From the Guidelines
A normal PR interval on an electrocardiogram (ECG) ranges from 120 to 200 milliseconds (0.12 to 0.20 seconds). This interval represents the time it takes for an electrical impulse to travel from the sinus node through the atria, AV node, bundle of His, and Purkinje fibers until the beginning of ventricular depolarization. The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex.
Key Points to Consider
- Values shorter than 120 milliseconds may indicate pre-excitation syndromes like Wolff-Parkinson-White syndrome, where an accessory pathway bypasses the normal conduction delay at the AV node.
- PR intervals longer than 200 milliseconds indicate first-degree AV block, which may be caused by medications (beta-blockers, calcium channel blockers, digoxin), increased vagal tone, electrolyte abnormalities, or structural heart disease, as noted in 1.
- The PR interval is an important diagnostic parameter that helps clinicians assess the heart's electrical conduction system and identify potential cardiac abnormalities.
Clinical Implications
- A prolonged PR interval, such as >300 ms, can lead to symptoms of fatigue or exertional intolerance due to loss of atrioventricular synchrony, resulting in decreased cardiac output and increased pulmonary capillary wedge pressure, as described in 1.
- Accurate measurement and interpretation of the PR interval are crucial for diagnosing and managing cardiac conduction disorders, as emphasized in 1.
Diagnostic Considerations
- The PR interval should be evaluated in the context of the patient's overall clinical presentation, including symptoms, medical history, and other electrocardiographic findings.
- Further evaluation, such as electrophysiological studies, may be necessary to determine the level and significance of AV block, particularly in cases with wide QRS complexes or high-degree AV block, as discussed in 1.
From the Research
Normal Range for PR Interval
The normal range for the PR interval is typically considered to be between 120-200 milliseconds 2, 3, 4, 5.
Definition of Prolonged PR Interval
A prolonged PR interval, also known as first-degree atrioventricular block, is generally defined as a PR interval exceeding 200 milliseconds 2, 3, 4, 5.
Variations in PR Interval
Some studies suggest that the upper limit of the normal PR interval may be higher, with one study proposing a cutoff of 240 milliseconds to define first-degree atrioventricular block 6.
Key Points
- The normal PR interval range is typically between 120-200 milliseconds.
- A PR interval exceeding 200 milliseconds is considered prolonged.
- The definition of a prolonged PR interval may vary slightly between studies.
- Some research suggests an upper limit of 240 milliseconds for the normal PR interval.
Associated Risks
Prolongation of the PR interval has been associated with increased risks of atrial fibrillation, pacemaker implantation, and all-cause mortality in some studies 2, although other studies have found no significant association with mortality 3, 4.
Clinical Significance
The clinical significance of PR interval prolongation is still being researched, with some studies suggesting it may be a biomarker for atrial cardiopathy in patients with cryptogenic stroke 5.