What is measured by the P-R interval on an ECG?

• A. Distance from P-wave to T-wave
• B. Distance from the beginning of the P-wave to the beginning of the Q wave or R-wave
• C. Distance between R-waves
• D. Duration of the QRS complex

Answer: (B)

The P-R interval on an ECG measures the time from the beginning of the P-wave, which represents atrial depolarization, to the beginning of the Q wave or R wave, marking the onset of ventricular depolarization. This interval indicates the time taken for an electrical impulse to travel from the sinus node through the atria, across the atrioventricular (AV) node, and into the ventricles. The normal range for the P-R interval is typically between 120 and 200 milliseconds, and it reflects the conduction time through the heart's electrical system. Understanding the P-R interval is crucial for identifying certain types of heart block or conduction delays, making it a key element in interpreting an ECG.

Other options do not describe the P-R interval accurately; the distance from the P-wave to the T-wave relates to a different part of the heart's electrical activity, the distance between R-waves refers to the R-R interval—which indicates the heart rate—and the duration of the QRS complex specifically measures the time for ventricular depolarization, not the conduction from atria to ventricles.