The cardiac electrical system has two bundle branches - the right and the left - and in BBB, one or the other of these bundle branches is no longer conducting electrical impulses normally. 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." The significance of having a BBB will vary from person to person, so it is important for you to discuss this ECG finding with your own doctor.
This article will help you understand BBB, so that when you talk to your doctor about it you should be better able to understand the issues, and know which questions to ask.
What Are the Bundle Branches, and What Do They Do?
The 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. 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 Normal Cardiac Electrical System
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.
Bundle Branch Block
BBB occurs when one of the bundle branches becomes diseased or damaged, and stops conducting electrical impulses; that is, one of the bundle branches becomes "blocked." The chief effect of a bundle branch block 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.The next page talks about right bundle branch block (RBBB) and left bundle branch block (LBBB).
