What is considered a prolonged P-R (PR) interval?

Prolonged PR Interval Definition and Clinical Significance

A PR interval greater than 200 milliseconds is considered prolonged and indicates first-degree atrioventricular block. 1

Definition and Measurement

• The PR interval represents the time from the onset of atrial depolarization to the onset of ventricular depolarization, measured from the earliest onset of the P wave to the earliest onset of the QRS complex 1
• Normal PR interval ranges from 120-200 ms; any PR interval >200 ms is classified as first-degree AV block 1
• Measurement requires examining multiple leads to identify the earliest P wave onset and earliest QRS onset 1

Causes of PR Prolongation

PR interval prolongation can result from various conditions affecting the conduction system:

• Most commonly occurs at the level of the AV node 2
• Physiological causes include increased vagal tone, particularly in athletes 2, 1
• Pathological causes include:
  • Medications: Beta-blockers, calcium channel blockers (e.g., verapamil) 2, 3
  • Congenital heart defects 2
  • Genetic mutations (e.g., SCN5A mutations) 2
  • Infectious causes (Lyme carditis, bacterial endocarditis, acute rheumatic fever) 2
  • Inflammatory/infiltrative conditions (myocarditis, amyloidosis) 2
  • Cardiomyopathies (including hypertrophic cardiomyopathy) 2
  • Ischemic heart disease 2
  • Degenerative diseases of the conduction system 2
  • Metabolic/endocrine disorders 2

Clinical Significance

• First-degree AV block with PR >200 ms has been associated with:

  • Increased risk of atrial fibrillation (HR 2.06) 4
  • Increased risk of pacemaker implantation (HR 2.89) 4
  • Increased all-cause mortality (HR 1.44) 4
  • Increased risk of heart failure (RR 1.39) 5

• Each 20 ms increment in PR interval is associated with:

  • 11% increased risk of atrial fibrillation 4
  • 22% increased risk of pacemaker implantation 4
  • 8% increased risk of all-cause mortality 4

• Extremely prolonged PR intervals (e.g., ≥0.30 s) may be associated with:

  • Pseudo-pacemaker syndrome 6, 7
  • Symptoms including exercise intolerance, palpitations, and syncope 6
  • Atrioventricular dissociation in severe cases 6

Special Considerations

• In athletes, PR intervals up to 400 ms may be normal due to increased vagal tone 1
• PR prolongation in the setting of bifascicular block is often at the AV node level and doesn't necessarily predict progression to higher-degree AV block 2
• Some studies suggest PR prolongation may normalize over time in a substantial proportion of individuals 8

Management Implications

• PR intervals >200 ms require monitoring for potential progression to higher-degree AV block 1
• PR intervals ≥400 ms warrant further evaluation 1
• In symptomatic patients with extremely prolonged PR intervals causing pseudo-pacemaker syndrome, pacemaker implantation may be considered 7
• Medications that prolong AV conduction (beta-blockers, calcium channel blockers) should be used cautiously in patients with pre-existing PR prolongation 3