Which AV block is defined by a sudden block in a P wave with no change in PR interval?

Which AV block is defined by a sudden block in a P wave with no change in PR interval?

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

Answer: (C)

Distinguishing AV blocks relies on how the PR interval behaves and whether a P wave is followed by a QRS. When a P wave is suddenly not followed by a QRS and the PR interval on the preceding conducted beats stays the same, the pattern points to a non-conducted atrial impulse occurring without any PR prolongation. This is classic Mobitz II. The key idea is that the block is typically below the AV node—in the His–Purkinje system—so the conduction of other impulses remains uniformly timed, giving a constant PR interval for the beats that do conduct. A dropped QRS occurs abruptly without the gradual PR-lengthening seen in other blocks. In contrast, first-degree AV block shows a consistently prolonged PR interval with every P wave followed by a QRS; no dropped beats. Wenckebach (Mobitz I) features progressively longer PR intervals followed by a dropped beat. Third-degree AV block has complete dissociation between atria and ventricles, with no fixed relationship between P waves and QRS. Mobitz II is more concerning because it can progress to complete heart block and often requires pacing.

Distinguishing AV blocks relies on how the PR interval behaves and whether a P wave is followed by a QRS. When a P wave is suddenly not followed by a QRS and the PR interval on the preceding conducted beats stays the same, the pattern points to a non-conducted atrial impulse occurring without any PR prolongation. This is classic Mobitz II.

The key idea is that the block is typically below the AV node—in the His–Purkinje system—so the conduction of other impulses remains uniformly timed, giving a constant PR interval for the beats that do conduct. A dropped QRS occurs abruptly without the gradual PR-lengthening seen in other blocks.

In contrast, first-degree AV block shows a consistently prolonged PR interval with every P wave followed by a QRS; no dropped beats. Wenckebach (Mobitz I) features progressively longer PR intervals followed by a dropped beat. Third-degree AV block has complete dissociation between atria and ventricles, with no fixed relationship between P waves and QRS.

Mobitz II is more concerning because it can progress to complete heart block and often requires pacing.