Which AV block is characterized by progressive PR interval lengthening until a P wave is dropped?

Which AV block is characterized by progressive PR interval lengthening until a P wave is dropped?

• A. First degree AV block
• B. Wenckebach (Mobitz I)
• C. Mobitz II (Mobitz II)
• D. Third-degree AV block

Answer: (B)

The main point is recognizing patterns of AV conduction on the ECG. This scenario shows a gradual lengthening of the PR interval with each beat, culminating in a P wave that is not followed by a QRS complex. That progressive PR prolongation until a dropped beat is the classic Wenckebach phenomenon, Mobitz type I, a second-degree AV block arising from conduction delay within the AV node. Because the block is at the AV node, the QRS complexes are usually narrow, and the rhythm can continue with another cycle after the dropped beat. This differs from other blocks: a constant but prolonged PR with every P wave conducted describes a first-degree AV block; a dropped beat that occurs without a preceding progressive PR prolongation and with a consistently normal PR before conducted beats points to Mobitz type II (often indicating a more distal, below-the-node block); complete AV dissociation with no reliable link between P waves and QRS complexes characterizes third-degree AV block, typically with a slow, often wide QRS escape rhythm.

The main point is recognizing patterns of AV conduction on the ECG. This scenario shows a gradual lengthening of the PR interval with each beat, culminating in a P wave that is not followed by a QRS complex. That progressive PR prolongation until a dropped beat is the classic Wenckebach phenomenon, Mobitz type I, a second-degree AV block arising from conduction delay within the AV node. Because the block is at the AV node, the QRS complexes are usually narrow, and the rhythm can continue with another cycle after the dropped beat.

This differs from other blocks: a constant but prolonged PR with every P wave conducted describes a first-degree AV block; a dropped beat that occurs without a preceding progressive PR prolongation and with a consistently normal PR before conducted beats points to Mobitz type II (often indicating a more distal, below-the-node block); complete AV dissociation with no reliable link between P waves and QRS complexes characterizes third-degree AV block, typically with a slow, often wide QRS escape rhythm.