What is a normal PR interval (P-R interval)?

From the Guidelines

The normal PR interval on an electrocardiogram (ECG) ranges from 120 to 200 milliseconds (0.12 to 0.20 seconds). This interval represents the time it takes for an electrical impulse to travel from the sinus node through the atria to the atrioventricular (AV) node and then to the ventricles. It is measured from the beginning of the P wave to the beginning of the QRS complex. A PR interval within this normal range indicates proper electrical conduction between the atria and ventricles. Values shorter than 120 milliseconds may suggest pre-excitation syndromes like Wolff-Parkinson-White syndrome, while intervals longer than 200 milliseconds indicate first-degree AV block, which may be caused by medications, electrolyte abnormalities, increased vagal tone, or underlying heart disease, as noted 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 is an important diagnostic parameter when evaluating cardiac rhythm disorders and conduction abnormalities.
• First-degree AV block is characterized by a PR interval longer than 200 milliseconds, where each P wave is conducted, but with a prolonged delay, as discussed in the guidelines for cardiac pacing and cardiac resynchronization therapy 1.
• The level of AV block can be classified as supra-Hisian, intra-Hisian, or infra-Hisian, depending on the anatomical point at which the conduction of the activation wavefront is impaired.
• A thorough evaluation of the ECG is required for the diagnosis of atrioventricular block, considering factors such as the relationship between P waves and QRS complexes, and the presence of other cardiac conditions.

Clinical Implications

• Accurate measurement and interpretation of the PR interval are crucial for diagnosing and managing cardiac conduction abnormalities.
• Patients with abnormal PR intervals may require further evaluation and treatment to prevent complications and improve quality of life, as emphasized in the 2019 ACC/AHA/HRS guideline 1.
• The PR interval should be considered in the context of the patient's overall clinical presentation, medical history, and other diagnostic findings.

From the Research

Definition of Normal PR Interval

• A normal PR interval is typically considered to be between 120-200 milliseconds 2.
• First-degree atrioventricular block is defined as a PR interval exceeding 200 milliseconds 2.
• Some studies suggest that a PR interval greater than 240 ms may be considered abnormal 3.

Clinical Significance of Prolonged PR Interval

• Prolongation of the PR interval is associated with increased risks of atrial fibrillation, pacemaker implantation, and all-cause mortality 2.
• A prolonged PR interval may be a marker of more advanced heart disease 4.
• Patients with first-degree AV block may experience more episodes of undetected "electrical desynchronization" during cardiac resynchronization therapy (CRT) 4.

Variability in PR Interval

• The PR interval can vary depending on factors such as age, sex, and heart rate 3, 5.
• Some individuals may have a prolonged PR interval at rest, but it may normalize with exercise or other conditions 3, 5.
• The prognosis of individuals with a prolonged PR interval may vary depending on the underlying cause and other factors 6, 5.

Diagnosis and Treatment

• Diagnosis of a prolonged PR interval is typically made using electrocardiography (ECG) 2, 3.
• Treatment of a prolonged PR interval depends on the underlying cause and may include pacing, medication, or other interventions 4, 6.