AV block can occur at which level of the conduction system?

• A. AV node, His bundle, or left/right bundle branches
• B. Only the AV node
• C. Only the SA node
• D. Only the Purkinje network

Answer: (A)

Conduction from the atria to the ventricles can be blocked at several sites along the AV conduction pathway. An AV block refers to impaired transmission anywhere from the AV node through the His bundle and into the bundle branches and Purkinje network. So blocks can occur at the AV node, at the His bundle, or in the left or right bundle branches (and their downstream Purkinje fibers). The SA node is the atrial pacemaker and its dysfunction causes sinus problems, not AV block. The site of block also helps predict the escape rhythm and QRS width: high (AV nodal) blocks often have narrow QRS escape rhythms, while infranodal blocks tend to have wider QRS escape rhythms.