From the Guidelines
A normal PR interval on an electrocardiogram (ECG) ranges from 120 to 200 milliseconds (0.12 to 0.20 seconds). This measurement represents the time it takes for an electrical impulse to travel from the sinus node through the atria to the atrioventricular (AV) node and into the ventricles. 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 1.
- 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 1.
- When interpreting PR intervals, it's essential to consider the patient's heart rate, as the PR interval can shorten slightly at faster heart rates and lengthen at slower rates, while still remaining within normal limits 1.
Clinical Considerations
- In athletes, a PR interval of 200–399 ms may be considered normal due to athletic conditioning 1.
- However, a PR interval >400 ms should prompt further evaluation for cardiac conduction disease, even in well-trained athletes 1.
- The 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay defines first-degree atrioventricular block as a PR interval >200 ms 1.
From the Research
Normal PR Interval
A normal PR interval is typically considered to be between 120-200 milliseconds 2.
Definition of Prolonged PR Interval
A prolonged PR interval, also known as first-degree atrioventricular block, is defined as a PR interval greater than 200 milliseconds 2.
Key Points
- The normal PR interval range is generally accepted as 120-200 milliseconds 2.
- A PR interval greater than 200 milliseconds is considered prolonged 2.
- Prolongation of the PR interval has been associated with increased risks of atrial fibrillation, pacemaker implantation, and all-cause mortality 2.
- However, some studies have found that a prolonged PR interval may not be associated with an increased risk of mortality or cardiovascular events in certain populations 3.
Studies on PR Interval
- A study published in 2017 found that a PR interval greater than 200 milliseconds was associated with an increased risk of cryptogenic stroke 4.
- Another study published in 2009 found that individuals with a PR interval greater than 200 milliseconds had a higher risk of atrial fibrillation, pacemaker implantation, and all-cause mortality 2.
- A study published in 2014 found that a prolonged PR interval was not associated with an increased risk of mortality or cardiovascular events in a middle-aged population 3.
- A study published in 1991 evaluated the PR intervals in normal and prolonged maximum limit with short QRS and suggested the term "first degree A-V block" for those cases with PR interval greater than 240 ms 5.