Which interval is used to determine rhythm regularity?

• A. R-R interval
• B. P-R interval
• C. QRS duration
• D. QT interval

Answer: (A)

Rhythm regularity is assessed by the consistency of the intervals between successive R waves on the ECG. The R-R interval measures the time between ventricular depolarizations, so if these gaps stay the same throughout the tracing, the rhythm is regular. If the R-R intervals vary, the rhythm is irregular (for example, atrial fibrillation often shows irregular R-R timing). The other intervals reflect conduction or duration of ventricular activity—P-R is AV conduction time, QRS duration is how long ventricular depolarization lasts, and QT interval covers depolarization and repolarization. They don’t directly indicate beat-to-beat timing, so they’re not used to judge rhythm regularity.