How can you differentiate Mobitz I (Wenckebach) from Mobitz II second-degree AV block on ECG?

• A. Mobitz II shows fixed PR with dropped QRS; Mobitz I shows progressive PR prolongation with intermittent nonconducted beats.
• B. Mobitz I shows fixed PR with intermittent QRS; Mobitz II shows progressive PR prolongation.
• C. Mobitz I shows dropped QRS with no PR change; Mobitz II shows dropped P waves.
• D. Mobitz I shows wide QRS; Mobitz II shows narrow QRS.

Answer: (B)

Distinguishing these two second-degree AV blocks comes down to how the atrioventricular conduction changes from beat to beat. In Wenckebach (Mobitz I), you’ll see the PR interval progressively lengthen with each subsequent beat until a P wave isn’t followed by a QRS complex. After that dropped beat, the cycle starts over with a relatively normal PR, and the pattern repeats. So the telltale sign is a gradually longer PR interval leading up to a nonconducted beat.

In Mobitz II, the PR interval stays essentially the same from beat to beat, even as occasionally a P wave fails to conduct and a QRS is dropped. There isn’t the gradual PR prolongation that characterizes Wenckebach. The dropped beats occur without prior slowing of conduction, which is why this form is more concerning and often suggests a block in the His-Purkinje system.

As a helpful clue, Mobitz II is more likely to be associated with wide QRS complexes if the block is below the AV node, whereas Wenckebach tends to have narrow QRS because the block is usually at the AV node.