What is a normal PR (P-R) interval?

Normal PR Interval

The normal PR interval on an electrocardiogram (ECG) ranges from 120 to 200 milliseconds. 1, 2

Definition and Measurement

• The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex on a standard 12-lead ECG 1
• A PR interval greater than 200 milliseconds is defined as first-degree atrioventricular (AV) block 1, 2
• A PR interval less than 120 milliseconds may indicate pre-excitation syndromes such as Wolff-Parkinson-White syndrome 1, 3

Clinical Significance of PR Interval Abnormalities

Prolonged PR Interval (>200 ms)

• First-degree AV block is defined as a PR interval greater than 200 ms 2
• In athletes, moderate prolongation of the PR interval (200-399 ms) is recognized as a feature of athletic conditioning 2
• However, a PR interval ≥400 ms, even in well-trained athletes, should prompt further evaluation for cardiac conduction disease 2
• Prolonged PR interval has been associated with:
  • Increased risk of atrial fibrillation 4, 5
  • Higher likelihood of pacemaker implantation 4
  • Increased all-cause mortality in some studies 4

Short PR Interval (<120 ms)

• May indicate pre-excitation syndromes or partial AV nodal bypass 1, 3
• Associated with shorter effective and functional refractory periods of the AV conducting system 3
• In women, short PR intervals (<121 ms) have been associated with increased risk of atrial fibrillation 5

Special Considerations

• In highly trained athletes, PR intervals between 200-399 ms may be normal due to increased vagal tone 2
• For athletes with PR intervals ≥400 ms, evaluation should include:
  • Assessment of chronotropic response to mild aerobic activity
  • Observation of PR interval normalization with exercise 2
• If the PR interval doesn't shorten appropriately with exertion, or if the athlete experiences pre-syncope/syncope, further evaluation is warranted 2

Clinical Pearls

• When evaluating narrow QRS tachycardias, the relationship between the P wave and QRS complex (RP versus PR intervals) helps determine the arrhythmia mechanism 1
• In first-degree AV block with narrow QRS complexes, the conduction delay is usually in the AV node 1
• The PR interval helps differentiate between various types of supraventricular tachycardias based on the relationship between P waves and QRS complexes 1