Can WPW Syndrome Be Misdiagnosed as Atrial Fibrillation?
Yes, WPW syndrome presenting with pre-excited atrial fibrillation is frequently misdiagnosed as typical atrial fibrillation, which is a potentially fatal error that can lead to administration of contraindicated medications and precipitate ventricular fibrillation. 1, 2
Why This Misdiagnosis Occurs
The electrocardiographic presentation of pre-excited atrial fibrillation in WPW syndrome mimics atrial fibrillation in several critical ways:
- Both present with irregular rhythms - The chaotic atrial activity conducts variably through the accessory pathway, creating an irregular ventricular response that appears identical to typical AFib 1
- Extremely rapid ventricular rates - Pre-excited AF commonly presents with ventricular rates of 240 ± 56 beats/min, with shortest R-R intervals averaging 194 ± 40 ms, which overlaps with the rapid ventricular response seen in typical AFib 3
- The wide QRS complex can be misinterpreted - The wide QRS (≥120 ms) in pre-excited AF results from anterograde conduction down the accessory pathway, but emergency providers may mistake this for AFib with aberrancy or bundle branch block 1, 4
Critical Distinguishing Features
A patient with wide complex irregular tachycardia should be assumed to have pre-excited atrial fibrillation due to WPW until proven otherwise 1:
- Delta waves may be visible between beats, representing the slurred initial QRS upstroke from ventricular pre-excitation 5, 4
- Variable QRS morphology - The QRS width and morphology change beat-to-beat depending on the degree of pre-excitation versus normal AV nodal conduction 1
- Extremely rapid rates exceeding 200-250 beats/min should raise immediate suspicion for WPW, as typical AFib rarely conducts this rapidly through the normal AV node 4, 6
- Younger patient age - WPW typically presents in the first two decades of life, whereas typical AFib is more common in older patients 5
Why This Misdiagnosis Is Dangerous
The consequences of misdiagnosing pre-excited AF as typical AFib are potentially fatal:
- AV nodal blocking agents are absolutely contraindicated in pre-excited AF but are standard therapy for typical AFib 7, 1
- Administering digoxin, diltiazem, verapamil, or beta-blockers blocks the AV node while leaving the accessory pathway unopposed, accelerating conduction through the bypass tract and precipitating ventricular fibrillation 7, 5, 1
- Adenosine is also contraindicated when the QRS is wide, as it can trigger the same dangerous acceleration 5
Correct Management Algorithm
For Hemodynamically Unstable Patients:
- Immediate direct-current cardioversion is the Class I recommendation to prevent ventricular fibrillation 7, 5, 1
For Hemodynamically Stable Patients:
- First-line pharmacological therapy: intravenous procainamide or ibutilide (Class I recommendation) to slow accessory pathway conduction and restore sinus rhythm 7, 5, 1
- Alternative options (Class IIb): intravenous flecainide, quinidine, or disopyramide 7, 1
- Amiodarone should be used with extreme caution as it can paradoxically accelerate ventricular conduction in WPW 1
Definitive Treatment:
- Catheter ablation of the accessory pathway is the Class I recommendation for all symptomatic patients with WPW who present with atrial fibrillation, with success rates of 93-98.5% 5, 1
Common Pitfalls to Avoid
- Never assume irregular wide complex tachycardia is AFib with aberrancy - Always consider pre-excited AF in the differential, especially in younger patients or those with rates >200 bpm 1, 4
- Do not rely on rate control with typical AFib medications - This approach is contraindicated and potentially lethal in WPW 7, 5
- Recognize that misdiagnosis can lead to cardiac arrest - Proper electrocardiographic interpretation is pivotal, as incorrect treatment can induce ventricular fibrillation 2
- Be aware that AF occurs in up to 50% of WPW patients - This is not a rare presentation and should always be on your differential 2