Which type of 2nd degree AV block involves progressive prolongation of the PR interval?

• A. Mobitz type 1
• B. Mobitz type 2
• C. 3rd degree AV block
• D. 1st degree AV block

Answer: (A)

The rationale for selecting Mobitz type 1 as the correct answer lies in its unique characteristic of progressively prolonging the PR interval prior to the conduction failure, which is represented by a dropped QRS complex. This pattern is known as the Wenckebach phenomenon, where each successive atrial impulse takes longer to conduct through the AV node until one impulse is completely blocked, resulting in a missed ventricular contraction.

In Mobitz type 1, the progressive lengthening of the PR interval indicates that the AV node is gradually becoming less responsive to incoming impulses, leading to this characteristic rhythm. Once the block occurs, the cycle restarts with a shorter PR interval for the subsequent conducted waves.

Other types of AV blocks do not exhibit this gradual prolongation: Mobitz type 2 has a fixed PR interval with intermittent dropped beats, while 3rd degree AV block presents complete dissociation between atrial and ventricular rhythms. A 1st degree AV block simply has a prolonged PR interval that is uniform, with no dropped beats or progressive changes. Understanding these distinctions is crucial in ECG interpretation, as they help distinguish between different rates and patterns of conduction disturbances in the heart's electrical system.