Wolff-Parkinson-White syndrome (WPW) occurs when an abnormal conduction pathway connects the atrium directly to the ventricles allowing conduction to bypass the AV node at times. This abnormal pathway is termed an "accessory pathway" or a "bypass tract".
The typical ECG finding of WPW is a short PR interval and a "delta wave". A delta wave is slurring of the upstroke of the QRS complex. This occurs since the action potential from the SA node is able to conduct to the ventricles very fast through the accessory pathway, so the QRS occurs immediately after the P wave making the delta wave.
When WPW occurs in the setting of atrial fibrillation, the fast atrial rate of 400-600 can conduct quite rapidly to the ventricles. If AV nodal blocking drugs such as beta-blockers or non-dihydropyridine calcium channel blockers are given in this setting, the conduction through the accessory pathway will be enhanced. This occurs since the accessory pathway is not subject to block by these medications, however the AV node is resulting in fewer action potentials traveling through the AV node and more through the accessory pathway. Procainamide or emergency cardioversion is the treatment of choice in the setting of WPW and atrial fibrillation. See a strip of how this "pre-excited atrial fibrillation": appears as a wide-QRS complex, irregular irregular tachycardia on the ECG:
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