What does the P-R interval measure?

The P-R interval measures the time from the beginning of atrial excitation, represented by the P wave, to the beginning of ventricular excitation, which is marked by the start of the Q wave in the QRS complex on an electrocardiogram (ECG). This interval reflects the time it takes for electrical impulses to travel from the atria through the atrioventricular (AV) node into the ventricles. Understanding this interval is crucial as it provides insight into the conduction time between these two essential phases of the cardiac cycle, and it is important for assessing the electrical health of the heart.

In the context of the other choices, the first option focuses on the duration from the beginning of ventricular excitation to the end of ventricular contraction, which pertains more to the Q-T interval than the P-R interval. The third option describes the time between two ventricular contractions, which is not what the P-R interval measures as it specifically starts with atrial excitation. Lastly, the fourth option refers to the overall cardiac cycle, including both atrial and ventricular contraction, which would be a more comprehensive measurement rather than the specific interval defined by the P-R measurement.