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Bundle Branch Blocks

Bundle branch blocks are disorders of electrical conduction distal to the bifurcation of the His bundle, causing changes in the depolarization of the ventricles.

This disturbance in ventricular depolarization causes marked changes in the QRS complex. In severe cases (Complete Bundle branch blocks), it causes an increase in the duration and changes in the morphology of the QRS complex.

The depolarization disorder also causes an alteration in ventricular repolarization. So the T wave also presents morphological changes in Complete Bundle Branch Block 1.

Anatomy of the Bundle Branch Blocks

Intraventricular Conduction

Rama Derecha, Rama Izquierda y Fascículos

1- Atrioventricular Node and Bundle of His. 2- Right Bundle. 3- Left Bundle.
4- Anterior Fascicle. 5- Posterior Fascicle.

The Bundle of His divides into two branches:

  • Right Branch stimulates the right ventricle and the right third of the interventricular septum.
  • The Left Branch stimulates the left ventricle and the left two-thirds of the interventricular septum.

While the right arm remains undivided, the left branch subdivides into two smaller branches (fascicles).

  • Anterior Fascicle, which transmits the impulse to the anterior-superior region of the left ventricle.
  • Posterior Fascicle, which transmits the impulse to the posterior-inferior region of the left ventricle.
  • .

Any disturbance in one of these three branches causes conduction disorder and electrocardiographic abnormalities.

Complete Bundle Branch Blocks

A Complete Bundle Branch Block occurs when there is a complete obstruction of the impulses in the right branch or left branch before the subdivision.

On the Electrocardiogram, the main characteristic of a Complete Bundle Branch Block is the QRS widening (greater than 0.12s).

This is caused because the electrical stimulation depolarizes first the ventricle with normal branch, and then, through myocardium, depolarizes the ventricle with affected branch.

This increases the time of ventricular depolarization and therefore a widening of the QRS complex.

More Information: Right Bundle Branch Block, Left Bundle Branch Block.

Left Fascicular Blocks or Hemiblocks

The fascicular blocks occur when the interruption of the impulse is in one of the fascicles of the Left Bundle Branch.

These blocks do not increase the time of ventricular depolarization. So the QRS complex is narrow.

Its main feature in the electrocardiogram is an important Axis Deviation.

More Information: Left Fascicular Blocks.

Incomplete Bundle Branch Blocks

In incomplete bundle branch block, there is no interruption of the stimulus. The impulse is conducted by both branches, but more slowly through the affected branch.

In Electrocardiogram are observed with minimal widening of the QRS (between 0.1s and 0.12s) and morphological changes similar to Complete Bundle Branch Blocks.

Incomplete Right Bundle Branch Block:

  • QRS complex duration between 0.1 and 0.12s.
  • Morphology of RSR 'in V1 or V2.

Incomplete Left Bundle Branch Block:

  • QRS duration between 0.1 and 0.12s.
  • High R wave in V4 and V5.
  • Absence of initial Q wave in I, aVL, V5 and V6.
  • Slurring of the initial part of R wave in V5-V6. 2

In both Incomplete bundle branch blocks could be ST segment or T Wave abnormalities.


  • 1. Goldberger A. Clinical Electrocardiography: A Simplified Approach. 7th ed. Philadelphia: Mosby Elservier; 2006.
  • 2. Schamroth L, Bradlow BA. Heart J. Incomplete Left Bundle-Branch Block. 1964 Mar; 26(2): 285–288.


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