Bundle Branch Block

What are the bundle branches, and what do they do?

The Normal Cardiac Electrical System

To summarize, the heart’s electrical impulse originates in the in the sinus node in the upper right atrium, then spreads across both atria, then travels through the AV node. Leaving the AV node, the electrical impulse penetrates into the ventricles via the His bundle. From the His bundle, the electrical impulse enters the two “bundle branches” (the right and the left). The right and left bundle branches send the electrical impulse to the right and left ventricle, respectively. When the bundle branches are functioning normally, the right and left ventricles contract nearly simultaneously.

Figure 2 - The Normal QRS Complex

In this image of a normal ECG, the QRS complex represents the electrical impulse as it is being distributed, via the bundle branch system, throughout the ventricles. Since normally both ventricles receive the electrical impulse at the same time, the normal QRS complex is relatively narrow (generally less than 0.1 second in duration.)

Figure 3 - Normal Bundle Branch Conduction

To form this normal QRS complex, the electrical impuse needs to travel to both ventricles at the same time. The in this figure, purple arrows indicate the electrical impulse, traveling down the bundle branches and causing the ventricles to beat simultaneously.

BBB occurs when one of the bundle branches becomes diseased or damaged, and stops conducting electrical impulses; that is, a bundle branch becomes “blocked.” The chief effect of a bundle branch block is to disrupt the normal, coordinated and simultaneous distribution of the electrical signal to the two ventricles.

Part 2 - Right and Left Bundle Branch Block

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