Normal PR Interval
The normal PR interval ranges from 120 to 200 milliseconds (ms), with first-degree AV block defined as a PR interval greater than 200 ms. 1, 2
Understanding the PR Interval
The PR interval represents the time required for the electrical impulse to travel from the sinus node through the atria to the AV node and into the ventricles. It is measured from the beginning of the P wave to the beginning of the QRS complex on an electrocardiogram (ECG).
Normal Range Breakdown:
- Lower limit: 120 ms
- Upper limit: 200 ms
- First-degree AV block: PR interval > 200 ms 1
Clinical Significance of PR Interval Abnormalities
Prolonged PR Interval (> 200 ms)
- Represents delayed conduction through the AV node
- May be a normal finding in well-trained athletes 1
- Moderate prolongation (200-399 ms) is often a benign finding 1
- Marked prolongation (≥ 400 ms) warrants further evaluation 1
- Associated with increased risk of:
- Atrial fibrillation
- Pacemaker implantation
- All-cause mortality 3
Short PR Interval (< 120 ms)
- May indicate pre-excitation syndromes (e.g., Wolff-Parkinson-White syndrome)
- Associated with increased mortality risk in some populations 4
Special Considerations
Age-Related Variations
- PR interval has an age-dependent relationship with heart rate 5
- For adults under 60 years: PRa = PR + 0.26 (HR - 70)
- For adults 60 years or older: PRa = PR + 0.42 (HR - 70) 5
Athletic Heart
- Sinus bradycardia and moderate PR prolongation (200-399 ms) are recognized features of athletic conditioning 1
- A PR interval ≥ 400 ms, even in well-trained athletes, should prompt further evaluation 1
Evaluation of Abnormal PR Intervals
For Prolonged PR Interval (First-Degree AV Block)
- Assess chronotropic response to mild aerobic activity
- If heart rate increases appropriately and PR interval normalizes with no symptoms, no further testing is necessary 1
- Further evaluation is indicated if:
- PR interval doesn't shorten appropriately with exertion
- Patient experiences pre-syncope/syncope
- Family history of cardiac disease or sudden death exists 1
For Markedly Prolonged PR Interval (≥ 400 ms)
- Echocardiogram or ambulatory ECG monitoring may be indicated 1
- Consider permanent pacemaker implantation if symptomatic 2
Pitfalls and Caveats
- PR interval can be influenced by autonomic tone, medications, and electrolyte abnormalities
- Inconsistent associations between PR interval and mortality may be explained by varying contributions of P-wave duration to PR interval length 4
- Recent research suggests PR interval prolongation may not be an independent risk factor for 1-year mortality in emergency department patients 6
- Atrial bigeminy with blocked premature atrial contractions can mimic AV block and should be carefully differentiated 2
Remember that while the normal PR interval is 120-200 ms, clinical context is essential when interpreting borderline or abnormal values.