From the Guidelines
The 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, and into the ventricles. It is measured from the beginning of the P wave to the beginning of the QRS complex. A PR interval 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. A PR interval longer than 200 milliseconds indicates first-degree AV block, which may be benign or could represent conduction system disease, as noted in the 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 1.
Some key points to consider when evaluating the PR interval include:
- The PR interval is important for assessing atrioventricular conduction and can help diagnose various cardiac conditions including heart blocks, pre-excitation syndromes, and effects of certain medications or electrolyte abnormalities on cardiac conduction.
- First-degree atrioventricular block is characterized by a PR interval >200 ms, but with 1:1 atrioventricular conduction, and is more accurately defined as atrioventricular delay because no P waves are blocked, as stated in the guideline 1.
- The guideline also notes that second-degree atrioventricular block is sub-classified into Mobitz I (Wenckebach conduction) and Mobitz II, and that high-grade, high-degree, or advanced atrioventricular block refers to situations where ≥2 consecutive P waves at a normal rate are not conducted without complete loss of atrioventricular conduction 1.
Overall, accurate measurement and interpretation of the PR interval are crucial for diagnosing and managing cardiac conduction abnormalities, and the 2018 ACC/AHA/HRS guideline provides valuable guidance on this topic 1.
From the Research
Normal PR Interval
The normal PR interval is typically considered to be between 120-200 milliseconds [ 2 ].
Prolonged PR Interval
A PR interval of more than 200 milliseconds is considered prolonged [ 2 ]. This can be an indicator of first-degree atrioventricular block.
Key Points
- A PR interval greater than 240 ms is associated with a higher incidence of abnormal Wenckebach period [ 3 ]
- Prolongation of the PR interval is associated with increased risks of atrial fibrillation, pacemaker implantation, and all-cause mortality [ 2 ]
- The PR interval represents a composite of distinct components, including P-wave onset to P-wave peak duration, P-wave peak to P-wave end duration, and PR-segment, which are not uniformly associated with atrial fibrillation [ 4 ]