What are the causes of a short PR (P-R) interval?

Causes of Short PR Interval

A short PR interval (<120 ms) is primarily caused by Wolff-Parkinson-White (WPW) syndrome due to an accessory pathway bypassing the AV node, though it can also represent a benign variant, enhanced AV nodal conduction, or rarely metabolic storage diseases like Fabry or Pompe disease. 1, 2

Primary Pathophysiologic Mechanisms

Wolff-Parkinson-White (WPW) Syndrome

• WPW syndrome is the most critical diagnosis to identify because it carries sudden cardiac death risk, with cardiac arrest being the first presentation in approximately 50% of patients who experience it. 2, 3
• The anatomical substrate is a direct muscular connection (accessory pathway) between atria and ventricles that bypasses the AV node, conducting the electrical impulse prematurely to the ventricles. 1
• Classic ECG findings include PR interval <120 ms, delta wave (slurred initial QRS upstroke), and widened QRS >120 ms. 1, 4
• WPW pattern occurs in approximately 1 in 250 athletes, with prevalence in the pediatric population estimated at 0.15-0.3%. 1
• The presence of a delta wave is the critical distinguishing feature that differentiates dangerous WPW from benign isolated short PR. 2, 3

Enhanced AV Nodal Conduction (Isolated Short PR)

• An isolated short PR interval without delta wave or QRS widening may represent a normal physiologic variant, particularly in young athletes or during increased sympathetic tone. 1, 2
• Shortening of PR interval during exercise or increased sympathetic activity is normal, with shortening of 0.10-0.11 seconds due to increased sympathetic tone and vagal withdrawal. 2
• Asymptomatic athletes with isolated short PR (<120 ms) without delta wave do not require further assessment according to the American College of Cardiology. 1, 3

Metabolic and Storage Diseases

• Fabry disease (X-linked lysosomal storage disorder) characteristically presents with short PR interval, sinus bradycardia, and left ventricular hypertrophy from glycosphingolipid accumulation. 2
• Fabry disease has an incidence of approximately 1 in 40,000-60,000 males and must be considered when short PR occurs with LVH and multisystem symptoms. 2
• Pompe disease (rare genetic disorder) can present with short PR interval, extremely tall QRS complexes, and cardiomegaly, particularly in infantile cases. 2

Neonatal and Pediatric Considerations

• In neonates, intermittent pre-excitation is not uncommon, and even continuous preexcitation may be subtle, detected only in mid-precordial leads. 1
• A high prevalence of WPW in newborns is indicated when two of four characteristics are present: PR <100 ms, QRS <80 ms, lack of Q wave in V6, and left axis deviation. 1

Diagnostic Algorithm

Step 1: Assess QRS Morphology

• First, determine if delta wave and QRS widening (>120 ms) are present to differentiate WPW pattern from isolated short PR. 1, 2
• Intermittent pre-excitation during sinus rhythm on resting ECG suggests a low-risk pathway. 1, 3

Step 2: Risk Stratification for WPW Pattern

• For confirmed WPW pattern, mandatory comprehensive evaluation includes:

  • Detailed symptom history (palpitations, syncope, chest pain) 2, 5
  • Echocardiography to exclude Ebstein's anomaly and cardiomyopathy 1, 2
  • Exercise stress testing to assess for abrupt loss of pre-excitation at higher heart rates (suggests low-risk pathway) 1, 3
  • Electrophysiology study if non-invasive testing is inconclusive or for competitive athletes 1, 2

• High-risk features requiring urgent intervention include:

  • Shortest pre-excited RR interval ≤250 ms during atrial fibrillation 1, 3
  • History of syncope or documented atrial fibrillation with pre-excitation 5
  • Symptomatic tachyarrhythmias 5

Step 3: Evaluate for Metabolic Causes

• Consider Fabry disease when short PR occurs with LVH, especially with multisystem symptoms (renal, neurologic, dermatologic manifestations). 2
• Consider Pompe disease in infants with short PR, extremely tall QRS voltages, and cardiomegaly. 2

Step 4: Age-Appropriate Assessment

• Use age-appropriate PR interval norms, particularly in pediatric populations where normal values differ from adults. 2
• In neonates with short PR, perform complete pediatric cardiologic work-up and ECG on parents and siblings. 1

Critical Pitfalls to Avoid

• Never dismiss WPW pattern as benign, even in asymptomatic patients, as sudden death can occur without warning and may be the first presentation. 2, 3
• Do not confuse isolated short PR with WPW pattern - the presence or absence of delta wave completely changes management and prognosis. 2, 3
• Never use AV nodal blocking agents (digoxin, diltiazem, verapamil, beta-blockers) in pre-excited atrial fibrillation as they can accelerate conduction through the accessory pathway and precipitate ventricular fibrillation. 5
• Do not attribute short PR with LVH solely to hypertension or athletic heart without excluding Fabry disease, especially with multisystem involvement. 2
• Ensure proper ECG calibration, particularly when extremely high QRS voltages are present (as in Pompe disease). 2
• Recognize that WPW pattern can mask ischemic changes on ECG and may increase arrhythmia risk during acute myocardial infarction. 6