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Home | ECG Library | ECG Basics | PR Interval

PR Interval

by Dr Mike Cadogan, last update February 1, 2019

↪  ECG Basics Homepage

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
ECG Prolonged PR 1st Degree 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.

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)
AJR AV nodal (junctional) rhythm

ECG Library Basics

  • P Wave
  • PR Interval
  • PR Segment
  • Q Wave
  • QRS Segment
  • QT Interval
  • J Point
  • ST Segment
  • R Wave
  • T Wave
  • U Wave
  • Osborn Wave
  • Epsilon Wave
  • Delta Wave

LITFL Further Reading

  • ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
  • ECG A to Z by diagnosis – ECG interpretation in clinical context
  • ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
  • 100 ECG Quiz – Self-assessment tool for examination practice
  • ECG Reference SITES and BOOKS – the best of the rest

Advanced Reading

  • Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography
  • Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric
  • Wagner GS. Marriott’s Practical Electrocardiography 12e
  • Chan TC. ECG in Emergency Medicine and Acute Care
  • Rawshani A. Clinical ECG Interpretation
  • Mattu A. ECG’s for the Emergency Physician
  • Hampton JR. The ECG In Practice, 6e

ECG LIBRARY 700

ECG LIBRARY

Electrocardiogram

EKG Library

About Dr Mike Cadogan

Emergency physician with a passion for medical informatics and medical education/textbooks. Asynchronous learning #FOAMed evangelist. | @sandnsurf | + Mike Cadogan | LinkedIn

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