Updated December 16, 2014.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Right bundle branch block (RBBB)

In RBBB, the right bundle branch no longer conducts electricity normally, and the heart's electrical impulse enters the ventricles using only (or mainly) the left bundle branch - which means the left ventricle receives the electrical impulse first. Then, from the left ventricle, the electrical impulse finally makes its way to the right ventricle. As a result, the two ventricles no longer receive the electrical impulse simultaneously.

First the left ventricle receives the electrical impulse, then the right.

Left Bundle Branch Block (LBBB)

Figure 4 depicts left bundle branch block, LBBB. In LBBB, the opposite occurs. Here, the left bundle branch no longer conducts electricity normally. So the electrical impulse enters the ventricles through the right bundle branch, and is carried first to the right ventricle (first panel). From there, it finally spreads to the left ventricle (second panel). Once again, the two ventricles no longer receive the electrical impulse simultaneously. First the right ventricle receives the electrical impulse, then the left.
With either type of BBB, therefore, the electrical signal spreads across the ventricles sequentially, as opposed to simultaneously. This sequential spread of the impulse (i.e., first the right ventricle, then the left, or vice versa) means that it takes longer for the impulse to spread throughout both ventricles.

And this means that, with either type of BBB, the QRS complex on the ECG is wider than normal.

The next page talks about the significance of RBBB and LBBB.