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

Short PR Interval: Causes and Clinical Implications

A short PR interval (<120 ms) is most commonly caused by ventricular pre-excitation via an accessory pathway (Wolff-Parkinson-White syndrome), though an isolated short PR without delta wave or QRS widening may represent a benign variant, particularly in athletes. 1, 2

Primary Causes

Wolff-Parkinson-White (WPW) Syndrome

• The classic triad consists of PR interval <120 ms, delta wave (slurred QRS upstroke), and QRS duration >120 ms 1, 2, 3
• The accessory pathway bypasses the AV node, allowing premature ventricular activation and creating the characteristic delta wave representing early ventricular depolarization 2, 3
• The widened QRS results from fusion of two wavefronts: one through the accessory pathway and one through the normal AV node-His-Purkinje system 3
• WPW carries sudden cardiac death risk because rapid conduction of atrial fibrillation across the accessory pathway can degenerate into ventricular fibrillation 1, 2

Isolated Short PR Interval (Without Delta Wave or Wide QRS)

• In asymptomatic athletes, an isolated short PR interval without widened QRS or delta wave should not be considered for further assessment 1
• May represent either a normal variant or Lown-Ganong-Levine syndrome 2
• Careful evaluation is still warranted as it may reflect underlying structural heart disease 2

Risk Stratification for WPW

High-Risk Features

• Shortest pre-excited RR interval ≤250 ms (≥240 beats/min) during atrial fibrillation indicates high risk and warrants catheter ablation 1, 2
• History of symptomatic tachycardia, multiple accessory pathways, or presence of Ebstein's anomaly 2
• Cardiac arrest is the first manifestation in approximately half of WPW cases, emphasizing the critical importance of proper identification 2

Low-Risk Indicators

• Intermittent pre-excitation during sinus rhythm on resting ECG suggests a low-risk pathway 1, 2
• Abrupt, complete loss of pre-excitation at higher heart rates during exercise stress testing indicates a low-risk accessory pathway 1

Evaluation Algorithm

For WPW Pattern (Short PR + Delta Wave + Wide QRS)

1. Obtain detailed symptom history (palpitations, syncope, near-syncope) and family history 2
2. Perform echocardiography to evaluate for Ebstein's anomaly and cardiomyopathy 1, 2
3. Exercise stress test to assess for abrupt loss of pre-excitation at higher heart rates 1, 2
4. If non-invasive testing cannot confirm low-risk pathway or is inconclusive, proceed to electrophysiological study to determine shortest pre-excited RR interval during atrial fibrillation 1, 2
5. Some physicians subject all competitive athletes in moderate/high-intensity sports to electrophysiological studies regardless of exercise test results, as high catecholamine levels during intense exercise may modify accessory pathway refractory periods unpredictably 1

For Isolated Short PR (Without Delta Wave or Wide QRS)

• In asymptomatic athletes, no further evaluation is required 1
• In symptomatic patients or those with concerning features, consider echocardiography and further cardiac evaluation 2

Management

Definitive Treatment

• Catheter ablation is recommended for high-risk WPW (shortest pre-excited RR interval ≤250 ms) 1, 2
• Ablation should be considered for symptomatic patients even with longer RR intervals 2

Critical Pitfalls

• Do not dismiss pre-excitation based solely on the presence of septal Q waves, as they can occasionally occur in WPW with anteroseptal accessory pathways 4
• Atrial fibrillation with rapid ventricular response in WPW may be misinterpreted as ventricular tachycardia due to grossly irregular rhythm with bizarre QRS complexes at rates exceeding 180/min 5
• Non-invasive tests are considered inferior to invasive electrophysiological assessment for determining sudden cardiac death risk 2
• Patients with intermittent pre-excitation, once thought to be at minimal risk, may still carry significant risk 6