Which statement BEST defines Second Degree AV Block Type II?

Which statement BEST defines Second Degree AV Block Type II?

• A. PRI lengthens until a QRS is dropped
• B. P waves and QRS dissociate completely
• C. Dropped QRS with no PRI prolongation
• D. QRS is narrow with regular rhythm

Answer: (C)

Mobitz II shows intermittent non-conducted P waves with dropped QRS complexes, but the PR interval on the beats that do conduct stays constant and does not progressively lengthen before a drop. This pattern points to a block below the AV node (often in the His-Purkinje system). The key feature is the sudden loss of QRS without prior PR prolongation, which distinguishes it from Mobitz I (where the PR interval lengthens before a drop). The QRS duration can be normal or widened depending on additional conduction disease, but the defining clue is the dropped QRS without a preceding change in PR. The other descriptions don’t fit Mobitz II: progressive PR prolongation before a drop describes Mobitz I; complete dissociation of P waves and QRS describes third-degree block; and simply noting a narrow QRS with regular rhythm doesn’t capture the characteristic intermittent dropped beats.

Mobitz II shows intermittent non-conducted P waves with dropped QRS complexes, but the PR interval on the beats that do conduct stays constant and does not progressively lengthen before a drop. This pattern points to a block below the AV node (often in the His-Purkinje system). The key feature is the sudden loss of QRS without prior PR prolongation, which distinguishes it from Mobitz I (where the PR interval lengthens before a drop). The QRS duration can be normal or widened depending on additional conduction disease, but the defining clue is the dropped QRS without a preceding change in PR.

The other descriptions don’t fit Mobitz II: progressive PR prolongation before a drop describes Mobitz I; complete dissociation of P waves and QRS describes third-degree block; and simply noting a narrow QRS with regular rhythm doesn’t capture the characteristic intermittent dropped beats.