Sinus arrhythmia is characterized by which description?

• A. Normal variation of sinus rhythm
• B. Beat-to-beat variation with breathing
• C. Constant heart rate
• D. Wide QRS complexes

Answer: (A)

Respiratory modulation of the heart rate is at play here. In a healthy person, the autonomic nervous system causes the SA node’s firing rate to vary with the breathing cycle: heart rate increases a bit during inspiration and slows during expiration due to changes in vagal tone. This makes the intervals between beats change in a regular, cyclical way, yet the rhythm remains sinus — with normal P waves and narrow QRS complexes. Because this variation is a normal physiological phenomenon tied to respiration, it is described as a normal variation of sinus rhythm, not a true pathological arrhythmia. It’s most noticeable in younger people and tends to lessen with age or heart disease. The other descriptions would imply a non-normal rhythm or conduction issue, which isn’t the case here.