In an ECG, what does the P-R interval measure?

In an ECG, what does the P-R interval measure?

• A. Time from the start of ventricular depolarization to the end of ventricular repolarization
• B. Time between atrial and ventricular systole
• C. Time from the start of atrial depolarization to the start of ventricular depolarization (conduction through the AV node)
• D. Time from the end of atrial depolarization to the end of ventricular depolarization

Answer: (C)

The main idea is that the PR interval measures the time it takes for an electrical impulse to travel from the atria to the ventricles, including the delay at the AV node. On an ECG, the P wave represents atrial depolarization, and the start of the QRS complex marks the onset of ventricular depolarization. Therefore, the PR interval is the time from the beginning of the P wave to the beginning of the QRS complex. This interval reflects atrial conduction plus the AV nodal delay before the ventricles are activated. It does not describe ventricular depolarization duration (that’s the QRS width) or ventricular repolarization (T wave/QT interval). A normal PR interval is about 0.12–0.20 seconds; a longer interval points to a first-degree AV block, while a shorter interval can be seen with pre-excitation syndromes.

The main idea is that the PR interval measures the time it takes for an electrical impulse to travel from the atria to the ventricles, including the delay at the AV node. On an ECG, the P wave represents atrial depolarization, and the start of the QRS complex marks the onset of ventricular depolarization. Therefore, the PR interval is the time from the beginning of the P wave to the beginning of the QRS complex. This interval reflects atrial conduction plus the AV nodal delay before the ventricles are activated. It does not describe ventricular depolarization duration (that’s the QRS width) or ventricular repolarization (T wave/QT interval). A normal PR interval is about 0.12–0.20 seconds; a longer interval points to a first-degree AV block, while a shorter interval can be seen with pre-excitation syndromes.