Which second degree AV block is characterized by the PR interval not lengthening before a dropped QRS?

The second degree AV block characterized by the PR interval remaining constant before a dropped QRS is known as Type 2 second degree block, or Mobitz type II. In this scenario, the impulses from the atria to the ventricles are intermittently blocked without any preceding lengthening of the PR interval. This produces a pattern where a QRS complex is occasionally dropped, but the timing of the PR interval does not change from one beat to the next.

Type 2 second degree block is usually associated with a more stable block compared to Wenkebach (Mobitz type I) where the PR interval progressively lengthens before a QRS is dropped. Because of this stability, Mobitz type II has a higher risk of progressing to complete heart block, making it clinically significant. Understanding this distinction is crucial for proper diagnosis and management in telemetry and cardiology settings.