PR Interval

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.

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

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
  • 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:

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.

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.

ECG LGL Lown-Ganong-Levine syndrome

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)

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BA MA (Oxon) MBChB (Edin) FACEM FFSEM. Emergency physician, Sir Charles Gairdner Hospital.  Passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | Twitter |

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