Which interval describes the time from the onset of atrial depolarization to the onset of ventricular depolarization?

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

Answer: (B)

The interval described is the PR interval. It runs from the start of the P wave, which marks atrial depolarization, to the start of the QRS complex, which marks the onset of ventricular depolarization. This interval captures the time it takes for the impulse to travel from the atria through the AV node and into the ventricles, including the AV nodal delay that coordinates atrial and ventricular contraction. A normal PR interval is about 0.12 to 0.20 seconds; a prolonged interval can indicate first-degree AV block. In contrast, the PR segment is the flat portion between the end of the P wave and the start of the QRS, reflecting AV nodal conduction without the atrial depolarization itself. The QT interval spans ventricular depolarization and repolarization, and the QRS duration reflects how long ventricular depolarization lasts.