bundle branch

The bundle branches are the two main forks of the heart's electrical highway. Where the bundle of His ends, it divides into a right bundle branch and a left bundle branch, which run down either side of the interventricular septum — the muscular wall between the two ventricles — carrying the impulse toward the apex of the heart and then out to the working muscle.

The right bundle branch is a slender single cord serving the right ventricle. The left bundle branch is broader and soon splits into fascicles (commonly an anterior and a posterior fascicle, plus septal fibers) to cover the larger left ventricle. All of these branches terminate in the Purkinje fibers, which spread the signal into the muscle itself. The whole arrangement exists to make both ventricles contract almost simultaneously, so the heart squeezes as a coordinated unit.

If one branch is blocked — a bundle branch block — the impulse can no longer travel quickly down that side. The affected ventricle then has to be activated more slowly, cell-to-cell through ordinary muscle, which widens the QRS complex on the ECG and creates the characteristic right or left bundle branch block patterns. A block can be a benign lifelong finding or a clue to underlying heart disease, depending on the context.