Which of the following describes the PR intervals on a second degree AV heart block, Type I?

In a second-degree AV heart block, Type I, also known as Mobitz Type I or Wenckebach phenomenon, the key characteristic of the PR interval is that it progressively lengthens with each successive heartbeat until a QRS complex is dropped. This phenomenon occurs due to a gradual increase in the duration of the PR interval as the conduction through the atrioventricular (AV) node becomes increasingly delayed.

After a certain point of gradual prolongation, the heart will fail to conduct the impulse to the ventricles, leading to a dropped QRS complex. This pattern of lengthening followed by a drop of the QRS is distinctive to this type of heart block. The progressive nature of the PR interval is what helps to differentiate it from other types of AV blocks, where the PR segments may remain consistent or demonstrate a different behavior. Therefore, the description of the PR intervals as progressively longer is accurate and reflects the typical behavior observed in this specific type of heart block.