What is the primary characteristic of sinus arrhythmia?

The primary characteristic of sinus arrhythmia is that it resembles normal sinus rhythm but is irregular. In a normal sinus rhythm, heartbeats occur at a consistent pace, typically ranging between 60 to 100 beats per minute. However, in sinus arrhythmia, although the underlying rhythm is still generated by the sinoatrial node, the intervals between heartbeats are not consistent, causing the rhythm to be irregular.

This irregularity is commonly associated with the respiratory cycle, where heart rates may increase slightly with inhalation and decrease during exhalation, leading to variations in the timing of beats. This pattern is a normal physiological response, especially in younger individuals, and typically does not indicate any underlying pathology.

In contrast to this, the other characteristics mentioned, such as a constant heart rate, limited occurrence only during inspiration, or associations with bradycardia, do not accurately describe sinus arrhythmia or its pattern. Sinus arrhythmia is defined by its irregularity while maintaining a sinus-originated heart rhythm.