Normal PR Interval on ECG
The normal PR interval on an electrocardiogram (ECG) ranges from 120 to 200 milliseconds (ms). 1, 2
Definition and Measurement
- The PR interval represents the time from the onset of atrial depolarization (beginning of P wave) to the onset of ventricular depolarization (beginning of QRS complex) 1
- It reflects the conduction time through the atria, AV node, bundle of His, and Purkinje fibers 1
- Accurate measurement requires identifying the earliest onset of the P wave and the earliest onset of the QRS complex across all leads 1
Measurement Technique
- Locate the first positive deflection before the QRS complex (P wave), typically most visible in leads II, III, and aVF 1
- Mark the earliest point where the baseline begins to deflect upward into the P wave across all leads 1
- Identify the earliest deflection of the QRS complex (Q wave or R wave) across all leads 1
- Count the small boxes between these two points (each small box = 40 ms) or use digital calipers on electronic ECG systems 1
Clinical Interpretation
- PR interval <120 ms: Short PR interval, may indicate pre-excitation syndromes or accelerated AV conduction 3
- PR interval 120-200 ms: Normal AV conduction 1, 2
- PR interval >200 ms: First-degree AV block 1, 4
- PR interval ≥400 ms: Marked first-degree AV block, requires further evaluation 1
Special Considerations in Athletes
- In well-trained athletes, PR intervals up to 400 ms may be normal due to increased vagal tone 5
- When evaluating profound sinus bradycardia or markedly increased PR interval in athletes, assess the chronotropic response to mild aerobic activity 5
- If heart rate increases appropriately and PR interval normalizes with exercise, and the athlete is asymptomatic, no further testing is necessary 5
Clinical Significance
- PR intervals >200 ms (first-degree AV block) have been associated with increased risk of atrial fibrillation, pacemaker implantation, and all-cause mortality in some studies 4
- However, other research suggests that in middle-aged populations, prolonged PR interval may normalize over time and is not associated with increased mortality risk 6
- PR intervals >240 ms may represent true first-degree AV block rather than physiological AV depression 7
Measurement Considerations
- For accurate measurements, the ECG should be recorded at standard calibration (10 mm/mV, 25 mm/s) 1
- In digitized ECGs, a sampling rate of 500 samples per second is recommended to ensure adequate resolution 1
- Variation in PR interval >30 ms across different leads on the same ECG may indicate ventricular pre-excitation 8
Remember that while the normal range is 120-200 ms, clinical context is essential for proper interpretation, particularly in athletes where longer PR intervals may be physiological rather than pathological.