In Mobitz II, what is observed in the PR interval of conducted beats?

• A. PR intervals shorten progressively
• B. PR intervals become extended leading to a dropped beat
• C. PR interval in conducted beats remains constant
• D. PR intervals vary randomly

Answer: (C)

In Mobitz II, the key feature is a constant PR interval for the beats that are conducted, with occasional P waves not followed by a QRS (dropped beats) happening without any preceding change in the PR duration. This means the signal from atria to ventricles is being blocked at a fixed point below the AV node—usually in the His-Purkinje system. Because the conduction path doesn’t progressively slow down, the PR interval of the conducted beats remains the same from one beat to the next.

This differs from Mobitz I (Wenckebach), where the PR interval lengthens step by step until a beat is dropped. So the correct observation is that the PR intervals in conducted beats stay constant, even though some P waves aren’t followed by QRS complexes. This pattern also warns that Mobitz II can progress to complete heart block, necessitating closer monitoring or pacing.