The PR interval on a standard 12-lead ECG represents the time interval from the onset of atrial depolarization to the onset of ventricular depolarization. Which of the following anatomical structures is responsible for the physiological delay that constitutes most of the PR interval duration?

Explanation

## PR Interval Physiology **Key Point:** The PR interval (measured from the start of the P wave to the start of the QRS complex) represents atrial depolarization and conduction through the AV node and bundle of His. ### Components of PR Interval | Structure | Contribution | Duration | |-----------|--------------|----------| | Atrial depolarization | P wave | ~80–100 ms | | AV node conduction | Delay (physiologic block) | ~100–120 ms | | Bundle of His & branches | Rapid conduction | ~20–40 ms | | Purkinje fibers | Rapid conduction | Minimal | **High-Yield:** The **atrioventricular (AV) node** is responsible for ~60–70% of the total PR interval duration because it has the slowest conduction velocity (~0.02–0.05 m/s) in the cardiac conduction system. This physiologic delay allows adequate time for atrial contraction to complete before ventricular depolarization begins, ensuring optimal ventricular filling (atrial "kick"). **Clinical Pearl:** Normal PR interval = 120–200 ms (3–5 small squares on standard ECG paper). Prolongation beyond 200 ms indicates first-degree AV block; shortening below 120 ms suggests pre-excitation (e.g., Wolff–Parkinson–White syndrome). **Mnemonic:** **SAVVY** — SA node (fast), AV node (Very slow), Bundle/Branches (fast), Ventricles (depolarize after QRS).