What is the normal range for the PR interval and what does prolongation indicate?

• A. 0.08–0.12 seconds
• B. 0.12–0.20 seconds
• C. 0.20–0.30 seconds
• D. 0.04–0.10 seconds

Answer: (B)

The PR interval measures the time from the start of atrial depolarization to the start of ventricular depolarization, reflecting conduction through the AV node. In a normal adult, this interval is 0.12 to 0.20 seconds. If it becomes longer than 0.20 seconds, that prolongation indicates slowed conduction through the AV node, most consistent with first-degree AV block. This slowing can result from factors like increased vagal tone, AV nodal blocking medications (such as certain beta-blockers or calcium channel blockers), electrolyte disturbances, ischemia or infarction involving the conduction system, or age-related degeneration of the conduction pathways. Shorter intervals (less than 0.12 seconds) are not considered prolongation and can be seen with rapid conduction or pre-excitation scenarios, while intervals longer than about 0.20 to 0.30 seconds reflect more significant AV conduction delay.