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A bundle branch block is a defect of the bundle branches or fascicles in the electrical conduction system of the heart. bundle branch block is a defect of the bundle branches or fascicles in the electrical conduction system of the heart.
The left bundle branch block can be further sub classified into: 1  Left anterior fascicular block. In this case only the anterior half of the left bundle branch (fascicle) is involved. 2  Left posterior fascicular block. Only the posterior part of the left bundle branch is involved.