The time from the beginning of the P wave, indicating atrial depolarization, to the beginning of the QRS complex, representing ventricular depolarization, is the PR interval.
This represents the time that it takes for the electrical impulse generated in the sinus node to travel through the atria and across the atrioventricular node to the ventricles. The normal PR interval is 0.12 to 0.20 seconds, or 120 to 200 milliseconds.
Multiple abnormalities of the PR interval — including prolongation of the PR interval, shortening of the PR interval and variation from beat to beat — can occur; these are discussed in detail in ECG Reviews and Criteria. A prolonged PR interval indicates delayed conduction of the sinoatrial, or SA, nodal impulse to the ventricles and is called first-degree AV block. A short PR interval can be seen when the AV node delay is bypassed, such as in Wolff-Parkinson-White syndrome or Lown-Ganong-Levine syndrome. The PR interval will vary in second-degree AV block type I (Wenkebach).
Note that the PR interval can be altered by changing sympathetic and parasympathetic tone. Because of this, medications such as beta-blockers can lengthen the PR interval and result in a first-degree AV block. Also, a longer PR interval can cause the first heart sound (S1) to sound soft on examination, and vice versa.