What distinguishes Mobitz Type I 2nd degree AV block?

Mobitz Type I second-degree AV block, also known as Wenckebach, is characterized by a progressive lengthening of the PR interval with each successive heartbeat until a QRS complex is eventually dropped. This phenomenon describes a situation where the electrical signal from the atria to the ventricles is increasingly delayed over several cycles, leading to the absence of a QRS complex after a certain number of P waves.

The gradual lengthening of the PR interval is distinctive and serves to differentiate Mobitz Type I from other types of blocks. Understanding this pattern is crucial for recognition and diagnosis during an ECG interpretation. When the PR interval is observed to consistently increase until a beat is dropped, it establishes the presence of this specific type of AV block.

Other options do not accurately reflect the behavior of PR intervals in Mobitz Type I. For instance, a constant PR interval suggests a different electrical conduction characteristic, while wide QRS complexes can indicate other types of conduction issues, and skipped P waves do not provide the characteristic pattern needed to diagnose Mobitz Type I.