Which AV block type is most likely to progress to complete heart block and may necessitate pacing?

• A. Mobitz I
• B. First-degree AV block
• C. Mobitz II
• D. Third-degree AV block

Answer: (C)

Progression to complete heart block depends on where the conduction failure is located. Mobitz II reflects a disease of the His-Purkinje system below the AV node, where conduction can fail abruptly, producing dropped beats without a preceding change in the PR interval. Because the block is in the distal conduction pathway, the risk of advancing to complete heart block is higher, making pacing often necessary even if pauses are infrequent or asymptomatic. By contrast, Mobitz I (Wenckebach) involves the AV node with progressively lengthening PR intervals before a dropped beat and is less likely to progress to complete block. First-degree AV block is a prolonged PR interval with 1:1 conduction and typically doesn’t progress to complete block, while third-degree block is already complete block and requires pacing, but the scenario described points to Mobitz II as the stage most likely to worsen.