In a bradycardic patient with a short PR interval, what is the likely diagnosis and how should it be confirmed and managed?

Bradycardia with Short PR Interval: Diagnosis and Management

Most Likely Diagnosis

The combination of bradycardia with a short PR interval most commonly suggests Wolff-Parkinson-White (WPW) syndrome with an accessory pathway, particularly if delta waves are present on ECG. 1

Critical Diagnostic Differentiation

The presence or absence of delta waves is the single most important ECG finding to guide your next steps:

If Delta Waves Are Present (Classic WPW Syndrome)

• Look for the classic triad: PR interval <120 ms, slurred QRS upstroke (delta wave), and widened QRS >120 ms, representing ventricular pre-excitation via an accessory pathway that bypasses the AV node 1, 2, 3
• This is WPW syndrome and requires immediate cardiology referral if the patient has any symptoms including palpitations, shortness of breath, chest pain, syncope, or near-syncope 1
• Note that intermittent pre-excitation exists where delta waves may not always be visible on every ECG, making diagnosis more challenging—maintain high suspicion if typical WPW findings appear on any ECG that were absent on prior studies 2

If Delta Waves Are Absent

• Consider sinus bradycardia with enhanced AV nodal conduction (physiologic short PR), which is often benign and seen in young individuals, athletes, and during sleep 4
• Evaluate for sinus node dysfunction if P-wave morphology changes are present, suggesting shifting atrial pacemaker sites 5
• First-degree AV block is defined as PR >200 ms, so a short PR interval by definition excludes this diagnosis 6

Immediate Risk Stratification for WPW Syndrome

High-Risk Features Requiring Urgent Intervention

• Shortest pre-excited RR interval <250 ms during atrial fibrillation 1, 7
• History of syncope or near-syncope 1
• Documented atrial fibrillation with pre-excitation 1, 7
• Multiple accessory pathways or posteroseptal location 1, 7
• Accessory pathway refractory period <240 ms 1
• Tachycardia rates exceeding 200/minute 3

Low-Risk Indicators

• Intermittent loss of pre-excitation on ambulatory monitoring 1
• Abrupt loss of pre-excitation during exercise testing 1

Diagnostic Workup

For Suspected WPW Syndrome

• Obtain 12-lead ECG during and immediately after any tachyarrhythmia episode—the diagnosis may only be apparent after termination of the arrhythmia 2
• 24-hour Holter monitoring to capture intermittent pre-excitation 1
• Exercise ECG to assess for loss of pre-excitation with increased sympathetic tone 1
• Electrophysiology study for definitive diagnosis and risk stratification 1, 3
• Echocardiography to exclude underlying structural heart disease 1

For Bradycardia Without Delta Waves

• Document temporal correlation between symptoms and bradycardia using Holter monitor, event recorder, or implantable cardiac monitor 4
• Review all negative chronotropic medications including beta-blockers, non-dihydropyridine calcium channel blockers, digoxin, and antiarrhythmic drugs 4
• Check thyroid function tests to exclude hypothyroidism 4
• Measure electrolytes (potassium, calcium, magnesium) 4
• Consider Lyme disease serology when epidemiologically appropriate 4

Management Algorithm

For Confirmed Symptomatic WPW Syndrome

Catheter ablation is the first-line definitive treatment with success rates of 95-98.5% and major complication rates of only 0.1-0.9%, and should be performed at experienced centers 1

Mandatory indications for ablation include:

• Symptomatic tachyarrhythmias 1
• History of syncope 1
• Documented atrial fibrillation with WPW 1
• High-risk features identified on electrophysiology study 1

Acute Management of WPW-Related Tachyarrhythmias

For hemodynamically unstable patients:

• Immediate synchronized DC cardioversion to prevent progression to ventricular fibrillation 1, 3

For hemodynamically stable patients with regular narrow-complex tachycardia:

• Vagal maneuvers and IV adenosine as first-line treatments 1

For hemodynamically stable patients with pre-excited atrial fibrillation:

• IV procainamide is the first-line treatment 1, 3
• Procainamide is the safest drug for stable WPW patients with any tachydysrhythmia, including wide-complex and irregular rhythms 3

For Asymptomatic Bradycardia with Short PR (No Delta Waves)

• No treatment is required for asymptomatic individuals with physiologic sinus bradycardia and short PR interval 4
• Permanent pacemaker implantation is not indicated in asymptomatic patients, as procedural risks outweigh any potential benefit 4

For Symptomatic Bradycardia with Short PR (No Delta Waves)

• Address all reversible causes first including medications and metabolic abnormalities 4
• For acute symptomatic patients with hemodynamic compromise: Administer atropine 0.5-1 mg IV as first-line treatment, repeated every 3-5 minutes to a maximum total dose of 3 mg 4
• Consider temporary transvenous pacing if medications fail 4
• Consider a trial of oral theophylline to increase heart rate and assess potential benefit of permanent pacing 4

Critical Medication Contraindications in WPW

AV nodal blocking agents are absolutely contraindicated in pre-excited atrial fibrillation due to the risk of accelerating conduction through the accessory pathway and precipitating ventricular fibrillation 1, 3

Never use in WPW with atrial fibrillation:

• Digoxin 1
• Diltiazem 1
• Verapamil 1, 3
• Beta-blockers 1

Note: Verapamil may be useful in narrow-complex, regular rhythms in WPW, but atrial fibrillation may be difficult to diagnose in very rapid tachycardia, making verapamil use potentially dangerous 3

Common Pitfalls to Avoid

• Do not dismiss pre-excitation based solely on the presence of septal Q waves—these can occasionally be present even with manifest anteroseptal accessory pathways 8
• Do not overlook intermittent pre-excitation—obtain ECGs after termination of any tachyarrhythmia episode 2
• Do not pace based solely on heart rate below an arbitrary cutoff without documented symptom correlation 4
• Do not proceed to permanent pacing in patients with documented symptoms occurring in the absence of bradycardia 4
• Do not use AV nodal blocking agents without first ruling out WPW syndrome in any patient with tachycardia rates >200/minute 3

Special Population Considerations

• Athletic individuals often exhibit physiologic sinus bradycardia and short PR intervals due to heightened vagal tone—no intervention is required unless symptoms develop 4
• Young patients require a higher threshold for permanent pacing given long-term device implications 4
• WPW occurs in approximately 0.1-3% of the general population, and the majority remain asymptomatic throughout their lives 9