What is the primary difference between second-degree AV block Type I and Type II?

The primary distinction between second-degree AV block Type I, also known as Wenckebach, and Type II lies in the behavior of the PR interval on an ECG. In Type I, there is a gradual lengthening of the PR interval with each successive heartbeat until a QRS complex is dropped; this pattern indicates a failure of conduction after the longest PR interval. This progressive increase in PR interval length characterizes the Type I block.

Conversely, Type II is characterized by a consistent PR interval that remains unchanged even when QRS complexes are dropped. This indicates a different mechanism of conduction failure and suggests a higher likelihood of progressing to complete heart block, given its more stable baseline compared to Type I.

This distinction is critical in clinical settings, as Type II often indicates a more serious underlying condition and requires closer monitoring and potential intervention. Understanding these differences aids in management and treatment decisions for patients exhibiting second-degree AV block.