What defines an irregular R-R interval?

An irregular R-R interval indicates that the time between successive R waves on an electrocardiogram (ECG) is not consistent, which is often a hallmark of certain arrhythmias. In the context of a second-degree type 2 block, also known as Mobitz type II, the conduction through the AV node is intermittently blocked without a preceding change in the R-R interval. This irregularity results in dropped beats that can lead to an unstable and erratic heart rhythm, which manifests as irregular R-R intervals on an ECG.

In contrast, normal sinus rhythm and sinus tachycardia typically present with regular R-R intervals as the heart maintains a consistent rhythm. A first-degree heart block is characterized by a prolonged PR interval but usually maintains a regular R-R interval without irregularity. Thus, the definition of an irregular R-R interval aligns clearly with the characteristics of a second-degree type 2 block.