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.

pr interval and pr segment

PR interval

  • The normal PR interval is between 120 – 200 ms 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.

 

First degree AV block (PR >200ms)

First degree AV block

first degree heart block

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

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

Prolonged PR due to second degree heart block

Second degree heart block, Mobitz type I (Wenckeback 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, broad QRS and a slurred upstroke to the QRS complex, the delta wave.

delta waves in wolff-parkinson-white syndrome

Short PR (<120ms), broad QRS and delta waves in WPW syndrome

Lown-Ganong-Levine syndrome

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

LGL syndrome

Short PR interval with normal QRS complexes in LGL 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).
accelerated junctional rhythm

Accelerated junctional rhythm demonstrating inverted P waves with a short PR interval (retrograde P waves)

 

Further Reading

References

  • Hampton, JR. The ECG in Practice (5th edition), Churchill Livingstone 2008.
  • Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice (6th edition), Saunders 2008.
  • Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.

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