What distinguishing feature helps differentiate a 2:1 Wenckebach from a 2:1 Mobitz block on EKG?

• A. A lengthened PR interval and widened QRS complex
• B. A lengthened PR interval with a normal QRS complex
• C. A normal PR interval with a widened QRS complex
• D. A normal PR interval with a narrow QRS complex

Answer: (B)

A 2:1 Wenckebach block, also known as Type I second-degree AV block, is characterized by a progressive lengthening of the PR interval until a QRS complex is dropped. In this scenario, after a dropped beat, the cycle restarts. Therefore, a key distinguishing feature of a 2:1 Wenckebach is that the PR interval becomes longer before the non-conducted beat, but during the 2:1 block itself, the PR interval that is conducted remains lengthened but is consistently the same for the conducted beats, which distinguishes it clinicially from a Mobitz block.

In contrast, a 2:1 Mobitz block (Type II) maintains a constant PR interval before the QRS complex drop occurs, which is a crucial factor in differentiating the two types of blocks. The Mobitz II block usually has a wider QRS complex due to the possible presence of an underlying bundle branch block, but in a 2:1 Wenckebach, it is common to maintain a normal QRS complex, specifically when no bundle branch block is present.

Thus, in the scenario of a 2:1 Wenckebach, the consistent lengthened PR interval with a normal Q