Understanding the PR Interval on ECG

Introduction

The PR interval is a key measurement on an electrocardiogram (ECG) that represents the time taken for electrical impulse to travel from the atria to the ventricles. It is measured from the beginning of the P wave to the start of the QRS complex.

Normal PR Interval Duration

• Normal range : 120–200 milliseconds (ms) (0.12–0.20 seconds)
• A short PR interval (< 120 ms) may indicate pre-excitation syndromes (e.g., Wolff-Parkinson-White syndrome) or a junctional rhythm.
• A prolonged PR interval (> 200 ms) suggests first-degree atrioventricular (AV) block .

What Does an Abnormal PR Interval Indicate?

1. Prolonged PR Interval (> 200 ms)

• First-degree AV block : Delayed conduction between atria and ventricles but without skipped beats.
• Mobitz Type I (Wenckebach) Block : Progressive lengthening of the PR interval until a beat is dropped.
• Mobitz Type II Block : PR interval remains constant, but some P waves fail to conduct.
• Third-degree AV Block (Complete Heart Block) : No relationship between P waves and QRS complexes.

2. Short PR Interval (< 120 ms)

• Wolff-Parkinson-White (WPW) Syndrome : Presence of an accessory pathway (Bundle of Kent) leads to early ventricular activation, often seen with a delta wave.
• Lown-Ganong-Levine (LGL) Syndrome : Short PR interval but without a delta wave, caused by AV nodal bypass through rapidly conducting fibers.
• Junctional Rhythms : The impulse originates near the AV node, leading to a short PR interval or even absent P waves.

Clinical Relevance

• A prolonged PR interval may be benign but can progress to higher-degree AV blocks, requiring pacemaker consideration in some cases.
• A short PR interval with delta waves might indicate WPW, which carries a risk of tachyarrhythmias and sudden cardiac death if atrial fibrillation occurs.

Diagnosis and Management

1. ECG Analysis : Identify PR interval duration and look for accompanying features (e.g., delta waves in WPW, block patterns in AV block).
2. Electrophysiology Study (EPS) : May be needed to assess conduction abnormalities more precisely.
3. Treatment Considerations :
   • First-degree AV block : Usually benign; no treatment unless symptomatic.
   • Mobitz II or third-degree AV block : Often require a pacemaker .
   • WPW syndrome with arrhythmias : May require catheter ablation to eliminate the accessory pathway.

Conclusion

The PR interval is a critical measurement in ECG interpretation. Recognizing abnormalities helps diagnose conditions like AV block and pre-excitation syndromes. If you have concerns about ECG findings, consult a cardiologist for evaluation and potential treatment.

Source recommendations

1. American College of Cardiology/American Heart Association (ACC/AHA) Guidelines for the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay

1. https://www.ahajournals.org/doi/10.1161/CIR.0000000000000628
2. https://pubmed.ncbi.nlm.nih.gov/30412710/
3. https://www.jacc.org/guidelines/bradycardia
4. https://pubmed.ncbi.nlm.nih.gov/30586772/
5. https://www.acc.org/latest-in-cardiology/ten-points-to-remember/2018/11/05/15/12/2018-acc-aha-hrs-guideline-on-bradycardia

2. European Society of Cardiology (ESC) Guidelines for the Management of Supraventricular Tachycardia

1. https://www.escardio.org/Guidelines/Clinical-Practice-Guidelines/Supraventricular-Tachycardia
2. https://academic.oup.com/eurheartj/article/41/5/655/5556821
3. https://pubmed.ncbi.nlm.nih.gov/31504425/
4. https://www.acc.org/Latest-in-Cardiology/ten-points-to-remember/2019/09/10/12/36/2019-ESC-Guidelines-for-Supraventricular-Tachycardia
5. https://pubmed.ncbi.nlm.nih.gov/14557344/

3. American Heart Association ECG Interpretation Guidelines

1. https://www.heart.org/en/health-topics/heart-attack/diagnosing-a-heart-attack/electrocardiogram-ecg-or-ekg
2. https://www.ahajournals.org/doi/10.1161/circulationaha.108.191095
3. https://www.heart.org/-/media/files/affiliates/mwa/kansas-city/kc-cardiac-and-stroke-symposium/2020-event-documents/cardiac-presentations/2-ecg-hockstad.pdf?la=en
4. https://www.ahajournals.org/doi/10.1161/circulationaha.108.191098
5. https://pubmed.ncbi.nlm.nih.gov/19281931/