In type 1 second degree heart block, how is the PR interval characterized?

In type 1 second degree heart block, also known as Mobitz type I or Wenckebach, the PR interval is characterized by a progressive lengthening 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, leading to a point where the electrical impulse fails to be conducted through the AV node, resulting in a missed (or dropped) QRS complex. This pattern can often be observed on an electrocardiogram (ECG) as a repetitive cycle of lengthening PR intervals followed by a QRS drop. The dropping of the QRS complex occurs after the longest PR interval, illustrating the characteristic trait of this type of heart block.