In second-degree AV block (2*AVB), how does the PR interval change?

In second-degree AV block, specifically type I (Wenckebach), the characteristic feature of the PR interval is that it progressively gets longer with each successive beat until a QRS complex is eventually dropped. This pattern reflects a gradual increase in the conduction time through the AV node, culminating in a failure to conduct the impulse and dropping a QRS complex.

This phenomenon occurs due to the refractory nature of the AV node, where each impulse encounters increased delay until it is unable to transmit the next one, resulting in the dropped QRS. Understanding the mechanics of this block is crucial, as it distinguishes type I from other types of heart block, particularly in the clinical context where observed heart rhythms can inform diagnosis and management strategies.

The other options do not reflect the behavior of the PR interval in 2*AVB type I: it does not remain constant, shorten progressively, nor does it vary widely in a specific or characteristic pattern. Recognizing these nuances helps in identifying and differentiating various types of AV block during diagnostic assessments.