essentials Step by Step
litfl.com
ecgwaves.com
metealpaslan.com
medmastery.com
uptodate.com
ecgpedia.org
wikipedia.org
Strong Medicine
Understanding Pacemakers

Home /

Left Anterior Fascicular Block (LAFB) - ECG

Left Anterior Hemiblock (LAHB), Left Anterior Fascicular Block

Left Bundle Branch
Left Anterior Fascicular Block
Electrical Vector and Left Anterior Fascicular Block
ECG and Left Anterior Fascicular Block
Differential Diagnosis
Left Anterior Fascicular Block and Old Anteroseptal Infarct

12-lead ECG

Smartwatch ECG

Have a doctor’s or smartwatch ECG?

Upload your 12-lead, smartwatch or smart device ECG.
Get a physician’s report within 24 hours.

Get my ECG reviewed →

Reviewed by a physician · Your questions answered · Not AI

Left Bundle Branch

The Left Bundle Branch (LBB) divides into:

Left Anterior Fascicle

It directs forward and laterally
It activates the anterior and lateral parts of the left ventricle

Left Posterior Fascicle

It directs backward and downward
It activates the posterior and inferior parts of the left ventricle

The Left Anterior Fascicle is thinner than the Left Posterior Fascicle

Left Anterior Fascicular Block is therefore more common (98%)
Left Posterior Fascicular Block is very rare (2%)

Left Anterior Fascicular Block

In left anterior Fascicular Block:

The lower part of the left ventricle is activated first

Through the posterior fascicle
A small initial vector (V_I) is created

Then the upper lateral part is activated

Through the myocardium, because the left anterior fascicle is blocked
A main cardiac vector (V_H) is created and it turns upward to the left

Electrical Vector and Left Anterior Fascicular Block

Left Anterior Fascicular Block (Quick Diagnosis)

Left anterior Fascicular Block is diagnosed using limb leads (I, II, III)
The main vector (V_H) points in the frontal plane upward to the left

Lead I is positive
Leads II and III are negative

ECG and Left Anterior Fascicular Block

Narrow QRS complex < 0.12s

The QRS is slightly widened by about 20ms but does not exceed 0.12s

Left axis deviation > -30º (most commonly -45º to -90º)

Lead I is positive
Leads II and III are negative

Prolonged R wave peak time in aVL > 45ms

The time from the beginning of the QRS complex to the peak of the R wave is more than 45ms
Due to prolonged activation of the left ventricle

Specifically, the part that should be activated through the blocked anterior fascicle

Differential Diagnosis

When diagnosing Left Anterior Fascicular Block (LAFB), it is crucial to exclude all causes of left axis deviation:

Left Anterior Fascicular Block and Old Anteroseptal Infarct

Precordial Leads (V1-V2) are placed parasternally in the 4th intercostal space

Physiological P wave (V1-V2) is equifasic (positive-negative)

Leads (V1-V2) are used for differential diagnosis:

Left Anterior Fascicular Block vs. Old Anteroseptal Infarct

Differential Diagnosis Using V1-V2

Properly placed V1-V2 electrodes in the 4th intercostal space

P wave is equifasic
Sometimes there is a QS configuration in V1-V2, which can indicate:
Left Anterior Fascicular Block, or Old Anteroseptal Infarct

If V1-V2 are placed lower (6th intercostal space)

P wave is monofasic positive
rS configuration (V1-V2) appears

This indicates Left Anterior Fascicular Block
Old infarct would still have QS configuration (without r wave)

If V1-V2 are placed higher (2nd intercostal space)

P wave is monofasic negative

Left Anterior Fascicular Block

Narrow QRS complexes (< 0.12s)
Left Axis Deviation (> -30º)

Positive QRS in lead I
Negative QRS in leads II and III

Left Anterior Fascicular Block

Narrow QRS complexes (< 0.12s)
Left Axis Deviation (> -30º)

Positive QRS in lead I
Negative QRS in leads II and III

Trifascicular Block (Left Anterior Fascicular Block + AV Block of 1st Degree + RBBB)

Trifascicular Block is a combination of 3 blocks:

Left Anterior Fascicular Block

Positive QRS in lead I
Negative QRS in leads II and III

AV Block of 1st Degree

Prolonged PQ interval > 0.2s

Right Bundle Branch Block (RBBB)

Wide QRS > 0.12s
rSR' in lead V1
Large S wave in lead V6

Sources

ECG from Basics to Essentials Step by Step
litfl.com
ecgwaves.com
metealpaslan.com
medmastery.com
uptodate.com
ecgpedia.org
wikipedia.org
Strong Medicine
Understanding Pacemakers