Normal PR Interval
The normal PR interval is 120-200 milliseconds (ms). 1
Definition and Measurement
- The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex on an electrocardiogram (ECG) 1
- It represents the time required for atrial depolarization and conduction through the atrioventricular (AV) node to the ventricles 1
- PR intervals are best measured in lead II or V5 1
Clinical Significance of PR Interval Abnormalities
Prolonged PR Interval (First-Degree AV Block)
- First-degree AV block is defined as a PR interval >200 ms with 1:1 atrioventricular conduction 1
- This represents a delay in conduction that may occur at the level of the AV node or His-Purkinje system 1
- If the QRS complex is narrow, the conduction delay is usually in the AV node and rarely within the His bundle 1
- If the QRS is wide, the conduction delay may be either in the AV node or in the His-Purkinje system 1
Short PR Interval
- PR intervals <120 ms are considered short 2
- Short PR intervals may be associated with pre-excitation syndromes or accelerated AV conduction 3
Special Considerations
Athletes
- Sinus bradycardia and first-degree AV block (PR interval 200-400 ms) are considered normal variants in athletes due to increased vagal tone 1
- These are physiological adaptations to regular exercise and do not require further evaluation in asymptomatic athletes with no significant family history 1
PR Interval and Heart Rate
- There is a negative correlation between PR interval and heart rate, though this correlation is weaker than that between QT interval and heart rate 4
- The association between PR interval and heart rate is age-dependent 4
- For individuals <60 years: PRa = PR + 0.26 (HR - 70) 4
- For individuals ≥60 years: PRa = PR + 0.42 (HR - 70) 4
Clinical Implications
- Markedly prolonged PR intervals (usually >300 ms) may cause symptoms due to inadequate left ventricular filling 1
- PR interval prolongation has been associated with increased risks of atrial fibrillation, pacemaker implantation, and all-cause mortality in some studies 3
- However, other studies have found no increased mortality risk with PR interval prolongation after adjustment for confounders 5, 6
- The clinical significance of PR interval abnormalities should be interpreted in the context of other clinical findings 1
Common Pitfalls
- Failing to recognize that PR interval prolongation can normalize over time in a substantial proportion of individuals 5
- Not considering that the significance of PR interval as a predictor may depend on the level of contribution of P-wave duration to its length 2
- Overlooking that first-degree AV block in athletes may be a normal physiological finding rather than pathological 1