Bundle branch blocks can take place in which structures?

• A. Left bundle branch and right bundle branch
• B. AV node only
• C. Purkinje fibers only
• D. SA node only

Answer: (A)

Bundle branch blocks arise when the ventricular conduction pathway is interrupted in one of the main pathways that carry impulses to the ventricles. The two structures that can be blocked are the left bundle branch and the right bundle branch, which respectively activate the left and right ventricles. When one of these branches is blocked, the ventricles activate asynchronously, leading to a widened QRS and the characteristic patterns seen on ECG. Other sites listed relate to different types of conduction problems: AV node blocks affect atrioventricular conduction, SA node issues affect pacing, and while Purkinje fibers are part of the ventricular conduction system, the classic bundle branch block specifically involves the left or right bundle branches.