Pitfall: Atrial Fibrillation in WPW Syndrome

There is a very important pitfall to be aware of when assessing and treating patients with WPW and a tachycardia. It is vital that you check the arrhythmia is not atrial fibrillation.

Why must you check for this?

In atrial fibrillation the atrial rate is 400-600bpm, but the AV node stops the majority of these signals reaching the ventricles. In WPW, the accessory pathway allows for rapid conduction directly to the ventricles, bypassing the AV node. Recognition is essential, as administering AV-nodal blocking drugs may cause the excessive electrical impulses to preferentially pass down the accessory pathway, precipitating ventricular arrhythmias and cardiac arrest.

ECG Features (figure 12):

• Rate >200bpm (may be >300bpm, which is too rapid to be conducted via the AV node)
• Irregular rhythm
• Delta wave
• Wide QRS (abnormal ventricular depolarisation via accessory pathway)
• Stable axis (differentiates for polymorphic VT)

Management involves DC cardioversion if the patient is unstable, or type I antiarrhythmics if stable such as procainamide.

Fig. 1 ‘Pre-excited’ Atrial Fibrillation in a patient with an accessory pathway

Learning bite

In patients with atrial fibrillation suspected of having an accessory pathway (e.g. WPW), all AV-nodal blocking drugs are contraindicated due to risk of precipitating ventricular arrhythmia or death. These include adenosine, beta-blockers, diltiazem, verapamil, digoxin and amiodarone.