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)

• 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

• 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.

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)

ECG Library Basics

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