What happens to the R-R interval in a second-degree Type 1 heart block?

In a second-degree Type 1 heart block, also known as Wenckebach, the R-R interval progressively lengthens until a QRS complex is dropped. This phenomenon explains why the correct choice is that the R-R interval gets longer over time before the cycle resets. The gradual lengthening occurs due to the progressive delay in conduction through the AV node, which eventually leads to a failure to conduct one of the impulses, resulting in a dropped beat.

The characteristic pattern that emerges is known as the "regularly irregular" cycle. While the individual R-R intervals may vary, the key feature is that there is a consistent lengthening before the drop occurs, distinguishing this block from other types. Understanding this pattern is crucial for recognizing second-degree Type 1 block on an ECG, as it helps in differentiating it from more stable rhythms or other types of heart block where the intervals may not change in this progressive manner.