The PR interval is the time from the beginning of the P wave (atrial depolarization) to the beginning of the QRS complex (ventricular depolarization). The normal PR interval measures 0.12 - 0.20 seconds (120-200 milliseconds). A prolonged or shortened PR interval can indicate certain disease. When prolonged, a first degree AV block is present. When shortened, Wolff-Parkinson-White syndrome or Lown-Ganong-Levine syndrome should be suspected.