A PR interval greater than 0.21 seconds indicates which block?

• A. PR interval
• B. QRS duration
• C. QT interval
• D. P-R segment

Answer: (A)

The key idea is that the PR interval measures the time it takes for electrical impulses to travel from the atria through the AV node to the ventricles. When this interval is longer than about 0.20 seconds (0.21 seconds in the prompt), it reveals slowed conduction through the AV node, which is called a first-degree AV block. Since the PR interval is the measurement that reflects AV nodal conduction time, it serves as the best indicator of this block.

QRS duration reflects ventricular conduction, QT interval reflects ventricular repolarization, and the P-R segment is the portion of the delay within the AV node but by itself doesn’t capture the full time from atrial depolarization to ventricular depolarization the way the PR interval does.