From the Guidelines
The PR interval length is considered normal when it measures between 120-200 milliseconds (0.12-0.20 seconds) in adults, and a prolonged PR interval greater than 200 ms indicates first-degree AV block, as stated in the 2019 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 1.
Key Points to Consider
- The PR interval represents the time it takes for electrical impulses to travel from the atria through the atrioventricular (AV) node to the ventricles.
- A prolonged PR interval may be caused by medications like beta-blockers, calcium channel blockers, digoxin, or conditions such as increased vagal tone, ischemia, or fibrosis.
- The 2019 ACC/AHA/HRS guideline provides a comprehensive overview of the evaluation and management of patients with bradycardia and cardiac conduction delay, including the diagnosis and treatment of first-degree AV block 1.
Importance of PR Interval
- The PR interval is important for assessing cardiac conduction and can help diagnose various rhythm abnormalities.
- Changes in the PR interval should be interpreted in the context of the patient's clinical presentation, medication use, and other ECG findings.
Clinical Implications
- A shortened PR interval (less than 120 ms) may indicate pre-excitation syndromes like Wolff-Parkinson-White syndrome, where an accessory pathway bypasses the normal conduction delay at the AV node.
- The diagnosis and treatment of AV block should be guided by the most recent and highest quality evidence, such as the 2019 ACC/AHA/HRS guideline 1.
From the Research
PR Interval Length
- The PR interval on the electrocardiogram represents the time that it takes an impulse to travel through the atrium and atrioventricular (AV) conduction system to the ventricles 2.
- Normally, activation is slowest in the AV node, and variations in PR interval most commonly parallel changes in AV nodal activation time 2.
- The AV nodal conduction time and effective refractory period are rate dependent and, in adult humans, are usually prolonged with increasing atrial paced rates 2.
Factors Affecting PR Interval
- Alterations in autonomic tone affect AV nodal conduction as well as sinus rate, with the effect usually in the same direction but often to different degrees 2.
- Drugs including the slow channel blockers and beta blockers affect AV nodal function, with slow channel blockers inhibiting the slow inward calcium current, which may prolong conduction and refractoriness in the AV node 2.
- Beta blockers have a negative dromotropic effect on the AV node, prolonging the AH interval and AV nodal refractory periods and may lengthen the PR interval 2.
Clinical Significance of Prolonged PR Interval
- Prolonged PR interval, or first degree AV block, has been traditionally regarded as a benign electrocardiographic finding in healthy individuals, but recent studies have suggested that it may be associated with increased mortality and morbidity 3.
- However, a study of 10,785 individuals found that prolonged PR interval was not associated with an increased risk of all-cause or cardiovascular mortality 3.
- The PR interval may be used as a selection criterion to identify the optimal physiological pacing mode, with persistent AF incidence lower in short PR patients treated by right ventricular pacing minimization and in long PR patients treated by standard dual-chamber pacing 4.
Evaluation of PR Interval
- Evaluation of the A-V node function by determining the Wenckebach period (WP) and atropine response can be used to assess PR interval 5.
- A strong linear decreasing correlation between the PR interval of the ECG and the WP was found in individuals with WP less than or equal to 110 ppm 5.
- PR interval greater than 240 ms had the greatest and significant incidence in the group with WP less than or equal to 110 ppm 5.