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 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:
- Preexcitation syndromes
- AV nodal (junctional) rhythm.
- 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.
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).
- 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
- 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
- Dubin D. Rapid Interpretation of EKG’s
- Mattu A. ECG’s for the Emergency Physician
- Hampton JR. The ECG In Practice, 6e