In which scenario may the PR Interval vary from beat to beat?

The PR interval represents the time taken for electrical impulses to travel from the atria to the ventricles through the AV node. In a normal sinus rhythm, the PR interval remains consistent because the conduction through the AV node is uniform. Similarly, in third-degree AV heart block, there is complete dissociation between the atrial and ventricular activities, which results in a fixed PR interval derived from the ventricular escape rhythm.

However, in second-degree AV block, particularly type 1 (Wenckebach), the PR interval progressively lengthens until a beat is dropped. In this case, the PR interval may vary from beat to beat, but only within the context of that pattern.

Atrial fibrillation, on the other hand, does not show a distinct PR interval because the atrial activity is chaotic, leading to an irregularly irregular rhythm. Consequently, while some aspects of conduction might change, the lack of coordinated atrial activity precludes a stable PR interval.

In summary, the second-degree AV block leads to varying PR intervals due to the pattern of progressively lengthening intervals and then a dropped beat, making this the correct scenario where PR intervals can vary from beat to beat.