On an ECG, what does the PR segment indicate?

The PR segment on an electrocardiogram (ECG) represents the period of time between the end of atrial depolarization and the beginning of ventricular depolarization. Specifically, it indicates the delay in the conduction of electrical impulses as they pass from the atria through the atrioventricular (AV) node and into the ventricles. This delay is crucial because it allows for proper filling of the ventricles with blood from the atria before the ventricles contract.

Understanding the importance of the PR segment involves recognizing the role of the AV node in this process. The AV node slows down the electrical impulse to ensure that the ventricles have adequate time to fill with blood. A normal PR interval takes about 120 to 200 milliseconds, and any deviation from this may suggest conduction abnormalities, such as first-degree heart block or other types of atrioventricular block.

While other segments and intervals on the ECG represent different phases of the cardiac cycle, the specific role of the PR segment is centered around this essential delay in conducting impulses to the ventricles, making "impulse delay in the ventricles" the accurate interpretation of what the PR segment indicates.