How do you identify a second-degree AV block, Type I (Wenckebach)?

A second-degree AV block, Type I, also known as Wenckebach, is characterized by a distinct pattern on the EKG. The hallmark of this type of block is the progressive lengthening of the PR interval with each successive heartbeat until a QRS complex is dropped. This means that after the dropped beat, the cycle starts over, and the PR interval begins to lengthen again.

This pattern is due to the gradual failure of the conduction through the AV node, typically resulting from increased vagal tone or drug effects. The progressive nature of the PR interval lengthening leading to a missed QRS distinguishes Wenckebach from other types of AV blocks.

Other options do not align with the characteristics of a second-degree AV block, Type I. For instance, a consistent PR interval does not describe any form of block, while a sudden drop of QRS complexes is more reminiscent of a different type of block, such as Type II. A fixed PR interval with varying QRS lengths indicates a different pathology, possibly associated with a higher degree of AV block. Thus, the progressive lengthening followed by a dropped QRS is the defining characteristic for identifying Wenckebach.