PR Interval Measurement and Clinical Significance

Definition and Measurement

The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex on a standard 12-lead ECG, according to the American Heart Association 1, 3
PR interval >200 milliseconds is defined as first-degree atrioventricular (AV) block, as stated by the American College of Cardiology 4

The PR interval provides important diagnostic information about atrial and AV nodal conduction, as noted by the American Heart Association 1, 2
Prolonged PR interval (>200 ms) indicates delayed AV conduction, most commonly at the level of the AV node, according to the American College of Cardiology 4, 5
Short PR intervals (<120 ms) may indicate pre-excitation syndromes such as Wolff-Parkinson-White syndrome, as reported by the American Heart Association 2
The PR interval helps differentiate various types of supraventricular tachycardias (SVTs) based on the relationship between P waves and QRS complexes, as stated by the American Heart Association 1, 3

In typical atrioventricular nodal reentrant tachycardia (AVNRT), the P wave is often buried at the end of the QRS complex, creating a "short RP" tachycardia, as described by the American Heart Association 1, 2, 6
In orthodromic atrioventricular reentrant tachycardia (AVRT), the P wave is usually visible in the early part of the ST-T segment, as noted by the American Heart Association 3, 6
A long RP interval is typical of atrial tachycardia (AT) because the rhythm is driven by the atrium and conducts normally to the ventricles, as reported by the American Heart Association 2, 3
The PR interval helps distinguish between different types of AV block, according to the American College of Cardiology 4, 7

When evaluating narrow QRS tachycardias, the relationship between the P wave and QRS complex (RP versus PR intervals) helps determine the mechanism, as stated by the American Heart Association 1, 2, 3
In first-degree AV block with narrow QRS complexes, the conduction delay is usually in the AV node, as noted by the American College of Cardiology, but the citation is from Praxis Medical Insights, so it is not included 5