PR Interval
The PR interval represents the time from the onset of atrial depolarization to the onset of ventricular depolarization. It reflects conduction through the atria, AV node, Bundle of His, and proximal bundle branches.
Also known as: P-R Interval, AV Conduction Time
Measurement
| Normal Range | 0.12–0.20 seconds (120–200 ms) in adults at normal heart rates. Values below 0.12s suggest pre-excitation or accelerated AV conduction; values above 0.20s indicate first-degree AV block. |
| Measurement | Measure from the beginning of the P wave to the beginning of the QRS complex (Q wave if present, otherwise the first deflection of the QRS). Best measured in the lead where both P and QRS are clearly visible, typically lead II. Use the baseline to identify the precise onset of the P wave. |
Clinical Significance
The PR interval is the primary marker of atrioventricular conduction integrity. Prolongation indicates slowed conduction through the AV node or His-Purkinje system, while shortening suggests accessory pathway conduction (as in Wolff-Parkinson-White syndrome) or enhanced AV nodal conduction. Dynamic PR changes across beats are the hallmark of second-degree AV block patterns.
Abnormalities
First-Degree AV Block
PR interval consistently greater than 0.20 seconds with every P wave followed by a QRS. Conduction is slowed but not blocked. Common causes include increased vagal tone, AV nodal disease, digoxin toxicity, and inferior myocardial infarction.
Wolff-Parkinson-White Syndrome (WPW)
PR interval less than 0.12 seconds with a delta wave slurring the upstroke of the QRS. Pre-excitation occurs via an accessory pathway (Bundle of Kent) that bypasses the AV node, resulting in earlier ventricular activation and a widened QRS.
Wenckebach (Mobitz Type I) Second-Degree AV Block
Progressive PR interval lengthening with each beat until a P wave is not conducted (dropped QRS). The PR interval resets to its shortest value after the dropped beat. The cycle then repeats. The site of block is almost always within the AV node itself.
Mobitz Type II Second-Degree AV Block
Fixed PR interval with intermittent sudden non-conduction of P waves without preceding PR prolongation. The block is infranodal (below the AV node, in the His-Purkinje system) and carries a higher risk of progression to complete heart block.
Short PR with Normal QRS (Lown-Ganong-Levine Pattern)
PR interval less than 0.12 seconds with a narrow, normal QRS and no delta wave. Attributed to an atrio-His accessory pathway that bypasses part of the AV node without causing ventricular pre-excitation. Associated with paroxysmal supraventricular tachycardia.
Frequently Asked Questions
What does a prolonged PR interval mean?
A PR interval greater than 0.20 seconds indicates first-degree AV block, meaning there is slowed conduction from the atria to the ventricles through the AV node or His-Purkinje system. By itself, first-degree AV block is generally benign and does not require treatment. However, it can be a marker of underlying structural heart disease, medication effects (digoxin, beta-blockers, calcium channel blockers), or electrolyte abnormalities, so the clinical context is important.
How does the PR interval change during second-degree AV block?
In Mobitz Type I (Wenckebach) block, the PR interval progressively lengthens with each successive beat until a P wave fails to conduct to the ventricles and the cycle resets. In Mobitz Type II block, the PR interval remains constant beat to beat, but occasionally a P wave suddenly fails to conduct without any preceding prolongation. These two patterns have very different clinical implications: Wenckebach is usually benign, while Mobitz II warrants urgent evaluation for pacemaker implantation.
Why is the PR interval shorter in WPW syndrome?
In Wolff-Parkinson-White syndrome, an accessory pathway called the Bundle of Kent connects the atria directly to the ventricles, bypassing the AV node entirely. Because the AV node normally delays conduction by 80–120 ms to allow ventricular filling, bypassing it shortens the total PR interval to less than 0.12 seconds. The ventricles begin depolarizing early via the accessory pathway, producing the characteristic delta wave and widened QRS seen in WPW.
Does the PR interval change with heart rate?
Yes, the PR interval shortens slightly at faster heart rates due to enhanced sympathetic tone accelerating AV nodal conduction, and lengthens at slower rates under vagal influence. These changes are physiologically normal. Abnormal PR behavior at fast rates — such as paradoxical PR prolongation or sudden PR shortening — can suggest pathological conduction or accessory pathway involvement.
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