Which type of block causes one P wave to precede each QRS complex until a QRS complex is dropped in a regular pattern?

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

Answer: (C)

Intermittent non-conducted P waves with a regular dropped QRS beat is characteristic of second-degree AV block, type II. In this pattern, impulses from the atria are sometimes not conducted to the ventricles, so you see P waves that are not followed by a QRS. The beats that do conduct have a constant, unchanging PR interval, and the dropped QRS occurs in a fixed ratio (for example, every second or third P wave is non-conducted). There is no progressive lengthening of the PR interval before a drop, which helps distinguish it from Mobitz I (Wenckebach). First-degree AV block would show a prolonged PR interval with every P followed by a QRS, and third-degree AV block shows complete AV dissociation with no relationship between P waves and QRS complexes.