PR Interval

The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. It comprises the P wave and the PR segment.

The interval should be measured in the lead with the largest, widest P wave and the longest QRS duration ¹.

The PR interval includes the atrial depolarization and the propagation of the impulse through the AV node and the Conduction System until the ventricular myocardium begins to depolarize ¹.

It does not include the duration of conduction from the Sinus Node to the right atrium (Sinoatrial conduction).

The PR interval also includes the atrial repolarization (atrial T wave), which is directed opposite to the P wave axis, but atrial repolarization usually has low amplitude and the PR segment is frequently isoelectric ¹.

Normal and Abnormal PR Interval

Normal PR Interval:

In adults the normal PR interval is 0.12 s to 0.20 s (3 to 5 small squares).

Normal PR interval (0.14 s).

It is generally shorter in children (see pediatric EKG) and in pregnant women, and it is longer in older persons.

Prolonged PR Interval:

Prolongation of the PR interval above 0.20 s (5 small squares) is called first degree AV block.

First degree AV Block with prolonged PR interval (0.36 sec).

It indicates a conduction delay from the sinus node to the ventricles. The atrioventricular node is the most commonly involved site in adults.

In type I 2nd degree AV block there is a progressive PR lengthening until a P wave is not conducted (Wenckebach phenomenon).

Short PR Interval

A short PR interval (<0.12 sec) may be caused by a pre-excitation syndrome (Wolff-Parkinson-White), ectopic atrial pacemaker or AV junctional rhythm.

Wolff-Parkinson-White with short PR interval (0.08 sec) and delta wave (red arrows).

The most important findings in Wolff-Parkinson-White syndrome are a short PR interval, the presence of the delta wave and wide QRS complex.

In ectopic atrial pacemaker or AV junctional rhythm may be seen a short PR interval with an abnormal P waves with narrow QRS complex.

Elements of the PR Interval

P Wave

Related articles: EKG waves, abnormal waves and intervals.

The P wave represents atrial depolarisation, it is the result of overlaying the electrical activity of both atria.

P wave duration is less that 0.10 s and its maximum voltage is 0.25 mV. Normal P wave is positive in all leads except aVR where it is negative, and V1, where P wave is biphasic.

More information: EKG waves, abnormal waves and intervals.

PR Segment

Related article: Segments and intervals.

PR segment is the isoelectric segment between the end of the P wave and the start of the QRS complex. It is included in the PR interval (read difference between segments and intervals).