Normally, the AV node is the only conduction pathway for impulses from the atria to the ventricles. Wolff-Parkinson-White syndrome is characterized by the presence of an accessory atrioventicular pathway located between the wall of the right or left atria and the ventricles, known as the Bundle of Kent. This pathway allows the impulse to bypass the AV node and activate the ventricles prematurely. Consequently, an initial slur to the QRS complex, known as a delta wave may be observed. The QRS complexes are wide, more than 0.11 sec, indicating that the impulse did not travel through the normal conducting system. The PR is shortened, to less than 0.12 sec, because the delay at the AV node is bypassed.

The EKG on the top shows normal sinus rhythm. The EKG at the bottom showsWolff-Parkinson-White Syndrome

The accessory pathway can cause a reentry circuit to be established. Reentry is initiated by a premature atrial or ventricular beat coupled with a unidirectional block in one of the pathways (because the normal impulse gets to pathway when it is refractory after the premature beat). The result is a continuous impulse conduction. Reentry causes two kinds of tachycardia.

Wolff-Parkinson-White syndrome is commonly associated with congenital heart abnormalities like Tetrology of Fallot, coarctation of the aorta, tricuspid atresia and transposition of the great vessels. In severe cases, treatment would involve surgical removal or ablation of one of the pathways.

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