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Clinical ECG Interpretation

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  1. Introduction to ECG Interpretation
    6 Chapters
  2. Arrhythmias and arrhythmology
    24 Chapters
  3. Myocardial Ischemia & Infarction
    22 Chapters
  4. Conduction Defects
    11 Chapters
  5. Cardiac Hypertrophy & Enlargement
    5 Chapters
  6. Drugs & Electrolyte Imbalance
    3 Chapters
  7. Genetics, Syndromes & Miscellaneous
    7 Chapters
  8. Exercise Stress Testing (Exercise ECG)
    6 Chapters
Section 4, Chapter 10
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Fascicular block (hemiblock): Left anterior & left posterior fascicular block

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Fascicular block (hemiblock): left anterior fascicular block & left posterior fascicular block

Fascicular blocks were previously referred to as hemiblocks, but the latter term has been deprecated. The left bundle branch is subdivided into the following two fascicles: (1) the anterior (anterosuperior) fascicle, which delivers the electrical impulse to the anterior wall of the left ventricle; (2) the posterior (posteroinferior) fascicle, which delivers the electrical impulse to the posterior and inferior walls of the left ventricle. Anatomical or functional block in the anterior fascicle leads to left anterior fascicular block. Similarly, left posterior fascicular block is due to block in the posterior fascicle. Approximately 5–10% of all individuals have a third fascicle – the median or centroseptal fascicle – which gives off Purkinje fibers to the interventricular septum.

Fascicular blocks occur due to anatomical or functional blocks in a fascicle. This alters the ECG curve in a characteristic fashion which is rather easy to spot. The hallmark of fascicular blocks is the deviation of the electrical axis. The QRS duration is only slightly prolonged but it does not reach 0.12 s.

Block in the anterior fascicle causes left anterior fascicular block (LAFB). Block in the posterior fascicle causes left posterior fascicular block (LPFB). In case of a fascicular block, the wall/walls without fascicular supply will depend on impulses spreading from the other part of the ventricle (where the fascicle is intact).

Left anterior fascicular block (anterior hemiblock) and left posterior fascicular block (posterior hemiblock).
Figure 1. Left anterior fascicular block (anterior hemiblock) and left posterior fascicular block (posterior hemiblock).

Left anterior fascicular block (LAFB)

Left anterior fascicular block is due to anatomical or functional block in the anterior fascicle. Depolarization of the left ventricle will depend entirely on the posterior fascicle. The initial vector will be directed inferiorly (Figure 1, panel A), yielding a small r-wave in inferior leads (II, III and aVF) and small q-wave in lateral leads (aVL, I and -aVR). The second vector, which is considerably stronger, is directed to the left, back and upwards; this results in a deep S-wave in inferior leads and large R-wave in left lateral leads. Hence, inferior leads show rS complex and lateral leads show qR complex. Occasionally the T-wave in lead aVL will be inverted and in some cases lead I will display a monophasic R-wave instead of qR complex. The electrical axis will be shifted to the left (left axis deviation), ranging between -45° and -90°. The QRS duration will be slightly prolonged (the prolongation ranges between 0.01 to 0.04 seconds).

ECG criteria for left anterior fascicular block (LAFB)

  • Electrical axis between -45° to -90°. If the electrical axis is -30° to -45, probable LAFB may be diagnosed.
  • QRS duration <0,12 seconds but slightly prolonged.
  • aVL shows qR complex. V5–V6 usually also shows qR complexes.
  • Leads II, III and aVF display rS complexes.

Causes of left anterior fascicular block (LAFB)

LAFB may occur in persons who are otherwise healthy. The majority of those with LAFB, however, have significant heart disease. Myocardial infarction, coronary artery disease, left ventricular hypertrophy, dilated cardiomyopathy, hypertrophic cardiomyopathy, degenerative disease, hypertension, hyperkalemia, myocarditis, amyloidosis may all cause LAFB.

Prognosis of left anterior fascicular block (LAFB)

Isolated LAFB is considered a benign conduction defect. Roughly 7% of cases progress to bifascicular block (which means that the LAFB is accompanied by a right bundle branch block), while 3% progress to third-degree AV block (complete heart block).

Noteworthy about left anterior fascicular block (LAFB)

  • LAFB may imitate anteroseptal infarction.
  • rS complexes in leads II, III and aVF may mask Q-waves from a prior inferior infarction.

Left posterior fascicular block (LPFB)

Left posterior fascicular block is much less common than LAFB. This is due to the fact that the posterior fascicle is larger and it has a greater arterial supply. Depolarization of the left ventricle will depend entirely on impulses from the anterior fascicle if the posterior one is defective. The vector is initially directed upwards and to the left, which yields a q-wave in lead aVF and R-wave in lead I. The second vector is directed downwards and to the right, which results in a prominent R-wave in lead aVF and an equally prominent S-wave in lead I. The electrical axis will be more positive than 90° (right axis deviation). As in LAFB, the QRS duration will be prolonged by approximately 0.01 to 0.04 s, but the total QRS duration will not reach 0.12 seconds. Refer to Figure 1.

ECG criteria for left posterior fascicular block (LPFB)

  • Electrical axis +90° to +180°.
  • rS complexes in leads I and aVL.
  • qR complexes in inferior leads (II, III and aVF).
  • Q-wave is mandatory in leads III and aVF.
  • QRS duration <0,12 seconds.

Causes of left posterior fascicular block (LPFB)

Degenerative processes, ischemic heart disease, hyperkalemia, myocarditis, amyloidosis and acute cor pulmonale may all cause LPFB. Importantly, LPFB is highly unusual in otherwise healthy individuals.

ECG criteria hemiblock fascicular block (left anterior, left posterior)


  • Establishing a diagnosis of LPFB requires that there are no clinical or ECG criteria of right ventricular hypertrophy present. Right ventricular hypertrophy is actually more common than LPFB and may cause ECG findings similar to LPFB.
  • LPFB may imitate inferior infarction.
  • LPFB may mask lateral infarction.
  • T-wave inversion may occur in inferior leads and simulate post-ischemic T-waves.

Next chapter

Non-specific Intraventricular Conduction Delay

Overview of Intraventricular Conduction Defects

Right Bundle Branch Block (RBBB)

Left Bundle Branch Block (LBBB)


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