What rhythm features two consecutive R to R intervals equal to the length of a block?

The identification of a sinus block is characterized by observing two consecutive R to R intervals that are equal to the length of a block on an electrocardiogram (ECG). In the context of a sinus block, this occurs when there is a temporary failure of the sinus node to generate an impulse, which causes a pause in the heart rhythm. The ECG will show a gap in the rhythm where the expected beats are missing, and when the rhythm resumes, the intervals between the R waves will align with the length of previously established intervals, reflecting the sudden absence of a normally generated beat.

This feature is specifically tied to the sinus node's failure, distinguishing it from other rhythms. For instance, a sinus pause refers to a longer pause without any beats and does not necessarily imply re-entry to the same baseline rhythm afterwards as seen in a complete heart block, where more sustained disruption of the conductive pathway occurs. Meanwhile, atrial fibrillation presents with irregularly timed intervals, making it impossible to identify consistent R to R intervals that fit the description. Thus, recognizing the distinct characteristics of the sinus block, with the two consecutive R to R intervals being equivalent to one block, confirms it as the correct answer.