AV blocks can take place in which structures?

• A. AV node, bundle of His, left and right bundle branches
• B. SA node and AV node
• C. Purkinje fibers only
• D. Atria and ventricles

Answer: (A)

AV blocks occur when the electrical signal cannot travel from the atria to the ventricles along the AV conduction axis. The sites along this pathway where a block can occur include the AV node, the bundle of His, and the left and right bundle branches. In other words, the block can happen anywhere from just after the AV node through the His-Purkinje system that connects to the ventricles. The SA node is the impulse origin, not a site of AV block, and blocks in the Purkinje fibers alone would cause intraventricular conduction issues rather than a failure of atrial-to-ventricular transmission. Blocks in the atria or in the ventricles themselves don’t describe AV conduction block.