PR interval measures from the beginning of the P wave to the beginning of the QRS complex; what does it represent?

• A. Measures from beginning of P wave to beginning of QRS complex; represents time for SA node stimulation, atrial depolarization, and conduction through AV node
• B. Measures the duration of the QRS complex
• C. Measures the duration of ventricular repolarization
• D. Measures the interval between QRS and QT

Answer: (A)

The PR interval reflects how long it takes for the electrical impulse to travel from the atria to the ventricles. The P wave shows atrial depolarization, and the beginning of the QRS complex marks the start of ventricular depolarization. The interval between these points includes the time the impulse spends in the atria and, importantly, the delay in the AV node that allows the ventricles to fill before they fire. So this interval measures atrial conduction plus AV nodal conduction up to the start of ventricular activation. It does not measure the duration of the QRS, which is ventricular depolarization, or ventricular repolarization (QT interval). Normal PR is about 0.12 to 0.20 seconds; longer PR suggests AV block, while shorter PR can occur with certain pre-excitation conditions.