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

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.

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.