The PR Interval on an EKG
The PR interval on an electrocardiogram (EKG) represents the time it takes for an electrical impulse to travel from the atria through the atrioventricular conduction system to the ventricles. This interval is measured from the beginning of the P wave to the beginning of the QRS complex.
Anatomical and Physiological Basis
The PR interval includes:
- Atrial depolarization (P wave)
- Conduction delay at the AV node
- Conduction through the His-Purkinje system
In normal individuals:
- PR interval ranges from 120-200 ms (0.12-0.20 seconds)
- Values >200 ms are considered first-degree AV block 1
Clinical Significance
First-Degree AV Block
- Every atrial stimulus is conducted to the ventricles, but with a PR interval >200 ms
- Conduction delay may occur at:
- AV node level (most common with narrow QRS)
- His-Purkinje system (more likely with wide QRS) 1
Diagnostic Implications
- PR interval helps classify AV blocks:
- First-degree: PR >200 ms with all P waves conducted
- Second-degree type I (Wenckebach): Progressive PR prolongation until a P wave is blocked
- Second-degree type II: Constant PR intervals before blocked P waves
- Third-degree (complete): No P waves conducted to ventricles 1
Factors Affecting PR Interval
- Rate dependence: PR interval typically prolongs with increasing atrial rates
- Autonomic tone:
- Parasympathetic stimulation (vagal tone) → PR prolongation
- Sympathetic stimulation → PR shortening 2
- Medications:
- Calcium channel blockers (verapamil, diltiazem) → PR prolongation
- Beta blockers → PR prolongation 2
Clinical Relevance in Diagnosis
The PR interval is crucial for:
- Differentiating supraventricular tachycardias by analyzing the RP interval (relationship between P waves and QRS complexes) 3
- Identifying conduction abnormalities that may require intervention
- Recognizing drug effects on cardiac conduction
Common Pitfalls in Interpretation
Failure to recognize marked first-degree AV block: PR intervals can be extremely prolonged (up to 560 ms) and may be associated with AV dissociation and pseudo-pacemaker syndrome 4
Overlooking PR variation: Significant variation in PR interval (>30 ms) across different leads may indicate pre-excitation syndromes like Wolff-Parkinson-White syndrome 5
Misinterpreting PR in pre-excitation: In WPW syndrome, the PR interval appears shortened due to conduction through an accessory pathway, bypassing the normal AV nodal delay 1
Remember that while most first-degree AV blocks are benign, extremely prolonged PR intervals (>300 ms) may cause symptoms due to inadequate left ventricular filling when left atrial systole occurs close to or simultaneous with the previous left ventricular systole 1.