In electrocardiography, the PR interval is the period, measured in milliseconds, that extends from the beginning of the P wave (the onset of atrial depolarization) until the beginning of the QRS complex (the onset of ventricular depolarization); it is normally between 120 and 200ms in duration. If a Q wave is measured by EKG, the PR interval is sometimes termed the PQ interval.
Variations in the PR interval can be associated with certain medical conditions:
- A long PR interval (of over 200 ms) may indicate a first degree heart block. Prolongation can be associated with hypokalemia, acute rheumatic fever,  or carditis associated with Lyme disease. 
- A short PR interval (of less than 120ms) may be associated with Wolff–Parkinson–White syndrome or Lown–Ganong–Levine syndrome, although it often is a harmless, normal variant seen on EKG.
- A variable PR interval may indicate other types of heart block.
- PR segment depression may indicate atrial injury or pericarditis.
- Variable morphologies of P waves in a single EKG lead is suggestive of an ectopic pacemaker rhythm such as wandering pacemaker or multifocal atrial tachycardia