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

Causes of Short PR Interval

A short PR interval (<120 ms) is primarily caused by either Wolff-Parkinson-White (WPW) syndrome with an accessory pathway bypassing the AV node, or represents a normal physiologic variant, particularly in young healthy individuals and athletes. 1, 2

Primary Pathophysiologic Causes

Wolff-Parkinson-White (WPW) Syndrome

• WPW pattern is the most critical cause to identify, characterized by a short PR interval, delta wave (slurred initial QRS upstroke), and wide QRS complex (>120 ms) resulting from an accessory pathway that bypasses the AV node and allows early ventricular activation 1, 2
• This occurs in approximately 1 in 250 athletes and carries a risk of sudden cardiac death, which can be the first manifestation in approximately 50% of cases 2, 3
• Associated structural abnormalities include Ebstein's anomaly and hypertrophic cardiomyopathy 2

Lown-Ganong-Levine (LGL) Syndrome

• Isolated short PR interval without delta wave or QRS widening may represent LGL syndrome, which involves an accessory pathway (James tract) that bypasses part of the AV node but does not cause ventricular pre-excitation 2, 4
• This condition is associated with paroxysmal supraventricular tachycardias but does not carry the same sudden death risk as WPW 4

Normal Physiologic Variant

• Shortening of the PR interval during exercise or increased sympathetic tone (by as much as 0.10-0.11 seconds) is normal in young, healthy individuals, probably due to increased sympathetic tone and vagal withdrawal 5
• Isolated short PR interval in asymptomatic athletes without delta wave represents a normal variant requiring no further evaluation 2, 3

Metabolic and Storage Diseases

Fabry Disease

• X-linked lysosomal storage disorder with glycosphingolipid accumulation in vascular endothelium, characterized by short PR interval, sinus bradycardia, and left ventricular hypertrophy 2
• Incidence is approximately 1 in 40,000 to 60,000 males and must be considered in patients with short PR and LVH, especially with multisystem symptoms 2

Pompe Disease

• Rare genetic disorder that can present with short PR interval (present in 75% of infantile cases), extremely tall QRS complexes, and cardiomegaly 2, 3
• When evaluating infants with cardiomegaly, short PR interval should prompt consideration of Pompe disease and measurement of serum creatine kinase 3

Coexisting Arrhythmia Substrates

Dual AV Nodal Physiology

• AVNRT can coexist with WPW syndrome, though uncommonly, representing dual pathology with both an accessory pathway and dual AV nodal physiology 6
• Patients with short PR intervals without delta waves demonstrate frequent arrhythmias involving multiple areas of the conduction system, including atrial premature beats (62%), paroxysmal supraventricular tachycardia (21%), and ventricular premature beats (58%) 7

Critical Diagnostic Algorithm

Step 1: Assess QRS Morphology

• Examine for delta wave and QRS duration to differentiate between WPW pattern (short PR + delta wave + wide QRS) and isolated short PR (short PR + normal QRS + no delta wave) 2, 3

Step 2: Risk Stratification Based on Findings

If WPW Pattern Present:

• Mandatory comprehensive evaluation regardless of symptoms, including detailed symptom history (palpitations, presyncope, syncope, family history of sudden death), 12-lead ECG, and echocardiography to evaluate for Ebstein's anomaly and structural cardiomyopathy 3
• Exercise stress testing as first-line non-invasive risk stratification to assess for abrupt loss of pre-excitation at higher heart rates (low-risk feature) 3
• Electrophysiological study when non-invasive testing is inconclusive or for competitive athletes, with high-risk criteria including shortest pre-excited RR interval ≤250 ms during induced atrial fibrillation 3

If Isolated Short PR (No Delta Wave):

• In asymptomatic athletes or patients without palpitations, no further cardiac evaluation is indicated as this represents either a normal variant or LGL syndrome, neither requiring intervention 2, 3
• Consider Fabry disease in patients with short PR and LVH before attributing findings to hypertension or athletic heart 2
• In infants with cardiomegaly and short PR, consider Pompe disease with proper ECG calibration to avoid missing extremely high QRS voltage 3

Common 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 manifestation 2, 3
• Do not confuse isolated short PR with WPW pattern, as the presence or absence of delta wave and QRS widening completely changes management 2, 3
• Ensure proper ECG calibration when evaluating for short PR, particularly in conditions like Pompe disease where QRS voltage may be extremely high and operators may decrease gain, missing this diagnostic clue 3
• Use age-appropriate PR interval norms, particularly adjusting for gestational age in infants, to ensure accurate diagnosis 3