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

• 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)

• 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

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

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, broad QRS and a slurred upstroke to the QRS complex, the delta wave.

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.

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).

 

Further Reading

• ECG BASICS — Waves, Intervals, Segments and Clinical Interpretation
• ECG CLINICAL CASES — Your favourite ECG’s placed in clinical context with a challenging Q&A approach
• ECG and Cardiology Eponymous Syndromes — Cheats guide to eponymous emancipation
• ECG Exam Template — a framework for the FACEM part 2 exam.
• ECG Reference Sites on the WEB — the best of the rest

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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