PR Interval: Normal Range, Short and Long PR, and Causes
The PR interval runs from the start of the P wave to the start of the QRS complex and measures the time the impulse takes to travel from the atria through the AV node and His-Purkinje system. In adults a normal PR is 120 to 200 ms, 3 to 5 small squares; a longer PR is first-degree AV block, and a shorter one raises the question of pre-excitation.
At a glance
- Normal adult PR: 120 to 200 ms, or 3 to 5 small squares at 25 mm/s.
- Over 200 ms is first-degree AV block. It is usually benign, but in the Framingham cohort it doubled the adjusted risk of atrial fibrillation and nearly tripled the risk of needing a pacemaker.
- Under 120 ms: look for a delta wave (Wolff-Parkinson-White), a low atrial or junctional rhythm, or a normal variant.
- PR depression in many leads with PR elevation in aVR suggests pericarditis.
- A PR that changes from beat to beat is not first-degree block: think of Mobitz I, AV dissociation or a wandering atrial pacemaker.
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Normal values
| Measure | Normal | Abnormal |
|---|---|---|
| PR interval, adults | 120 to 200 ms | Over 200 ms: first-degree AV block. Under 120 ms: short PR |
| Small squares at 25 mm/s | 3 to 5 | More than 5 is long; fewer than 3 is short |
| PR segment | On the baseline | Depressed in many leads, and raised in aVR, in pericarditis |
| Children | Shorter, and varies with age and heart rate | Use age-specific pediatric ranges |
What it looks like
Normal PR
Short PR, delta wave
Long PR
How to measure it
- Use the lead with the clearest P wave, often II, or measure across simultaneous leads from the earliest P wave onset to the earliest QRS onset.
- Start at the beginning of the P wave.
- End at the first deflection of the QRS, whether a Q wave, an R wave or a delta wave.
- Measure several beats. The PR is constant in normal conduction and in first-degree block.
- Count small squares: each is 40 ms at the standard 25 mm/s.
Causes
Long PR: first-degree AV block
- High vagal tone and athletic training
- AV nodal blocking drugs such as beta-blockers, verapamil, diltiazem, digoxin and amiodarone
- degenerative conduction disease with age
- inferior myocardial infarction
- myocarditis, Lyme carditis and rheumatic fever
- hyperkalemia
- infiltrative heart disease
Short PR
- Pre-excitation (Wolff-Parkinson-White)
- fast AV nodal conduction as a normal variant
- low atrial or junctional rhythm
- storage diseases such as Pompe and Fabry disease
PR segment changes
- Pericarditis: depression in many leads with elevation in aVR
- atrial infarction, rarely
What to do
- First-degree AV block without symptoms: no treatment. Review AV nodal blocking drugs, and look for a cause if the long PR is new.
- A very long PR, over 300 ms, can cause symptoms like pacemaker syndrome; pacing can be considered when symptoms clearly come from the block.
- A long PR with a bundle branch block needs follow-up for progressive conduction disease, sooner if there is syncope.
- A short PR with a delta wave: assess for Wolff-Parkinson-White syndrome and ask about palpitations and syncope.
Pitfalls and mimics
- Measuring to the peak of the R wave instead of the start of the QRS.
- Missing a P wave hidden in the T wave at fast heart rates.
- Calling a PR that varies first-degree block.
- In AV dissociation the PR varies at random and there is no true PR interval.
Key studies
Cheng 2009, JAMA (Framingham Heart Study)
Among 7,575 adults followed from the early 1970s to 2007, a PR over 200 ms was linked to a 2-fold adjusted risk of atrial fibrillation, a nearly 3-fold risk of pacemaker implantation and a 1.4-fold risk of death.
Nielsen 2013, Heart Rhythm (Copenhagen ECG Study)
In 288,181 primary care patients, a PR at or above the 95th percentile (196 ms in women, 204 ms in men) raised the risk of atrial fibrillation in both sexes, and a very short PR raised it in women.
Holmqvist 2013, Ann Noninvasive Electrocardiol
A review of why marked first-degree AV block is not always benign and when it can cause symptoms that pacing relieves.
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Questions clinicians ask
What is a normal PR interval?
In adults, 120 to 200 ms: 3 to 5 small squares at the standard paper speed of 25 mm/s. Children have shorter, age-dependent ranges.
What does a long PR interval mean?
A PR over 200 ms is first-degree AV block: every impulse reaches the ventricles, but slowly, usually because of slow conduction in the AV node. It is often benign but is linked to a higher long-term risk of atrial fibrillation and pacemaker need.
What does a short PR interval mean?
A PR under 120 ms can be a normal variant, but look for a delta wave (Wolff-Parkinson-White), a low atrial or junctional rhythm, and rarely a storage disease.
How many small boxes is a normal PR interval?
3 to 5 small squares at 25 mm/s, because each small square is 40 ms.
Is first-degree heart block dangerous?
Usually not, and it needs no treatment by itself. Check drugs that slow the AV node, and follow patients with a very long PR, symptoms or an accompanying bundle branch block.
Sources
- Cheng S, Keyes MJ, Larson MG, et al. Long-term outcomes in individuals with prolonged PR interval or first-degree atrioventricular block. JAMA. 2009;301(24):2571-2577. doi.org/10.1001/jama.2009.888
- Nielsen JB, Pietersen A, Graff C, et al. Risk of atrial fibrillation as a function of the electrocardiographic PR interval: results from the Copenhagen ECG Study. Heart Rhythm. 2013;10(9):1249-1256. doi.org/10.1016/j.hrthm.2013.04.012
- Holmqvist F, Daubert JP. First-degree AV block: an entirely benign finding or a potentially curable cause of cardiac disease? Ann Noninvasive Electrocardiol. 2013;18(3):215-224. doi.org/10.1111/anec.12062
- Kusumoto FM, Schoenfeld MH, Barrett C, et al. 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay. Circulation. 2019;140(8):e382-e482. doi.org/10.1161/CIR.0000000000000628
- Mason JW, Hancock EW, Gettes LS. Recommendations for the standardization and interpretation of the electrocardiogram: part II: electrocardiography diagnostic statement list. Circulation. 2007;115(10):1325-1332. doi.org/10.1161/CIRCULATIONAHA.106.180201
Written in our own words by 2 Minute Medicine's physician editors from the sources listed here. The diagrams are schematic drawings made for 2MM, not patient tracings. This page supports, and does not replace, a full ECG reading and clinical judgment.
For health care professionals. Values are for adults unless a page says otherwise.
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