What is the PR (P-R) interval?

The PR Interval

The PR interval is the time from the onset of the P wave to the beginning of the QRS complex, representing the time required for atrial depolarization and conduction through the AV node, and is normally between 120-200 milliseconds. 1

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 1
• A normal PR interval ranges from 120-200 milliseconds in adults 2
• PR interval >200 milliseconds is defined as first-degree atrioventricular (AV) block 1, 2
• The PR interval primarily reflects conduction time through the atria and AV node, with the AV node typically being the site of greatest delay 3

Clinical Significance

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

PR Interval in Arrhythmia Diagnosis

• 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 1
• In orthodromic atrioventricular reentrant tachycardia (AVRT), the P wave is usually visible in the early part of the ST-T segment 1
• A long RP interval is typical of atrial tachycardia (AT) because the rhythm is driven by the atrium and conducts normally to the ventricles 1
• The PR interval helps distinguish between different types of AV block 1

Prognostic Implications

• Each 20-millisecond increment in PR interval is associated with increased risk of atrial fibrillation (adjusted HR 1.11), pacemaker implantation (adjusted HR 1.22), and all-cause mortality (adjusted HR 1.08) 2
• First-degree AV block (PR >200 ms) is associated with a 2-fold risk of atrial fibrillation, 3-fold risk of pacemaker implantation, and 1.4-fold risk of all-cause mortality 2
• However, some studies have found no increased risk of mortality or cardiovascular hospitalizations with prolonged PR interval in middle-aged populations 5
• The PR interval has age-dependent variations, with different rate-adjustment formulas needed for adults younger than 60 years versus those 60 years or older 6

Factors Affecting PR Interval

• Autonomic tone significantly influences PR interval - increased parasympathetic tone prolongs it while sympathetic stimulation shortens it 3
• Medications including beta-blockers and calcium channel blockers (particularly verapamil and diltiazem) can prolong the PR interval 3
• The PR interval is inversely related to heart rate, though this relationship is weaker than that between QT interval and heart rate 6
• Age is an important factor affecting PR interval, with older adults typically having longer PR intervals 6

Clinical Pearls

• When evaluating narrow QRS tachycardias, the relationship between the P wave and QRS complex (RP versus PR intervals) helps determine the mechanism 1
• In first-degree AV block with narrow QRS complexes, the conduction delay is usually in the AV node 4
• In first-degree AV block with wide QRS complexes, the conduction delay may be in either the AV node or the His-Purkinje system 4
• A prolonged PR interval that normalizes over time is observed in approximately 30% of individuals 5