Hemiblocks (Fascicular Blocks): A Practical ECG-Based Guide
Hemiblocks refer to conduction delays or blocks in one of the fascicles of the left bundle branch. The left bundle divides into two main fascicles—anterior and posterior—so involvement of one results in a left anterior hemiblock (LAHB) or left posterior hemiblock (LPHB). When two conduction pathways are involved simultaneously, the pattern is termed a bifascicular block. Recognition of these patterns on ECG is clinically important because they often reflect underlying structural heart disease and may predict progression to higher-degree atrioventricular (AV) block.
Left Anterior Hemiblock (LAHB)
Left anterior hemiblock is the most common fascicular block. It occurs due to delayed conduction through the left anterior fascicle, with impulses traveling preferentially down the posterior fascicle and then upward and leftward through the myocardium.
Key ECG features
Left axis deviation (usually −45° to −90°)
Small q waves in leads I and aVL
Small r waves in leads II, III, and aVF
Normal or near-normal QRS duration (<120 ms)
No ECG evidence of inferior myocardial infarction or left ventricular hypertrophy
Memory aid
“Q1 S3”: Q wave in lead I and S wave in lead III
Clinical relevance LAHB is often associated with ischemic heart disease, hypertension, and degenerative conduction system disease. On its own, it is usually benign, but in the presence of other conduction abnormalities, it gains prognostic significance.
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Left Posterior Hemiblock (LPHB)
Left posterior hemiblock is much less common because the posterior fascicle is thicker, shorter, and has a dual blood supply. Its diagnosis requires exclusion of other causes of right axis deviation.
Key ECG features
Right axis deviation (+90° to +120°)
Small r wave with deep S wave in lead I
Small q wave with tall R wave in lead III
Normal QRS duration
Absence of right ventricular hypertrophy or lateral myocardial infarction
Clinical relevance LPHB almost always indicates underlying heart disease, commonly ischemic or fibrotic. Its presence should prompt careful clinical correlation.
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Bifascicular Block
A bifascicular block occurs when two of the three main intraventricular conduction pathways are impaired. The most common combination is right bundle branch block (RBBB) with left anterior hemiblock.
Common patterns
RBBB + Left Anterior Hemiblock (most frequent)
RBBB + Left Posterior Hemiblock
ECG characteristics
Wide QRS complex due to bundle branch block
Axis deviation reflecting the involved fascicle
Typical RBBB morphology in V1 with secondary axis shift
Clinical significance Bifascicular block suggests advanced conduction system disease and carries an increased risk of progression to complete (third-degree) AV block, especially if associated with syncope or PR prolongation. Such patients may require close monitoring and, in selected cases, pacemaker evaluation.
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Why Hemiblocks Matter in Practice
They help localize conduction disease within the His–Purkinje system
They may be the first ECG clue to underlying structural or ischemic heart disease
In combination with other conduction abnormalities, they have prognostic and therapeutic implications
A systematic ECG approach—assessing QRS width, frontal plane axis, and exclusion of mimicking conditions—is essential for accurate diagnosis.
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Key takeaway:
Left anterior hemiblock is common and often benign in isolation, left posterior hemiblock is rare and usually pathologic, and bifascicular block warrants vigilance due to the risk of progression to advanced AV block.
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