What does the PR interval measure in an ECG?

The PR interval on an ECG is defined as the time span from the beginning of the P wave to the beginning of the QRS complex. It reflects the period during which electrical impulses travel from the atria through the AV node and into the ventricles. The significance of this interval lies primarily in its ability to indicate any delays in conduction through the AV node, which is crucial for allowing adequate time for the ventricles to fill with blood before they contract.

Option C correctly describes the PR interval as measuring the time delay for the AV node, as this interval is essential in assessing proper cardiac function. If the PR interval is prolonged, it may suggest an impairment in conduction through the AV node, known as a first-degree AV block.

In contrast, the other options focus on different aspects of the cardiac cycle. The first choice refers to the duration of ventricular activity (QRS to T wave), which does not pertain to the PR interval. The second choice inaccurately implies that the PR interval solely measures the contraction time of the atria, neglecting the importance of the conduction through the AV node. The fourth choice discusses ventricular depolarization duration, which is represented by the QRS complex and not the PR interval. Thus, the correct understanding of