Can bundle branch block be fatal?

Not usually: Right bundle branch block is pretty benign. Most cases of left bundle branch block are associated with significant heart disease. It is the severity of that heart disease and the type of disease which could lead to a fatal outcome. The bundle branch block itself is not likely to be fatal. If it progressed to complete heart block, then a pacemaker would be needed and conceivably it could be fatal.

Bundle branch block: Right bbb is usually benign and seen in about 15-20% of healthy individuals, but combined with fasicular block - anterior or posterior - it may be an indication of electrical/structural disease of the heart. Left bbb tends to indicate structural heart disease - needs evaluation. Bbb may progress to complete heart block - slow heart rhythms - and could require a pacemaker. Bbb fatal <2%. See md.