PR Interval

The PR interval is the time from the onset of the P wave to the start of the QRS complex. It reflects conduction through the AV node.

The normal PR interval is between 120 – 200 ms (0.12-0.20s) in duration (three to five small squares).
If the PR interval is > 200 ms, first degree heart block is said to be present.
PR interval < 120 ms suggests pre-excitation (the presence of an accessory pathway between the atria and ventricles) or AV nodal (junctional) rhythm.

ECG basics: waves, segments and intervals LITFL ECG library

Prolonged PR Interval – AV block (PR >200ms)

Delayed conduction through the AV node
May occur in isolation or co-exist with other blocks (e.g., second-degree AV block, trifascicular block)

First degree AV block

Sinus rhythm with marked 1st degree heart block (PR interval 340ms)

Second degree AV block (Mobitz I) with prolonged PR interval

ECG 2nd Degree AV block Mobitz I Wenckebach phenomenon

Second degree heart block, Mobitz type I (Wenckebach phenomenon).
Note how the baseline PR interval is prolonged, and then further prolongs with each successive beat, until a QRS complex is dropped.
The PR interval before the dropped beat is the longest (340ms), while the PR interval after the dropped beat is the shortest (280ms).

Short PR interval (<120ms)

A short PR interval is seen with:

Preexcitation syndromes
AV nodal (junctional) rhythm.

Pre-excitation syndromes

Wolff-Parkinson-White (WPW) and Lown-Ganong-Levine (LGL) syndromes.
These involve the presence of an accessory pathway connecting the atria and ventricles.
The accessory pathway conducts impulses faster than normal, producing a short PR interval.
The accessory pathway also acts as an anatomical re-entry circuit, making patients susceptible to re-entry tachyarrhythmias.
Patients present with episodes of paroxsymal supraventricular tachycardia (SVT), specifically atrioventricular re-entry tachycardia (AVRT), and characteristic features on the resting 12-lead ECG.

Wolff-Parkinson-White syndrome

The characteristic features of Wolff-Parkinson-White syndrome are a short PR interval (<120ms), broad QRS and a slurred upstroke to the QRS complex, the delta wave.

ECG Wolff-Parkinson-White WPW Delta wave

Lown-Ganong-Levine syndrome

The features of Lown-Ganong-Levine syndrome LGL syndrome are a very short PR interval with normal P waves and QRS complexes and absent delta waves.

AV nodal (junctional) rhythm

Junctional rhythms are narrow complex, regular rhythms arising from the AV node.
P waves are either absent or abnormal (e.g. inverted) with a short PR interval (=retrograde P waves).
ECG: Accelerated junctional rhythm demonstrating inverted P waves with a short PR interval (retrograde P waves)