The history of the Wolff-Parkinson-White (WPW) syndrome is a 50-year–long tale of speculation and discussion among physiologists, anatomists, and clinicians about how to explain the frequently occurring tachycardias in patients showing a strange ECG.1 That riddle was solved in 1967 in Amsterdam when Durrer and associates2 showed that the WPW syndrome was based on a second connection between the atrium and ventricle apart from the normal AV node–His pathway. The presence of conduction over such an accessory AV pathway was demonstrated by epicardial mapping during sinus rhythm. The crucial role of that structure in the tachycardia mechanism was proven by programmed electrical stimulation of the heart and mapping of cardiac activation by intracardiac catheters.3 It was shown that an impulse circulating in a circuit (consisting of atrium–AV node–His pathway–ventricle–accessory AV pathway) was responsible for the tachycardias usually found in these patients. Essential for initiation of the tachycardia is a difference in the electrophysiological properties of the 2 AV connections, which allows the occurrence of unidirectional block in 1 of the 2 structures. This difference can be exposed not only by critically timed atrial and ventricular premature beats but also by changes in the sinus rate (eg, during exercise). For perpetuation of the tachycardia, the circulation time of the impulse in the tachycardia circuit must be longer than the duration of the refractory period of the different parts of the tachycardia circuit.4 See p 3030 It also became clear that the anterograde refractory period of the accessory AV pathway determines the ventricular rate during atrial fibrillation.5 This helps to identify WPW patients at risk for dying suddenly when atrial fibrillation supervenes. The demonstration that an accessory AV pathway is an essential link both in the reentrant tachycardia mechanism and the ventricular rate during atrial fibrillation has …