- 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
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.
- 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).
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