Which statement best describes sinus arrhythmia?

• A. A non-variant rhythm with fixed R-R interval
• B. An irregular rhythm with irregular P waves
• C. A constant R-R interval regardless of breathing
• D. A normal variant with R-R interval changes with breathing

Answer: (D)

Sinus arrhythmia is a normal physiologic variation in heart rate tied to breathing, produced by changes in vagal tone. With inspiration, vagal influence drops and the heart rate increases, shortening the R-R interval; with expiration, vagal influence rises and the rate slows, lengthening the R-R interval. On an ECG you see a sinus rhythm with normal P waves and consistent overall rhythm, but the spacing between beats varies in step with respiration. This pattern is especially common in healthy young people and doesn’t indicate disease or require treatment. So the best description is a normal variant with R-R interval changes during breathing. The other descriptions describe a fixed rhythm or abnormal P waves, which aren’t characteristic of sinus arrhythmia.