During fetal life, numerous connections link the
atria to the ventricles, but they all disappear before birth except for the
bundle of His. Sometimes, some of the connections do not disappear. This
accessory pathway(s) characterizes the Wolff-Parkinson-White syndrome. An
accessory atrioventricular connection (bundle of Kent) bypasses the
atrioventricular node, and inserts directly into myocardium (consistent with
bypass tracts in Lown-Ganong-Levine syndrome). The accessory bundle may
conduct anteor retrograde. Antegrade conduction causes early commencement
of ventricular depolarization, seen as a delta wave on the ECG. Retrograde
conduction allows a reentry circuit to develop and is a mechanism for
paroxysmal tachycardia generation. The accessory connection may only conduct
in a retrograde direction; therefore a delta wave will not appear on ECG.
Such a tract is described as concealed. In atrial fibrillation, antegrade
conduction along the accessory pathway may result in rapid ventricular rate,
which is poorly tolerated.