The PR interval measures the time from the start of which two cardiac events?

• A. Time from the start of the P wave to the start of the QRS complex
• B. Time from the start of the QRS complex to the end of the T wave
• C. Duration of the P wave
• D. Duration of the QRS complex

Answer: (A)

The PR interval represents the time it takes for the electrical impulse to travel from the atria to the ventricles. It is measured from the start of the P wave, which reflects atrial depolarization, to the start of the QRS complex, which marks the onset of ventricular depolarization. This interval thus includes atrial depolarization plus the delay through the AV node and conducting pathways, giving the time from the beginning of atrial activation to the beginning of ventricular activation. The QRS duration itself is the time for ventricular depolarization, and the QT interval spans from the start of ventricular depolarization to the end of ventricular repolarization (QRS to T). The duration of just the P wave is not the interval in question.