The Normal Cardiac Electrical SystemThe bundle branches are an important part of the cardiac electrical system, the system that regulates the heart rhythm and coordinates the pumping action of the heart. Here is a quick primer on the normal cardiac electrical system. You will be able to better follow this discussion on BBB if you have a look at this primer.
The normal cardiac electrical system is also depicted in Figure 1 on this page.
Figure 1 - The Normal Electrical System: AVN = AV node, His = His bundle, RBB = right bundle branch, LBB = left bundle branch, RA = right atrium, RV = right ventricle, LA = left atrium, LV = left ventricle
To review, 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 ventricles, respectively. When the bundle branches are functioning normally, the right and left ventricles contract nearly simultaneously.
Figure 2 shows the normal QRS complex on a normal ECG. The ECG is merely a visual representation of the heart's electrical impulse as it moves through the heart. On the ECG, the QRS complex represents the electrical impulse as it is being distributed, by the bundle branch system, throughout the ventricles. Since normally both ventricles receive the electrical impulse at the same time, the normal QRS complex, as depicted in this figure, is relatively narrow (generally less than 0.1 second in duration.)
Figure 3 shows, in more detail, how the bundle branches normally distribute the heart's electrical impulse to the two ventricles. A normal QRS complex is formed when the electrical impulse reaches both ventricles at the same time. In this figure, purple arrows indicate the electrical impulse as it travels down the bundle branches and causes the ventricles to beat simultaneously.
What Is Bundle Branch Block?
As we have just seen, the bundle branches work to evenly distribute the spread of the cardiac electrical impulse across the ventricles, so that when the ventricles contract (to eject blood out of the heart), they do so in a coordinated and efficient fashion. The right bundle branch delivers the electrical impulse to the right ventricle, and the left bundle branch delivers the impulse to the left ventricle.
In BBB, one or the other (or both) of these bundle branches no longer conduct electrical impulses normally. This can occur from disease or damage to one of the bundle branches, or it may occur for no apparent reason in completely healthy people. When the electrical impulse is delayed in reaching its respective ventricle, the delay shows up as a distinctive pattern on the ECG called a BBB. The chief effect of a BBB is to disrupt the normal, coordinated and simultaneous contraction of the two ventricles. The contraction of one ventricle (the one whose bundle branch is blocked) occurs slightly after the contraction of the other.
People with BBB usually will have either right bundle branch block (RBBB) or left bundle branch block (LBBB), depending on which of the two bundle branches is "blocked." Sometimes both bundle branches are affected, and the BBB pattern on the ECG is not clearly identifiable as either right or left BBB - in this case, the BBB is referred to as an intraventricular conduction delay (IVCD).
The next page talks about right bundle branch block (RBBB) and left bundle branch block (LBBB).