Which of the following heart rhythms is characterized by an irregularity which exceeds .16 seconds in the longest R-R cycle?

In the context of heart rhythms, sinus arrhythmia is characterized by a variation in the interval between heartbeats that can exceed .16 seconds in the longest R-R cycle. This condition occurs due to the influence of respiratory cycles on heart rhythm; specifically, it is common in young, healthy individuals and reflects an increase in heart rate during inhalation and a decrease during exhalation.

In sinus arrhythmia, because the rhythm is inherently irregular due to the variations in timing related to the respiratory cycle, it often leads to broader discrepancies in the timing of successive R-R intervals. The variation can be significant enough to exceed the .16-second threshold mentioned, distinguishing it from other rhythms that typically exhibit more consistent interval lengths.

Other rhythms like sinus bradycardia, third-degree AV block, and normal sinus rhythm do not exhibit this level of variability in R-R intervals. Sinus bradycardia has a regular rhythm but at a slower rate, third-degree AV block shows a dissociation between atrial and ventricular rates, and normal sinus rhythm has consistent R-R intervals typically featuring minimal variation. This makes sinus arrhythmia the only rhythm fitting the specified characteristic of irregularity greater than .16 seconds in length.