Mobitz Type I (Wenckebach) AV block is characterized by which pattern?

• A. Progressive PR interval prolongation with occasional dropped QRS
• B. Constant PR with dropped QRS
• C. No P waves with wide QRS
• D. Fixed PR with constant QRS

Answer: (A)

Mobitz Type I Wenckebach block is a second-degree AV block pattern where conduction through the AV node progressively slows with each successive atrial impulse. This shows up on the ECG as a stair-step increase in the PR interval from beat to beat until a P wave is not followed by a QRS complex. After that dropped beat, the PR interval resets and the cycle begins again. The QRS that do occur are usually normal in width because the block is at the AV node, not below it.

This pattern—progressively longer PR intervals followed by a nonconducted P wave (dropped QRS) and then a reset—is what makes Wenckebach distinct. It differs from Mobitz II, where the PR interval stays the same even though some P waves are not conducted, and from rhythms with no P waves or wide QRS, which indicate different origins of rhythm disturbances.