Which type of second-degree AV block is considered more serious?

• A. Mobitz 1
• B. Mobitz 2
• C. First-degree AV block
• D. Third-degree AV block

Answer: (B)

Mobitz type II second-degree AV block is considered more serious than Mobitz type I and other types of blocks because it is associated with a higher risk of progressing to complete heart block. In Mobitz type II, the QRS complexes are dropped without prior lengthening of the PR interval, indicating an unpredictable block that can lead to significant cardiovascular complications. This pattern suggests a more severe underlying issue with the conduction system than Mobitz type I, where the block is more predictable and often less dangerous.

Additionally, Mobitz type II often occurs below the AV node, involving the bundle of His or the right and left bundle branches. The inconsistency in conduction can lead to decreased cardiac output and symptoms such as syncope or even sudden cardiac arrest.

First-degree AV block is typically regarded as benign, with a consistent PR interval that does not cause significant concern. Third-degree AV block, while serious, is classified differently than Mobitz II and represents complete heart block where no impulses from the atria reach the ventricles.

Thus, Mobitz type II's propensity to deteriorate and its characteristics contribute to its classification as a more serious condition among these options.