What happens to P-R intervals in Type 1 second-degree atrioventricular block?

In Type 1 second-degree atrioventricular block, also known as Mobitz Type I or Wenckebach, the characteristic behavior of the P-R intervals is a gradual lengthening until a P wave is not conducted, resulting in a dropped QRS complex. This phenomenon occurs due to a progressive delay in conduction through the AV node.

Initially, the P-R interval becomes longer with each successive heartbeat as the conduction through the AV node deteriorates. This lengthening continues until a threshold is reached where the atrial impulse is not able to successfully trigger a ventricular response, leading to a missed QRS complex. After this dropped beat, the process resets, and the cycle starts again with a shorter P-R interval, which repeats the gradual lengthening.

Understanding this pattern is crucial for identifying Type 1 second-degree AV block on an ECG, as it helps in differentiating it from other types of heart block. This progression of the P-R interval is a distinctive feature of Mobitz Type I and is critical for accurate diagnosis and management of the condition.