Which interval on an ECG is used to estimate AV conduction time?

• A. P-R interval
• B. QRS duration
• C. T wave interval
• D. P wave duration

Answer: (A)

The P-R interval is the measure that reflects AV conduction time. It spans from the start of atrial depolarization (the P wave) to the start of ventricular depolarization (the QRS complex). This interval includes the time the impulse spends traveling through the AV node, where a brief delay slows conduction so the atria can finish contracting before the ventricles beat. That AV nodal delay is what you’re estimating when you look at the P-R interval.

In a normal adult, the P-R interval is about 0.12 to 0.20 seconds. If it’s prolonged, it indicates slowed AV conduction (like first-degree AV block). If it’s unusually short, AV conduction is faster than normal, though that’s less common clinically. The other intervals don’t measure AV conduction time: the QRS duration reflects how long ventricular depolarization takes; the T wave interval relates to ventricular repolarization; and the P wave duration only shows how long atrial depolarization lasts.