What characterizes sinus arrhythmia?

Sinus arrhythmia is characterized by a variation in heart rate that correlates with the respiratory cycle—specifically, the heart rate increases during inspiration and decreases during expiration. This phenomenon occurs due to the influence of the autonomic nervous system on the heart. During inhalation, there is a reduction in intrathoracic pressure, which enhances venous return to the heart and subsequently raises the heart rate. Conversely, during exhalation, the heart rate slows down as the pressure returns to normal. This normal physiological response predominantly occurs in healthy individuals, especially in younger populations, and represents a benign condition without pathological significance.

The other options do not accurately describe sinus arrhythmia. Consistent heart rates below 60 bpm indicate bradycardia, while heart rates exceeding 100 bpm suggest tachycardia. Irregular and rapid ventricular contractions are a sign of arrhythmias that may require intervention and differ vastly from the benign nature of sinus arrhythmia.