Left Anterior Fascicular Block
The fascicular block alone: QRS under 120 ms
Left Anterior Fascicular Block
cautionThe upper, front wall of the left ventricle is reached a little late, from below, turning the axis to −45° or beyond: qR in I and aVL, rS in II, III and aVF, with a QRS under 120 ms.
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.
- Measure the QRS.
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96 ms: not wide. A fascicular block changes the direction of the QRS far more than its length. - Find the axis.
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Lead I is positive and aVF negative, so the axis is to the left; II is negative too, so it is beyond −30°. It measures −54° here. See −54° on the axis page. - Look at I and aVL.
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A small q, then a tall R: qR. The q is the first forces pointing down; the R is the late upper wall pulling up and to the left. - Look at II, III and aVF.
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A small r, then a deep S: rS. The same two phases, seen from below. The S in III, 15.6 mm, is almost twice the R in I, 8.2 mm, which is the quick way to see an axis beyond −45°. - Check the R-peak time in aVL.
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50 ms, at or over the 45 ms the criteria ask for: the upper wall is reached late. - Look for what is not there.
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No wide QRS, no R′ in V1, normal T waves. Choose With RBBB beside the title to add a right bundle branch block.
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| Frontal axis | −45° to −90° | −54° here. −30° to −45° is left axis deviation, but not enough on its own for LAFB. |
| I and aVL | qR | q 1.1 mm and R 8.2 mm in I. |
| II, III and aVF | rS | S 15.6 mm in III. The S in III is usually deeper than the S in II. |
| R-peak time in aVL | 45 ms or more | 50 ms here. |
| QRS duration | Under 120 ms | 96 ms here. |
| T waves | Normal |
These are the criteria of the 2009 AHA/ACCF/HRS recommendations on intraventricular conduction.
Mechanism
The left bundle is not one pathway but two. The anterior fascicle runs up to the anterior and lateral walls of the left ventricle, near the base; the posterior fascicle, broader, runs down to the inferior and posterior walls. Normally both fire together, and their forces largely balance, leaving a mean axis near +60°.
When the anterior fascicle fails, the impulse enters the left ventricle through the posterior fascicle alone. The lower, back wall depolarises first, pointing down and slightly right: the small q in I and aVL, the small r in II, III and aVF. The upper, front wall is reached a few tens of milliseconds later, spreading upwards from below, and its large, late force points up and to the left: the tall R in aVL and I, the deep S in the inferior leads.
The axis turns towards the late forces, which is the rule for every fascicular block: the QRS points towards the part of the ventricle activated last. The delay is short because the rest of the conduction system still works, so the QRS stays under 120 ms.
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Why the axis alone is not enough
Many things move the axis left: a large left ventricle, an inferior myocardial infarction, a horizontal heart, Wolff-Parkinson-White. LAFB is the one that does it with this shape: a q in I and aVL, an initial r in the inferior leads, and a late R in aVL. An axis of −30° to −45° with a normal shape is left axis deviation, not LAFB.
LAFB and an inferior infarct
An inferior myocardial infarction also turns the axis left, by removing the inferior forces. The difference is the start of the QRS: LAFB keeps a small initial r in II, III and aVF; an inferior infarct replaces it with a Q (QS or Qr). The two can coexist, and then the Q waves win.
Bifascicular block
The right bundle and the left anterior fascicle share a blood supply and often fail together: right bundle branch block with the axis of LAFB, bifascicular block, with only the left posterior fascicle still conducting. Choose With RBBB beside the title for its tracing.
Masking and mimicking
LAFB can hide an anterior or inferior myocardial infarction and alter the voltage criteria for left ventricular hypertrophy. It can produce small q waves in V2 and V3 when the chest leads are placed high, which should not be read as an old infarct.
Clinical impact
LAFB is the most common conduction block, and becomes more common with age. It goes with hypertension, coronary artery disease, aortic valve disease, cardiomyopathy and age-related fibrosis of the conduction system, and it can appear during an anterior myocardial infarction. Isolated, it rarely leads to anything further. Its importance grows when it is joined by right bundle branch block, because then only one fascicle is left.
Management
Look for the cause
Isolated LAFB needs no treatment. Look at the rest of the tracing and the patient for hypertension, left ventricular hypertrophy and coronary disease; an echocardiogram when the history suggests structural heart disease.
Count the fascicles
With RBBB, it is bifascicular block. With no symptoms, no pacing is needed, and the tracing is worth repeating over time.
Take syncope seriously
Syncope with bifascicular block raises the question of intermittent complete heart block. Ambulatory monitoring or an electrophysiology study answers it, and a pacemaker is indicated when high-grade block is found.
Differential
Also a left axis, but with Q waves (QS or Qr) in II, III and aVF instead of the small initial r of LAFB.
Can come with a left axis, but the QRS is 120 ms or more, with no septal q in I and V6 and a deep QS in V1.
Can turn the axis left, usually not past −30°, with tall voltages and a strain pattern rather than a q in aVL and deep rS in the inferior leads.
The mirror image: an axis of +110° or more, rS in I and aVL, qR in II, III and aVF.
References
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, Part III: Intraventricular Conduction Disturbances — Circulation, 2009
- Hemiblocks revisited — Circulation, 2007
- 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay — Circulation, 2019
- 2021 ESC Guidelines on cardiac pacing and cardiac resynchronization therapy — European Heart Journal, 2021