PR interval

The PR interval is the stretch of the ECG from the start of the P wave to the start of the QRS complex. In plain terms, it is the time it takes for the electrical signal to travel from the atria down to the ventricles — a brief pause at the gateway between the heart's upper and lower chambers.

Most of that time is spent in the AV node, the relay station that deliberately slows the impulse before passing it on. This built-in delay lets the atria finish emptying their blood into the ventricles before the ventricles contract, which improves the efficiency of each beat. A normal PR interval is roughly one fifth of a second.

Changes in the PR interval reveal how well that gateway is conducting. A long PR interval means the signal is being held up — the first sign of heart block. A very short PR interval can suggest the signal is bypassing the normal delay through an extra electrical pathway. Measuring the PR interval is thus a standard step in assessing the conduction system.