Normal PR Interval
The normal PR interval ranges from 120 to 200 milliseconds, measured from the beginning of the P wave to the beginning of the QRS complex on a standard 12-lead ECG. 1, 2
Standard Definition
- PR interval >200 ms is defined as first-degree atrioventricular (AV) block, representing delayed AV conduction most commonly at the level of the AV node 1, 2
- PR interval <120 ms may indicate pre-excitation syndromes such as Wolff-Parkinson-White (WPW) syndrome, where an accessory pathway bypasses the AV node 1, 2
Special Populations: Athletes
The normal range differs in highly trained athletes due to increased vagal tone:
- PR intervals of 200-400 ms are considered normal variants in asymptomatic athletes and do not require further evaluation in the absence of symptoms or family history of cardiac disease 1
- Profound first-degree AV block (PR ≥400 ms) warrants evaluation even in athletes, including assessment of chronotropic response to mild aerobic activity 1
- If the PR interval normalizes with exercise and the athlete is asymptomatic, no further testing is necessary 1
Athletes Requiring Further Evaluation
Asymptomatic athletes with first-degree AV block who develop Mobitz type I second-degree AV block with exercise should undergo electrophysiologic study (EPS) to evaluate for possible intra-His or infra-His block 1
Fetal Assessment
- In fetal echocardiography, the mechanical PR interval (time between atrial and ventricular contraction) normally measures 120 ± 10 ms 1
- Lengthening of the mechanical PR interval may identify fetuses at risk for progression to complete heart block, particularly in pregnancies complicated by maternal systemic lupus erythematosus or Sjögren syndrome 1
Clinical Significance of Prolonged PR
While traditionally considered benign, prolonged PR interval (>200 ms) is associated with increased long-term risks including atrial fibrillation, pacemaker requirement, and mortality in population studies 3. However, in middle-aged populations, PR interval normalizes in approximately 30% of individuals over time and may not confer increased mortality risk 4, highlighting the importance of clinical context when interpreting isolated PR prolongation.