What is unique about sinus arrhythmia?

• A. Regular rhythm with variable rate
• B. Irregular rhythm, can be brady (<60) or normal; PR and QRS are normal
• C. QRS wider than normal
• D. P waves inverted before QRS

Answer: (B)

Sinus arrhythmia is a normal, respiration-related variation in heart rate. The rhythm is irregular because the intervals between beats change with breathing—usually speeding up a bit during inspiration and slowing during expiration. Despite this irregularity, the heart’s electrical conduction stays normal: P waves precede every QRS, and both the PR interval and QRS duration are within normal limits. The rate may be under 60 (bradycardic) or within the normal range, but the pattern remains a benign sinus rhythm. This combination—irregular rhythm with normal conduction and P waves—distinguishes sinus arrhythmia from other rhythm disturbances that involve abnormal conduction or ectopic beats.